Auto merge of #6824 - Y-Nak:refactor_loops_module, r=flip1995

Refactor: organize loops file into loops module (Delegated)

`@flip1995` `@nahuakang`

closes #6693
r? `@flip1995`

 As we talked about in the PM of Zulip,  this PR is a delegated PR from `@nahuakang.`

Changes from the last commit of #6693:
1. Unify the name of the main entries of all modules to check, that was pointed out [here](https://github.com/rust-lang/rust-clippy/pull/6693#discussion_r582897713)
2. Simplify ` check_for_loop_arg`, that was pointed out [here](https://github.com/rust-lang/rust-clippy/pull/6693#discussion_r582914044) and [here](https://github.com/rust-lang/rust-clippy/pull/6693#discussion_r582929910)
3. Resolve conflicts

changelog: Refactor `loops.rs` file into `loops` module.

21 files changed, 3386 insertions, 3173 deletions

diff --git a/clippy_lints/src/loops.rs b/clippy_lints/src/loops.rs
deleted file mode 100644
index 711cd5b3b15..00000000000
--- a/clippy_lints/src/loops.rs
+++ /dev/null
@@ -1,3173 +0,0 @@
-use crate::consts::constant;
-use crate::utils::sugg::Sugg;
-use crate::utils::usage::mutated_variables;
-use crate::utils::visitors::LocalUsedVisitor;
-use crate::utils::{
-    contains_name, get_enclosing_block, get_parent_expr, get_trait_def_id, has_iter_method, higher, implements_trait,
-    indent_of, is_in_panic_handler, is_integer_const, is_no_std_crate, is_ok_ctor, is_refutable, is_some_ctor,
-    is_type_diagnostic_item, last_path_segment, match_trait_method, match_type, multispan_sugg, path_to_local,
-    path_to_local_id, paths, single_segment_path, snippet, snippet_with_applicability, snippet_with_macro_callsite,
-    span_lint, span_lint_and_help, span_lint_and_sugg, span_lint_and_then, sugg, SpanlessEq,
-};
-use if_chain::if_chain;
-use rustc_ast::ast;
-use rustc_data_structures::fx::{FxHashMap, FxHashSet};
-use rustc_errors::Applicability;
-use rustc_hir::def::{DefKind, Res};
-use rustc_hir::intravisit::{walk_block, walk_expr, walk_pat, walk_stmt, NestedVisitorMap, Visitor};
-use rustc_hir::{
-    def_id, BinOpKind, BindingAnnotation, Block, BorrowKind, Expr, ExprKind, GenericArg, HirId, InlineAsmOperand,
-    Local, LoopSource, MatchSource, Mutability, Node, Pat, PatKind, QPath, Stmt, StmtKind,
-};
-use rustc_infer::infer::TyCtxtInferExt;
-use rustc_lint::{LateContext, LateLintPass, LintContext};
-use rustc_middle::hir::map::Map;
-use rustc_middle::lint::in_external_macro;
-use rustc_middle::middle::region;
-use rustc_middle::ty::{self, Ty, TyS};
-use rustc_session::{declare_lint_pass, declare_tool_lint};
-use rustc_span::source_map::Span;
-use rustc_span::symbol::{sym, Ident, Symbol};
-use rustc_typeck::expr_use_visitor::{ConsumeMode, Delegate, ExprUseVisitor, PlaceBase, PlaceWithHirId};
-use std::iter::{once, Iterator};
-use std::mem;
-
-declare_clippy_lint! {
-    /// **What it does:** Checks for for-loops that manually copy items between
-    /// slices that could be optimized by having a memcpy.
-    ///
-    /// **Why is this bad?** It is not as fast as a memcpy.
-    ///
-    /// **Known problems:** None.
-    ///
-    /// **Example:**
-    /// ```rust
-    /// # let src = vec![1];
-    /// # let mut dst = vec![0; 65];
-    /// for i in 0..src.len() {
-    ///     dst[i + 64] = src[i];
-    /// }
-    /// ```
-    /// Could be written as:
-    /// ```rust
-    /// # let src = vec![1];
-    /// # let mut dst = vec![0; 65];
-    /// dst[64..(src.len() + 64)].clone_from_slice(&src[..]);
-    /// ```
-    pub MANUAL_MEMCPY,
-    perf,
-    "manually copying items between slices"
-}
-
-declare_clippy_lint! {
-    /// **What it does:** Checks for looping over the range of `0..len` of some
-    /// collection just to get the values by index.
-    ///
-    /// **Why is this bad?** Just iterating the collection itself makes the intent
-    /// more clear and is probably faster.
-    ///
-    /// **Known problems:** None.
-    ///
-    /// **Example:**
-    /// ```rust
-    /// let vec = vec!['a', 'b', 'c'];
-    /// for i in 0..vec.len() {
-    ///     println!("{}", vec[i]);
-    /// }
-    /// ```
-    /// Could be written as:
-    /// ```rust
-    /// let vec = vec!['a', 'b', 'c'];
-    /// for i in vec {
-    ///     println!("{}", i);
-    /// }
-    /// ```
-    pub NEEDLESS_RANGE_LOOP,
-    style,
-    "for-looping over a range of indices where an iterator over items would do"
-}
-
-declare_clippy_lint! {
-    /// **What it does:** Checks for loops on `x.iter()` where `&x` will do, and
-    /// suggests the latter.
-    ///
-    /// **Why is this bad?** Readability.
-    ///
-    /// **Known problems:** False negatives. We currently only warn on some known
-    /// types.
-    ///
-    /// **Example:**
-    /// ```rust
-    /// // with `y` a `Vec` or slice:
-    /// # let y = vec![1];
-    /// for x in y.iter() {
-    ///     // ..
-    /// }
-    /// ```
-    /// can be rewritten to
-    /// ```rust
-    /// # let y = vec![1];
-    /// for x in &y {
-    ///     // ..
-    /// }
-    /// ```
-    pub EXPLICIT_ITER_LOOP,
-    pedantic,
-    "for-looping over `_.iter()` or `_.iter_mut()` when `&_` or `&mut _` would do"
-}
-
-declare_clippy_lint! {
-    /// **What it does:** Checks for loops on `y.into_iter()` where `y` will do, and
-    /// suggests the latter.
-    ///
-    /// **Why is this bad?** Readability.
-    ///
-    /// **Known problems:** None
-    ///
-    /// **Example:**
-    /// ```rust
-    /// # let y = vec![1];
-    /// // with `y` a `Vec` or slice:
-    /// for x in y.into_iter() {
-    ///     // ..
-    /// }
-    /// ```
-    /// can be rewritten to
-    /// ```rust
-    /// # let y = vec![1];
-    /// for x in y {
-    ///     // ..
-    /// }
-    /// ```
-    pub EXPLICIT_INTO_ITER_LOOP,
-    pedantic,
-    "for-looping over `_.into_iter()` when `_` would do"
-}
-
-declare_clippy_lint! {
-    /// **What it does:** Checks for loops on `x.next()`.
-    ///
-    /// **Why is this bad?** `next()` returns either `Some(value)` if there was a
-    /// value, or `None` otherwise. The insidious thing is that `Option<_>`
-    /// implements `IntoIterator`, so that possibly one value will be iterated,
-    /// leading to some hard to find bugs. No one will want to write such code
-    /// [except to win an Underhanded Rust
-    /// Contest](https://www.reddit.com/r/rust/comments/3hb0wm/underhanded_rust_contest/cu5yuhr).
-    ///
-    /// **Known problems:** None.
-    ///
-    /// **Example:**
-    /// ```ignore
-    /// for x in y.next() {
-    ///     ..
-    /// }
-    /// ```
-    pub ITER_NEXT_LOOP,
-    correctness,
-    "for-looping over `_.next()` which is probably not intended"
-}
-
-declare_clippy_lint! {
-    /// **What it does:** Checks for `for` loops over `Option` or `Result` values.
-    ///
-    /// **Why is this bad?** Readability. This is more clearly expressed as an `if
-    /// let`.
-    ///
-    /// **Known problems:** None.
-    ///
-    /// **Example:**
-    /// ```rust
-    /// # let opt = Some(1);
-    ///
-    /// // Bad
-    /// for x in opt {
-    ///     // ..
-    /// }
-    ///
-    /// // Good
-    /// if let Some(x) = opt {
-    ///     // ..
-    /// }
-    /// ```
-    ///
-    /// // or
-    ///
-    /// ```rust
-    /// # let res: Result<i32, std::io::Error> = Ok(1);
-    ///
-    /// // Bad
-    /// for x in &res {
-    ///     // ..
-    /// }
-    ///
-    /// // Good
-    /// if let Ok(x) = res {
-    ///     // ..
-    /// }
-    /// ```
-    pub FOR_LOOPS_OVER_FALLIBLES,
-    correctness,
-    "for-looping over an `Option` or a `Result`, which is more clearly expressed as an `if let`"
-}
-
-declare_clippy_lint! {
-    /// **What it does:** Detects `loop + match` combinations that are easier
-    /// written as a `while let` loop.
-    ///
-    /// **Why is this bad?** The `while let` loop is usually shorter and more
-    /// readable.
-    ///
-    /// **Known problems:** Sometimes the wrong binding is displayed ([#383](https://github.com/rust-lang/rust-clippy/issues/383)).
-    ///
-    /// **Example:**
-    /// ```rust,no_run
-    /// # let y = Some(1);
-    /// loop {
-    ///     let x = match y {
-    ///         Some(x) => x,
-    ///         None => break,
-    ///     };
-    ///     // .. do something with x
-    /// }
-    /// // is easier written as
-    /// while let Some(x) = y {
-    ///     // .. do something with x
-    /// };
-    /// ```
-    pub WHILE_LET_LOOP,
-    complexity,
-    "`loop { if let { ... } else break }`, which can be written as a `while let` loop"
-}
-
-declare_clippy_lint! {
-    /// **What it does:** Checks for functions collecting an iterator when collect
-    /// is not needed.
-    ///
-    /// **Why is this bad?** `collect` causes the allocation of a new data structure,
-    /// when this allocation may not be needed.
-    ///
-    /// **Known problems:**
-    /// None
-    ///
-    /// **Example:**
-    /// ```rust
-    /// # let iterator = vec![1].into_iter();
-    /// let len = iterator.clone().collect::<Vec<_>>().len();
-    /// // should be
-    /// let len = iterator.count();
-    /// ```
-    pub NEEDLESS_COLLECT,
-    perf,
-    "collecting an iterator when collect is not needed"
-}
-
-declare_clippy_lint! {
-    /// **What it does:** Checks `for` loops over slices with an explicit counter
-    /// and suggests the use of `.enumerate()`.
-    ///
-    /// **Why is it bad?** Using `.enumerate()` makes the intent more clear,
-    /// declutters the code and may be faster in some instances.
-    ///
-    /// **Known problems:** None.
-    ///
-    /// **Example:**
-    /// ```rust
-    /// # let v = vec![1];
-    /// # fn bar(bar: usize, baz: usize) {}
-    /// let mut i = 0;
-    /// for item in &v {
-    ///     bar(i, *item);
-    ///     i += 1;
-    /// }
-    /// ```
-    /// Could be written as
-    /// ```rust
-    /// # let v = vec![1];
-    /// # fn bar(bar: usize, baz: usize) {}
-    /// for (i, item) in v.iter().enumerate() { bar(i, *item); }
-    /// ```
-    pub EXPLICIT_COUNTER_LOOP,
-    complexity,
-    "for-looping with an explicit counter when `_.enumerate()` would do"
-}
-
-declare_clippy_lint! {
-    /// **What it does:** Checks for empty `loop` expressions.
-    ///
-    /// **Why is this bad?** These busy loops burn CPU cycles without doing
-    /// anything. It is _almost always_ a better idea to `panic!` than to have
-    /// a busy loop.
-    ///
-    /// If panicking isn't possible, think of the environment and either:
-    ///   - block on something
-    ///   - sleep the thread for some microseconds
-    ///   - yield or pause the thread
-    ///
-    /// For `std` targets, this can be done with
-    /// [`std::thread::sleep`](https://doc.rust-lang.org/std/thread/fn.sleep.html)
-    /// or [`std::thread::yield_now`](https://doc.rust-lang.org/std/thread/fn.yield_now.html).
-    ///
-    /// For `no_std` targets, doing this is more complicated, especially because
-    /// `#[panic_handler]`s can't panic. To stop/pause the thread, you will
-    /// probably need to invoke some target-specific intrinsic. Examples include:
-    ///   - [`x86_64::instructions::hlt`](https://docs.rs/x86_64/0.12.2/x86_64/instructions/fn.hlt.html)
-    ///   - [`cortex_m::asm::wfi`](https://docs.rs/cortex-m/0.6.3/cortex_m/asm/fn.wfi.html)
-    ///
-    /// **Known problems:** None.
-    ///
-    /// **Example:**
-    /// ```no_run
-    /// loop {}
-    /// ```
-    pub EMPTY_LOOP,
-    style,
-    "empty `loop {}`, which should block or sleep"
-}
-
-declare_clippy_lint! {
-    /// **What it does:** Checks for `while let` expressions on iterators.
-    ///
-    /// **Why is this bad?** Readability. A simple `for` loop is shorter and conveys
-    /// the intent better.
-    ///
-    /// **Known problems:** None.
-    ///
-    /// **Example:**
-    /// ```ignore
-    /// while let Some(val) = iter() {
-    ///     ..
-    /// }
-    /// ```
-    pub WHILE_LET_ON_ITERATOR,
-    style,
-    "using a `while let` loop instead of a for loop on an iterator"
-}
-
-declare_clippy_lint! {
-    /// **What it does:** Checks for iterating a map (`HashMap` or `BTreeMap`) and
-    /// ignoring either the keys or values.
-    ///
-    /// **Why is this bad?** Readability. There are `keys` and `values` methods that
-    /// can be used to express that don't need the values or keys.
-    ///
-    /// **Known problems:** None.
-    ///
-    /// **Example:**
-    /// ```ignore
-    /// for (k, _) in &map {
-    ///     ..
-    /// }
-    /// ```
-    ///
-    /// could be replaced by
-    ///
-    /// ```ignore
-    /// for k in map.keys() {
-    ///     ..
-    /// }
-    /// ```
-    pub FOR_KV_MAP,
-    style,
-    "looping on a map using `iter` when `keys` or `values` would do"
-}
-
-declare_clippy_lint! {
-    /// **What it does:** Checks for loops that will always `break`, `return` or
-    /// `continue` an outer loop.
-    ///
-    /// **Why is this bad?** This loop never loops, all it does is obfuscating the
-    /// code.
-    ///
-    /// **Known problems:** None
-    ///
-    /// **Example:**
-    /// ```rust
-    /// loop {
-    ///     ..;
-    ///     break;
-    /// }
-    /// ```
-    pub NEVER_LOOP,
-    correctness,
-    "any loop that will always `break` or `return`"
-}
-
-declare_clippy_lint! {
-    /// **What it does:** Checks for loops which have a range bound that is a mutable variable
-    ///
-    /// **Why is this bad?** One might think that modifying the mutable variable changes the loop bounds
-    ///
-    /// **Known problems:** None
-    ///
-    /// **Example:**
-    /// ```rust
-    /// let mut foo = 42;
-    /// for i in 0..foo {
-    ///     foo -= 1;
-    ///     println!("{}", i); // prints numbers from 0 to 42, not 0 to 21
-    /// }
-    /// ```
-    pub MUT_RANGE_BOUND,
-    complexity,
-    "for loop over a range where one of the bounds is a mutable variable"
-}
-
-declare_clippy_lint! {
-    /// **What it does:** Checks whether variables used within while loop condition
-    /// can be (and are) mutated in the body.
-    ///
-    /// **Why is this bad?** If the condition is unchanged, entering the body of the loop
-    /// will lead to an infinite loop.
-    ///
-    /// **Known problems:** If the `while`-loop is in a closure, the check for mutation of the
-    /// condition variables in the body can cause false negatives. For example when only `Upvar` `a` is
-    /// in the condition and only `Upvar` `b` gets mutated in the body, the lint will not trigger.
-    ///
-    /// **Example:**
-    /// ```rust
-    /// let i = 0;
-    /// while i > 10 {
-    ///     println!("let me loop forever!");
-    /// }
-    /// ```
-    pub WHILE_IMMUTABLE_CONDITION,
-    correctness,
-    "variables used within while expression are not mutated in the body"
-}
-
-declare_clippy_lint! {
-    /// **What it does:** Checks whether a for loop is being used to push a constant
-    /// value into a Vec.
-    ///
-    /// **Why is this bad?** This kind of operation can be expressed more succinctly with
-    /// `vec![item;SIZE]` or `vec.resize(NEW_SIZE, item)` and using these alternatives may also
-    /// have better performance.
-    /// **Known problems:** None
-    ///
-    /// **Example:**
-    /// ```rust
-    /// let item1 = 2;
-    /// let item2 = 3;
-    /// let mut vec: Vec<u8> = Vec::new();
-    /// for _ in 0..20 {
-    ///    vec.push(item1);
-    /// }
-    /// for _ in 0..30 {
-    ///     vec.push(item2);
-    /// }
-    /// ```
-    /// could be written as
-    /// ```rust
-    /// let item1 = 2;
-    /// let item2 = 3;
-    /// let mut vec: Vec<u8> = vec![item1; 20];
-    /// vec.resize(20 + 30, item2);
-    /// ```
-    pub SAME_ITEM_PUSH,
-    style,
-    "the same item is pushed inside of a for loop"
-}
-
-declare_clippy_lint! {
-    /// **What it does:** Checks whether a for loop has a single element.
-    ///
-    /// **Why is this bad?** There is no reason to have a loop of a
-    /// single element.
-    /// **Known problems:** None
-    ///
-    /// **Example:**
-    /// ```rust
-    /// let item1 = 2;
-    /// for item in &[item1] {
-    ///     println!("{}", item);
-    /// }
-    /// ```
-    /// could be written as
-    /// ```rust
-    /// let item1 = 2;
-    /// let item = &item1;
-    /// println!("{}", item);
-    /// ```
-    pub SINGLE_ELEMENT_LOOP,
-    complexity,
-    "there is no reason to have a single element loop"
-}
-
-declare_clippy_lint! {
-    /// **What it does:** Check for unnecessary `if let` usage in a for loop
-    /// where only the `Some` or `Ok` variant of the iterator element is used.
-    ///
-    /// **Why is this bad?** It is verbose and can be simplified
-    /// by first calling the `flatten` method on the `Iterator`.
-    ///
-    /// **Known problems:** None.
-    ///
-    /// **Example:**
-    ///
-    /// ```rust
-    /// let x = vec![Some(1), Some(2), Some(3)];
-    /// for n in x {
-    ///     if let Some(n) = n {
-    ///         println!("{}", n);
-    ///     }
-    /// }
-    /// ```
-    /// Use instead:
-    /// ```rust
-    /// let x = vec![Some(1), Some(2), Some(3)];
-    /// for n in x.into_iter().flatten() {
-    ///     println!("{}", n);
-    /// }
-    /// ```
-    pub MANUAL_FLATTEN,
-    complexity,
-    "for loops over `Option`s or `Result`s with a single expression can be simplified"
-}
-
-declare_lint_pass!(Loops => [
-    MANUAL_MEMCPY,
-    MANUAL_FLATTEN,
-    NEEDLESS_RANGE_LOOP,
-    EXPLICIT_ITER_LOOP,
-    EXPLICIT_INTO_ITER_LOOP,
-    ITER_NEXT_LOOP,
-    FOR_LOOPS_OVER_FALLIBLES,
-    WHILE_LET_LOOP,
-    NEEDLESS_COLLECT,
-    EXPLICIT_COUNTER_LOOP,
-    EMPTY_LOOP,
-    WHILE_LET_ON_ITERATOR,
-    FOR_KV_MAP,
-    NEVER_LOOP,
-    MUT_RANGE_BOUND,
-    WHILE_IMMUTABLE_CONDITION,
-    SAME_ITEM_PUSH,
-    SINGLE_ELEMENT_LOOP,
-]);
-
-impl<'tcx> LateLintPass<'tcx> for Loops {
-    #[allow(clippy::too_many_lines)]
-    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
-        if let Some((pat, arg, body, span)) = higher::for_loop(expr) {
-            // we don't want to check expanded macros
-            // this check is not at the top of the function
-            // since higher::for_loop expressions are marked as expansions
-            if body.span.from_expansion() {
-                return;
-            }
-            check_for_loop(cx, pat, arg, body, expr, span);
-        }
-
-        // we don't want to check expanded macros
-        if expr.span.from_expansion() {
-            return;
-        }
-
-        // check for never_loop
-        if let ExprKind::Loop(ref block, _, _, _) = expr.kind {
-            match never_loop_block(block, expr.hir_id) {
-                NeverLoopResult::AlwaysBreak => span_lint(cx, NEVER_LOOP, expr.span, "this loop never actually loops"),
-                NeverLoopResult::MayContinueMainLoop | NeverLoopResult::Otherwise => (),
-            }
-        }
-
-        // check for `loop { if let {} else break }` that could be `while let`
-        // (also matches an explicit "match" instead of "if let")
-        // (even if the "match" or "if let" is used for declaration)
-        if let ExprKind::Loop(ref block, _, LoopSource::Loop, _) = expr.kind {
-            // also check for empty `loop {}` statements, skipping those in #[panic_handler]
-            if block.stmts.is_empty() && block.expr.is_none() && !is_in_panic_handler(cx, expr) {
-                let msg = "empty `loop {}` wastes CPU cycles";
-                let help = if is_no_std_crate(cx.tcx.hir().krate()) {
-                    "you should either use `panic!()` or add a call pausing or sleeping the thread to the loop body"
-                } else {
-                    "you should either use `panic!()` or add `std::thread::sleep(..);` to the loop body"
-                };
-                span_lint_and_help(cx, EMPTY_LOOP, expr.span, msg, None, help);
-            }
-
-            // extract the expression from the first statement (if any) in a block
-            let inner_stmt_expr = extract_expr_from_first_stmt(block);
-            // or extract the first expression (if any) from the block
-            if let Some(inner) = inner_stmt_expr.or_else(|| extract_first_expr(block)) {
-                if let ExprKind::Match(ref matchexpr, ref arms, ref source) = inner.kind {
-                    // ensure "if let" compatible match structure
-                    match *source {
-                        MatchSource::Normal | MatchSource::IfLetDesugar { .. } => {
-                            if arms.len() == 2
-                                && arms[0].guard.is_none()
-                                && arms[1].guard.is_none()
-                                && is_simple_break_expr(&arms[1].body)
-                            {
-                                if in_external_macro(cx.sess(), expr.span) {
-                                    return;
-                                }
-
-                                // NOTE: we used to build a body here instead of using
-                                // ellipsis, this was removed because:
-                                // 1) it was ugly with big bodies;
-                                // 2) it was not indented properly;
-                                // 3) it wasn’t very smart (see #675).
-                                let mut applicability = Applicability::HasPlaceholders;
-                                span_lint_and_sugg(
-                                    cx,
-                                    WHILE_LET_LOOP,
-                                    expr.span,
-                                    "this loop could be written as a `while let` loop",
-                                    "try",
-                                    format!(
-                                        "while let {} = {} {{ .. }}",
-                                        snippet_with_applicability(cx, arms[0].pat.span, "..", &mut applicability),
-                                        snippet_with_applicability(cx, matchexpr.span, "..", &mut applicability),
-                                    ),
-                                    applicability,
-                                );
-                            }
-                        },
-                        _ => (),
-                    }
-                }
-            }
-        }
-        if let ExprKind::Match(ref match_expr, ref arms, MatchSource::WhileLetDesugar) = expr.kind {
-            let pat = &arms[0].pat.kind;
-            if let (
-                &PatKind::TupleStruct(ref qpath, ref pat_args, _),
-                &ExprKind::MethodCall(ref method_path, _, ref method_args, _),
-            ) = (pat, &match_expr.kind)
-            {
-                let iter_expr = &method_args[0];
-
-                // Don't lint when the iterator is recreated on every iteration
-                if_chain! {
-                    if let ExprKind::MethodCall(..) | ExprKind::Call(..) = iter_expr.kind;
-                    if let Some(iter_def_id) = get_trait_def_id(cx, &paths::ITERATOR);
-                    if implements_trait(cx, cx.typeck_results().expr_ty(iter_expr), iter_def_id, &[]);
-                    then {
-                        return;
-                    }
-                }
-
-                let lhs_constructor = last_path_segment(qpath);
-                if method_path.ident.name == sym::next
-                    && match_trait_method(cx, match_expr, &paths::ITERATOR)
-                    && lhs_constructor.ident.name == sym::Some
-                    && (pat_args.is_empty()
-                        || !is_refutable(cx, &pat_args[0])
-                            && !is_used_inside(cx, iter_expr, &arms[0].body)
-                            && !is_iterator_used_after_while_let(cx, iter_expr)
-                            && !is_nested(cx, expr, &method_args[0]))
-                {
-                    let mut applicability = Applicability::MachineApplicable;
-                    let iterator = snippet_with_applicability(cx, method_args[0].span, "_", &mut applicability);
-                    let loop_var = if pat_args.is_empty() {
-                        "_".to_string()
-                    } else {
-                        snippet_with_applicability(cx, pat_args[0].span, "_", &mut applicability).into_owned()
-                    };
-                    span_lint_and_sugg(
-                        cx,
-                        WHILE_LET_ON_ITERATOR,
-                        expr.span.with_hi(match_expr.span.hi()),
-                        "this loop could be written as a `for` loop",
-                        "try",
-                        format!("for {} in {}", loop_var, iterator),
-                        applicability,
-                    );
-                }
-            }
-        }
-
-        if let Some((cond, body)) = higher::while_loop(&expr) {
-            check_infinite_loop(cx, cond, body);
-        }
-
-        check_needless_collect(expr, cx);
-    }
-}
-
-enum NeverLoopResult {
-    // A break/return always get triggered but not necessarily for the main loop.
-    AlwaysBreak,
-    // A continue may occur for the main loop.
-    MayContinueMainLoop,
-    Otherwise,
-}
-
-#[must_use]
-fn absorb_break(arg: &NeverLoopResult) -> NeverLoopResult {
-    match *arg {
-        NeverLoopResult::AlwaysBreak | NeverLoopResult::Otherwise => NeverLoopResult::Otherwise,
-        NeverLoopResult::MayContinueMainLoop => NeverLoopResult::MayContinueMainLoop,
-    }
-}
-
-// Combine two results for parts that are called in order.
-#[must_use]
-fn combine_seq(first: NeverLoopResult, second: NeverLoopResult) -> NeverLoopResult {
-    match first {
-        NeverLoopResult::AlwaysBreak | NeverLoopResult::MayContinueMainLoop => first,
-        NeverLoopResult::Otherwise => second,
-    }
-}
-
-// Combine two results where both parts are called but not necessarily in order.
-#[must_use]
-fn combine_both(left: NeverLoopResult, right: NeverLoopResult) -> NeverLoopResult {
-    match (left, right) {
-        (NeverLoopResult::MayContinueMainLoop, _) | (_, NeverLoopResult::MayContinueMainLoop) => {
-            NeverLoopResult::MayContinueMainLoop
-        },
-        (NeverLoopResult::AlwaysBreak, _) | (_, NeverLoopResult::AlwaysBreak) => NeverLoopResult::AlwaysBreak,
-        (NeverLoopResult::Otherwise, NeverLoopResult::Otherwise) => NeverLoopResult::Otherwise,
-    }
-}
-
-// Combine two results where only one of the part may have been executed.
-#[must_use]
-fn combine_branches(b1: NeverLoopResult, b2: NeverLoopResult) -> NeverLoopResult {
-    match (b1, b2) {
-        (NeverLoopResult::AlwaysBreak, NeverLoopResult::AlwaysBreak) => NeverLoopResult::AlwaysBreak,
-        (NeverLoopResult::MayContinueMainLoop, _) | (_, NeverLoopResult::MayContinueMainLoop) => {
-            NeverLoopResult::MayContinueMainLoop
-        },
-        (NeverLoopResult::Otherwise, _) | (_, NeverLoopResult::Otherwise) => NeverLoopResult::Otherwise,
-    }
-}
-
-fn never_loop_block(block: &Block<'_>, main_loop_id: HirId) -> NeverLoopResult {
-    let stmts = block.stmts.iter().map(stmt_to_expr);
-    let expr = once(block.expr.as_deref());
-    let mut iter = stmts.chain(expr).flatten();
-    never_loop_expr_seq(&mut iter, main_loop_id)
-}
-
-fn stmt_to_expr<'tcx>(stmt: &Stmt<'tcx>) -> Option<&'tcx Expr<'tcx>> {
-    match stmt.kind {
-        StmtKind::Semi(ref e, ..) | StmtKind::Expr(ref e, ..) => Some(e),
-        StmtKind::Local(ref local) => local.init.as_deref(),
-        _ => None,
-    }
-}
-
-fn never_loop_expr(expr: &Expr<'_>, main_loop_id: HirId) -> NeverLoopResult {
-    match expr.kind {
-        ExprKind::Box(ref e)
-        | ExprKind::Unary(_, ref e)
-        | ExprKind::Cast(ref e, _)
-        | ExprKind::Type(ref e, _)
-        | ExprKind::Field(ref e, _)
-        | ExprKind::AddrOf(_, _, ref e)
-        | ExprKind::Struct(_, _, Some(ref e))
-        | ExprKind::Repeat(ref e, _)
-        | ExprKind::DropTemps(ref e) => never_loop_expr(e, main_loop_id),
-        ExprKind::Array(ref es) | ExprKind::MethodCall(_, _, ref es, _) | ExprKind::Tup(ref es) => {
-            never_loop_expr_all(&mut es.iter(), main_loop_id)
-        },
-        ExprKind::Call(ref e, ref es) => never_loop_expr_all(&mut once(&**e).chain(es.iter()), main_loop_id),
-        ExprKind::Binary(_, ref e1, ref e2)
-        | ExprKind::Assign(ref e1, ref e2, _)
-        | ExprKind::AssignOp(_, ref e1, ref e2)
-        | ExprKind::Index(ref e1, ref e2) => never_loop_expr_all(&mut [&**e1, &**e2].iter().cloned(), main_loop_id),
-        ExprKind::Loop(ref b, _, _, _) => {
-            // Break can come from the inner loop so remove them.
-            absorb_break(&never_loop_block(b, main_loop_id))
-        },
-        ExprKind::If(ref e, ref e2, ref e3) => {
-            let e1 = never_loop_expr(e, main_loop_id);
-            let e2 = never_loop_expr(e2, main_loop_id);
-            let e3 = e3
-                .as_ref()
-                .map_or(NeverLoopResult::Otherwise, |e| never_loop_expr(e, main_loop_id));
-            combine_seq(e1, combine_branches(e2, e3))
-        },
-        ExprKind::Match(ref e, ref arms, _) => {
-            let e = never_loop_expr(e, main_loop_id);
-            if arms.is_empty() {
-                e
-            } else {
-                let arms = never_loop_expr_branch(&mut arms.iter().map(|a| &*a.body), main_loop_id);
-                combine_seq(e, arms)
-            }
-        },
-        ExprKind::Block(ref b, _) => never_loop_block(b, main_loop_id),
-        ExprKind::Continue(d) => {
-            let id = d
-                .target_id
-                .expect("target ID can only be missing in the presence of compilation errors");
-            if id == main_loop_id {
-                NeverLoopResult::MayContinueMainLoop
-            } else {
-                NeverLoopResult::AlwaysBreak
-            }
-        },
-        ExprKind::Break(_, ref e) | ExprKind::Ret(ref e) => e.as_ref().map_or(NeverLoopResult::AlwaysBreak, |e| {
-            combine_seq(never_loop_expr(e, main_loop_id), NeverLoopResult::AlwaysBreak)
-        }),
-        ExprKind::InlineAsm(ref asm) => asm
-            .operands
-            .iter()
-            .map(|(o, _)| match o {
-                InlineAsmOperand::In { expr, .. }
-                | InlineAsmOperand::InOut { expr, .. }
-                | InlineAsmOperand::Const { expr }
-                | InlineAsmOperand::Sym { expr } => never_loop_expr(expr, main_loop_id),
-                InlineAsmOperand::Out { expr, .. } => never_loop_expr_all(&mut expr.iter(), main_loop_id),
-                InlineAsmOperand::SplitInOut { in_expr, out_expr, .. } => {
-                    never_loop_expr_all(&mut once(in_expr).chain(out_expr.iter()), main_loop_id)
-                },
-            })
-            .fold(NeverLoopResult::Otherwise, combine_both),
-        ExprKind::Struct(_, _, None)
-        | ExprKind::Yield(_, _)
-        | ExprKind::Closure(_, _, _, _, _)
-        | ExprKind::LlvmInlineAsm(_)
-        | ExprKind::Path(_)
-        | ExprKind::ConstBlock(_)
-        | ExprKind::Lit(_)
-        | ExprKind::Err => NeverLoopResult::Otherwise,
-    }
-}
-
-fn never_loop_expr_seq<'a, T: Iterator<Item = &'a Expr<'a>>>(es: &mut T, main_loop_id: HirId) -> NeverLoopResult {
-    es.map(|e| never_loop_expr(e, main_loop_id))
-        .fold(NeverLoopResult::Otherwise, combine_seq)
-}
-
-fn never_loop_expr_all<'a, T: Iterator<Item = &'a Expr<'a>>>(es: &mut T, main_loop_id: HirId) -> NeverLoopResult {
-    es.map(|e| never_loop_expr(e, main_loop_id))
-        .fold(NeverLoopResult::Otherwise, combine_both)
-}
-
-fn never_loop_expr_branch<'a, T: Iterator<Item = &'a Expr<'a>>>(e: &mut T, main_loop_id: HirId) -> NeverLoopResult {
-    e.map(|e| never_loop_expr(e, main_loop_id))
-        .fold(NeverLoopResult::AlwaysBreak, combine_branches)
-}
-
-fn check_for_loop<'tcx>(
-    cx: &LateContext<'tcx>,
-    pat: &'tcx Pat<'_>,
-    arg: &'tcx Expr<'_>,
-    body: &'tcx Expr<'_>,
-    expr: &'tcx Expr<'_>,
-    span: Span,
-) {
-    let is_manual_memcpy_triggered = detect_manual_memcpy(cx, pat, arg, body, expr);
-    if !is_manual_memcpy_triggered {
-        check_for_loop_range(cx, pat, arg, body, expr);
-        check_for_loop_explicit_counter(cx, pat, arg, body, expr);
-    }
-    check_for_loop_arg(cx, pat, arg, expr);
-    check_for_loop_over_map_kv(cx, pat, arg, body, expr);
-    check_for_mut_range_bound(cx, arg, body);
-    check_for_single_element_loop(cx, pat, arg, body, expr);
-    detect_same_item_push(cx, pat, arg, body, expr);
-    check_manual_flatten(cx, pat, arg, body, span);
-}
-
-// this function assumes the given expression is a `for` loop.
-fn get_span_of_entire_for_loop(expr: &Expr<'_>) -> Span {
-    // for some reason this is the only way to get the `Span`
-    // of the entire `for` loop
-    if let ExprKind::Match(_, arms, _) = &expr.kind {
-        arms[0].body.span
-    } else {
-        unreachable!()
-    }
-}
-
-/// a wrapper of `Sugg`. Besides what `Sugg` do, this removes unnecessary `0`;
-/// and also, it avoids subtracting a variable from the same one by replacing it with `0`.
-/// it exists for the convenience of the overloaded operators while normal functions can do the
-/// same.
-#[derive(Clone)]
-struct MinifyingSugg<'a>(Sugg<'a>);
-
-impl<'a> MinifyingSugg<'a> {
-    fn as_str(&self) -> &str {
-        let Sugg::NonParen(s) | Sugg::MaybeParen(s) | Sugg::BinOp(_, s) = &self.0;
-        s.as_ref()
-    }
-
-    fn into_sugg(self) -> Sugg<'a> {
-        self.0
-    }
-}
-
-impl<'a> From<Sugg<'a>> for MinifyingSugg<'a> {
-    fn from(sugg: Sugg<'a>) -> Self {
-        Self(sugg)
-    }
-}
-
-impl std::ops::Add for &MinifyingSugg<'static> {
-    type Output = MinifyingSugg<'static>;
-    fn add(self, rhs: &MinifyingSugg<'static>) -> MinifyingSugg<'static> {
-        match (self.as_str(), rhs.as_str()) {
-            ("0", _) => rhs.clone(),
-            (_, "0") => self.clone(),
-            (_, _) => (&self.0 + &rhs.0).into(),
-        }
-    }
-}
-
-impl std::ops::Sub for &MinifyingSugg<'static> {
-    type Output = MinifyingSugg<'static>;
-    fn sub(self, rhs: &MinifyingSugg<'static>) -> MinifyingSugg<'static> {
-        match (self.as_str(), rhs.as_str()) {
-            (_, "0") => self.clone(),
-            ("0", _) => (-rhs.0.clone()).into(),
-            (x, y) if x == y => sugg::ZERO.into(),
-            (_, _) => (&self.0 - &rhs.0).into(),
-        }
-    }
-}
-
-impl std::ops::Add<&MinifyingSugg<'static>> for MinifyingSugg<'static> {
-    type Output = MinifyingSugg<'static>;
-    fn add(self, rhs: &MinifyingSugg<'static>) -> MinifyingSugg<'static> {
-        match (self.as_str(), rhs.as_str()) {
-            ("0", _) => rhs.clone(),
-            (_, "0") => self,
-            (_, _) => (self.0 + &rhs.0).into(),
-        }
-    }
-}
-
-impl std::ops::Sub<&MinifyingSugg<'static>> for MinifyingSugg<'static> {
-    type Output = MinifyingSugg<'static>;
-    fn sub(self, rhs: &MinifyingSugg<'static>) -> MinifyingSugg<'static> {
-        match (self.as_str(), rhs.as_str()) {
-            (_, "0") => self,
-            ("0", _) => (-rhs.0.clone()).into(),
-            (x, y) if x == y => sugg::ZERO.into(),
-            (_, _) => (self.0 - &rhs.0).into(),
-        }
-    }
-}
-
-/// a wrapper around `MinifyingSugg`, which carries a operator like currying
-/// so that the suggested code become more efficient (e.g. `foo + -bar` `foo - bar`).
-struct Offset {
-    value: MinifyingSugg<'static>,
-    sign: OffsetSign,
-}
-
-#[derive(Clone, Copy)]
-enum OffsetSign {
-    Positive,
-    Negative,
-}
-
-impl Offset {
-    fn negative(value: Sugg<'static>) -> Self {
-        Self {
-            value: value.into(),
-            sign: OffsetSign::Negative,
-        }
-    }
-
-    fn positive(value: Sugg<'static>) -> Self {
-        Self {
-            value: value.into(),
-            sign: OffsetSign::Positive,
-        }
-    }
-
-    fn empty() -> Self {
-        Self::positive(sugg::ZERO)
-    }
-}
-
-fn apply_offset(lhs: &MinifyingSugg<'static>, rhs: &Offset) -> MinifyingSugg<'static> {
-    match rhs.sign {
-        OffsetSign::Positive => lhs + &rhs.value,
-        OffsetSign::Negative => lhs - &rhs.value,
-    }
-}
-
-#[derive(Debug, Clone, Copy)]
-enum StartKind<'hir> {
-    Range,
-    Counter { initializer: &'hir Expr<'hir> },
-}
-
-struct IndexExpr<'hir> {
-    base: &'hir Expr<'hir>,
-    idx: StartKind<'hir>,
-    idx_offset: Offset,
-}
-
-struct Start<'hir> {
-    id: HirId,
-    kind: StartKind<'hir>,
-}
-
-fn is_slice_like<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'_>) -> bool {
-    let is_slice = match ty.kind() {
-        ty::Ref(_, subty, _) => is_slice_like(cx, subty),
-        ty::Slice(..) | ty::Array(..) => true,
-        _ => false,
-    };
-
-    is_slice || is_type_diagnostic_item(cx, ty, sym::vec_type) || is_type_diagnostic_item(cx, ty, sym!(vecdeque_type))
-}
-
-fn fetch_cloned_expr<'tcx>(expr: &'tcx Expr<'tcx>) -> &'tcx Expr<'tcx> {
-    if_chain! {
-        if let ExprKind::MethodCall(method, _, args, _) = expr.kind;
-        if method.ident.name == sym::clone;
-        if args.len() == 1;
-        if let Some(arg) = args.get(0);
-        then { arg } else { expr }
-    }
-}
-
-fn get_details_from_idx<'tcx>(
-    cx: &LateContext<'tcx>,
-    idx: &Expr<'_>,
-    starts: &[Start<'tcx>],
-) -> Option<(StartKind<'tcx>, Offset)> {
-    fn get_start<'tcx>(e: &Expr<'_>, starts: &[Start<'tcx>]) -> Option<StartKind<'tcx>> {
-        let id = path_to_local(e)?;
-        starts.iter().find(|start| start.id == id).map(|start| start.kind)
-    }
-
-    fn get_offset<'tcx>(cx: &LateContext<'tcx>, e: &Expr<'_>, starts: &[Start<'tcx>]) -> Option<Sugg<'static>> {
-        match &e.kind {
-            ExprKind::Lit(l) => match l.node {
-                ast::LitKind::Int(x, _ty) => Some(Sugg::NonParen(x.to_string().into())),
-                _ => None,
-            },
-            ExprKind::Path(..) if get_start(e, starts).is_none() => Some(Sugg::hir(cx, e, "???")),
-            _ => None,
-        }
-    }
-
-    match idx.kind {
-        ExprKind::Binary(op, lhs, rhs) => match op.node {
-            BinOpKind::Add => {
-                let offset_opt = get_start(lhs, starts)
-                    .and_then(|s| get_offset(cx, rhs, starts).map(|o| (s, o)))
-                    .or_else(|| get_start(rhs, starts).and_then(|s| get_offset(cx, lhs, starts).map(|o| (s, o))));
-
-                offset_opt.map(|(s, o)| (s, Offset::positive(o)))
-            },
-            BinOpKind::Sub => {
-                get_start(lhs, starts).and_then(|s| get_offset(cx, rhs, starts).map(|o| (s, Offset::negative(o))))
-            },
-            _ => None,
-        },
-        ExprKind::Path(..) => get_start(idx, starts).map(|s| (s, Offset::empty())),
-        _ => None,
-    }
-}
-
-fn get_assignment<'tcx>(e: &'tcx Expr<'tcx>) -> Option<(&'tcx Expr<'tcx>, &'tcx Expr<'tcx>)> {
-    if let ExprKind::Assign(lhs, rhs, _) = e.kind {
-        Some((lhs, rhs))
-    } else {
-        None
-    }
-}
-
-/// Get assignments from the given block.
-/// The returned iterator yields `None` if no assignment expressions are there,
-/// filtering out the increments of the given whitelisted loop counters;
-/// because its job is to make sure there's nothing other than assignments and the increments.
-fn get_assignments<'a, 'tcx>(
-    Block { stmts, expr, .. }: &'tcx Block<'tcx>,
-    loop_counters: &'a [Start<'tcx>],
-) -> impl Iterator<Item = Option<(&'tcx Expr<'tcx>, &'tcx Expr<'tcx>)>> + 'a {
-    // As the `filter` and `map` below do different things, I think putting together
-    // just increases complexity. (cc #3188 and #4193)
-    stmts
-        .iter()
-        .filter_map(move |stmt| match stmt.kind {
-            StmtKind::Local(..) | StmtKind::Item(..) => None,
-            StmtKind::Expr(e) | StmtKind::Semi(e) => Some(e),
-        })
-        .chain((*expr).into_iter())
-        .filter(move |e| {
-            if let ExprKind::AssignOp(_, place, _) = e.kind {
-                path_to_local(place).map_or(false, |id| {
-                    !loop_counters
-                        .iter()
-                        // skip the first item which should be `StartKind::Range`
-                        // this makes it possible to use the slice with `StartKind::Range` in the same iterator loop.
-                        .skip(1)
-                        .any(|counter| counter.id == id)
-                })
-            } else {
-                true
-            }
-        })
-        .map(get_assignment)
-}
-
-fn get_loop_counters<'a, 'tcx>(
-    cx: &'a LateContext<'tcx>,
-    body: &'tcx Block<'tcx>,
-    expr: &'tcx Expr<'_>,
-) -> Option<impl Iterator<Item = Start<'tcx>> + 'a> {
-    // Look for variables that are incremented once per loop iteration.
-    let mut increment_visitor = IncrementVisitor::new(cx);
-    walk_block(&mut increment_visitor, body);
-
-    // For each candidate, check the parent block to see if
-    // it's initialized to zero at the start of the loop.
-    get_enclosing_block(&cx, expr.hir_id).and_then(|block| {
-        increment_visitor
-            .into_results()
-            .filter_map(move |var_id| {
-                let mut initialize_visitor = InitializeVisitor::new(cx, expr, var_id);
-                walk_block(&mut initialize_visitor, block);
-
-                initialize_visitor.get_result().map(|(_, initializer)| Start {
-                    id: var_id,
-                    kind: StartKind::Counter { initializer },
-                })
-            })
-            .into()
-    })
-}
-
-fn build_manual_memcpy_suggestion<'tcx>(
-    cx: &LateContext<'tcx>,
-    start: &Expr<'_>,
-    end: &Expr<'_>,
-    limits: ast::RangeLimits,
-    dst: &IndexExpr<'_>,
-    src: &IndexExpr<'_>,
-) -> String {
-    fn print_offset(offset: MinifyingSugg<'static>) -> MinifyingSugg<'static> {
-        if offset.as_str() == "0" {
-            sugg::EMPTY.into()
-        } else {
-            offset
-        }
-    }
-
-    let print_limit = |end: &Expr<'_>, end_str: &str, base: &Expr<'_>, sugg: MinifyingSugg<'static>| {
-        if_chain! {
-            if let ExprKind::MethodCall(method, _, len_args, _) = end.kind;
-            if method.ident.name == sym!(len);
-            if len_args.len() == 1;
-            if let Some(arg) = len_args.get(0);
-            if path_to_local(arg) == path_to_local(base);
-            then {
-                if sugg.as_str() == end_str {
-                    sugg::EMPTY.into()
-                } else {
-                    sugg
-                }
-            } else {
-                match limits {
-                    ast::RangeLimits::Closed => {
-                        sugg + &sugg::ONE.into()
-                    },
-                    ast::RangeLimits::HalfOpen => sugg,
-                }
-            }
-        }
-    };
-
-    let start_str = Sugg::hir(cx, start, "").into();
-    let end_str: MinifyingSugg<'_> = Sugg::hir(cx, end, "").into();
-
-    let print_offset_and_limit = |idx_expr: &IndexExpr<'_>| match idx_expr.idx {
-        StartKind::Range => (
-            print_offset(apply_offset(&start_str, &idx_expr.idx_offset)).into_sugg(),
-            print_limit(
-                end,
-                end_str.as_str(),
-                idx_expr.base,
-                apply_offset(&end_str, &idx_expr.idx_offset),
-            )
-            .into_sugg(),
-        ),
-        StartKind::Counter { initializer } => {
-            let counter_start = Sugg::hir(cx, initializer, "").into();
-            (
-                print_offset(apply_offset(&counter_start, &idx_expr.idx_offset)).into_sugg(),
-                print_limit(
-                    end,
-                    end_str.as_str(),
-                    idx_expr.base,
-                    apply_offset(&end_str, &idx_expr.idx_offset) + &counter_start - &start_str,
-                )
-                .into_sugg(),
-            )
-        },
-    };
-
-    let (dst_offset, dst_limit) = print_offset_and_limit(&dst);
-    let (src_offset, src_limit) = print_offset_and_limit(&src);
-
-    let dst_base_str = snippet(cx, dst.base.span, "???");
-    let src_base_str = snippet(cx, src.base.span, "???");
-
-    let dst = if dst_offset == sugg::EMPTY && dst_limit == sugg::EMPTY {
-        dst_base_str
-    } else {
-        format!(
-            "{}[{}..{}]",
-            dst_base_str,
-            dst_offset.maybe_par(),
-            dst_limit.maybe_par()
-        )
-        .into()
-    };
-
-    format!(
-        "{}.clone_from_slice(&{}[{}..{}]);",
-        dst,
-        src_base_str,
-        src_offset.maybe_par(),
-        src_limit.maybe_par()
-    )
-}
-
-/// Checks for for loops that sequentially copy items from one slice-like
-/// object to another.
-fn detect_manual_memcpy<'tcx>(
-    cx: &LateContext<'tcx>,
-    pat: &'tcx Pat<'_>,
-    arg: &'tcx Expr<'_>,
-    body: &'tcx Expr<'_>,
-    expr: &'tcx Expr<'_>,
-) -> bool {
-    if let Some(higher::Range {
-        start: Some(start),
-        end: Some(end),
-        limits,
-    }) = higher::range(arg)
-    {
-        // the var must be a single name
-        if let PatKind::Binding(_, canonical_id, _, _) = pat.kind {
-            let mut starts = vec![Start {
-                id: canonical_id,
-                kind: StartKind::Range,
-            }];
-
-            // This is one of few ways to return different iterators
-            // derived from: https://stackoverflow.com/questions/29760668/conditionally-iterate-over-one-of-several-possible-iterators/52064434#52064434
-            let mut iter_a = None;
-            let mut iter_b = None;
-
-            if let ExprKind::Block(block, _) = body.kind {
-                if let Some(loop_counters) = get_loop_counters(cx, block, expr) {
-                    starts.extend(loop_counters);
-                }
-                iter_a = Some(get_assignments(block, &starts));
-            } else {
-                iter_b = Some(get_assignment(body));
-            }
-
-            let assignments = iter_a.into_iter().flatten().chain(iter_b.into_iter());
-
-            let big_sugg = assignments
-                // The only statements in the for loops can be indexed assignments from
-                // indexed retrievals (except increments of loop counters).
-                .map(|o| {
-                    o.and_then(|(lhs, rhs)| {
-                        let rhs = fetch_cloned_expr(rhs);
-                        if_chain! {
-                            if let ExprKind::Index(base_left, idx_left) = lhs.kind;
-                            if let ExprKind::Index(base_right, idx_right) = rhs.kind;
-                            if is_slice_like(cx, cx.typeck_results().expr_ty(base_left))
-                                && is_slice_like(cx, cx.typeck_results().expr_ty(base_right));
-                            if let Some((start_left, offset_left)) = get_details_from_idx(cx, &idx_left, &starts);
-                            if let Some((start_right, offset_right)) = get_details_from_idx(cx, &idx_right, &starts);
-
-                            // Source and destination must be different
-                            if path_to_local(base_left) != path_to_local(base_right);
-                            then {
-                                Some((IndexExpr { base: base_left, idx: start_left, idx_offset: offset_left },
-                                    IndexExpr { base: base_right, idx: start_right, idx_offset: offset_right }))
-                            } else {
-                                None
-                            }
-                        }
-                    })
-                })
-                .map(|o| o.map(|(dst, src)| build_manual_memcpy_suggestion(cx, start, end, limits, &dst, &src)))
-                .collect::<Option<Vec<_>>>()
-                .filter(|v| !v.is_empty())
-                .map(|v| v.join("\n    "));
-
-            if let Some(big_sugg) = big_sugg {
-                span_lint_and_sugg(
-                    cx,
-                    MANUAL_MEMCPY,
-                    get_span_of_entire_for_loop(expr),
-                    "it looks like you're manually copying between slices",
-                    "try replacing the loop by",
-                    big_sugg,
-                    Applicability::Unspecified,
-                );
-                return true;
-            }
-        }
-    }
-    false
-}
-
-// Scans the body of the for loop and determines whether lint should be given
-struct SameItemPushVisitor<'a, 'tcx> {
-    should_lint: bool,
-    // this field holds the last vec push operation visited, which should be the only push seen
-    vec_push: Option<(&'tcx Expr<'tcx>, &'tcx Expr<'tcx>)>,
-    cx: &'a LateContext<'tcx>,
-}
-
-impl<'a, 'tcx> Visitor<'tcx> for SameItemPushVisitor<'a, 'tcx> {
-    type Map = Map<'tcx>;
-
-    fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
-        match &expr.kind {
-            // Non-determinism may occur ... don't give a lint
-            ExprKind::Loop(..) | ExprKind::Match(..) => self.should_lint = false,
-            ExprKind::Block(block, _) => self.visit_block(block),
-            _ => {},
-        }
-    }
-
-    fn visit_block(&mut self, b: &'tcx Block<'_>) {
-        for stmt in b.stmts.iter() {
-            self.visit_stmt(stmt);
-        }
-    }
-
-    fn visit_stmt(&mut self, s: &'tcx Stmt<'_>) {
-        let vec_push_option = get_vec_push(self.cx, s);
-        if vec_push_option.is_none() {
-            // Current statement is not a push so visit inside
-            match &s.kind {
-                StmtKind::Expr(expr) | StmtKind::Semi(expr) => self.visit_expr(&expr),
-                _ => {},
-            }
-        } else {
-            // Current statement is a push ...check whether another
-            // push had been previously done
-            if self.vec_push.is_none() {
-                self.vec_push = vec_push_option;
-            } else {
-                // There are multiple pushes ... don't lint
-                self.should_lint = false;
-            }
-        }
-    }
-
-    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
-        NestedVisitorMap::None
-    }
-}
-
-// Given some statement, determine if that statement is a push on a Vec. If it is, return
-// the Vec being pushed into and the item being pushed
-fn get_vec_push<'tcx>(cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'_>) -> Option<(&'tcx Expr<'tcx>, &'tcx Expr<'tcx>)> {
-    if_chain! {
-            // Extract method being called
-            if let StmtKind::Semi(semi_stmt) = &stmt.kind;
-            if let ExprKind::MethodCall(path, _, args, _) = &semi_stmt.kind;
-            // Figure out the parameters for the method call
-            if let Some(self_expr) = args.get(0);
-            if let Some(pushed_item) = args.get(1);
-            // Check that the method being called is push() on a Vec
-            if is_type_diagnostic_item(cx, cx.typeck_results().expr_ty(self_expr), sym::vec_type);
-            if path.ident.name.as_str() == "push";
-            then {
-                return Some((self_expr, pushed_item))
-            }
-    }
-    None
-}
-
-/// Detects for loop pushing the same item into a Vec
-fn detect_same_item_push<'tcx>(
-    cx: &LateContext<'tcx>,
-    pat: &'tcx Pat<'_>,
-    _: &'tcx Expr<'_>,
-    body: &'tcx Expr<'_>,
-    _: &'tcx Expr<'_>,
-) {
-    fn emit_lint(cx: &LateContext<'_>, vec: &Expr<'_>, pushed_item: &Expr<'_>) {
-        let vec_str = snippet_with_macro_callsite(cx, vec.span, "");
-        let item_str = snippet_with_macro_callsite(cx, pushed_item.span, "");
-
-        span_lint_and_help(
-            cx,
-            SAME_ITEM_PUSH,
-            vec.span,
-            "it looks like the same item is being pushed into this Vec",
-            None,
-            &format!(
-                "try using vec![{};SIZE] or {}.resize(NEW_SIZE, {})",
-                item_str, vec_str, item_str
-            ),
-        )
-    }
-
-    if !matches!(pat.kind, PatKind::Wild) {
-        return;
-    }
-
-    // Determine whether it is safe to lint the body
-    let mut same_item_push_visitor = SameItemPushVisitor {
-        should_lint: true,
-        vec_push: None,
-        cx,
-    };
-    walk_expr(&mut same_item_push_visitor, body);
-    if same_item_push_visitor.should_lint {
-        if let Some((vec, pushed_item)) = same_item_push_visitor.vec_push {
-            let vec_ty = cx.typeck_results().expr_ty(vec);
-            let ty = vec_ty.walk().nth(1).unwrap().expect_ty();
-            if cx
-                .tcx
-                .lang_items()
-                .clone_trait()
-                .map_or(false, |id| implements_trait(cx, ty, id, &[]))
-            {
-                // Make sure that the push does not involve possibly mutating values
-                match pushed_item.kind {
-                    ExprKind::Path(ref qpath) => {
-                        match cx.qpath_res(qpath, pushed_item.hir_id) {
-                            // immutable bindings that are initialized with literal or constant
-                            Res::Local(hir_id) => {
-                                if_chain! {
-                                    let node = cx.tcx.hir().get(hir_id);
-                                    if let Node::Binding(pat) = node;
-                                    if let PatKind::Binding(bind_ann, ..) = pat.kind;
-                                    if !matches!(bind_ann, BindingAnnotation::RefMut | BindingAnnotation::Mutable);
-                                    let parent_node = cx.tcx.hir().get_parent_node(hir_id);
-                                    if let Some(Node::Local(parent_let_expr)) = cx.tcx.hir().find(parent_node);
-                                    if let Some(init) = parent_let_expr.init;
-                                    then {
-                                        match init.kind {
-                                            // immutable bindings that are initialized with literal
-                                            ExprKind::Lit(..) => emit_lint(cx, vec, pushed_item),
-                                            // immutable bindings that are initialized with constant
-                                            ExprKind::Path(ref path) => {
-                                                if let Res::Def(DefKind::Const, ..) = cx.qpath_res(path, init.hir_id) {
-                                                    emit_lint(cx, vec, pushed_item);
-                                                }
-                                            }
-                                            _ => {},
-                                        }
-                                    }
-                                }
-                            },
-                            // constant
-                            Res::Def(DefKind::Const, ..) => emit_lint(cx, vec, pushed_item),
-                            _ => {},
-                        }
-                    },
-                    ExprKind::Lit(..) => emit_lint(cx, vec, pushed_item),
-                    _ => {},
-                }
-            }
-        }
-    }
-}
-
-/// Checks for looping over a range and then indexing a sequence with it.
-/// The iteratee must be a range literal.
-#[allow(clippy::too_many_lines)]
-fn check_for_loop_range<'tcx>(
-    cx: &LateContext<'tcx>,
-    pat: &'tcx Pat<'_>,
-    arg: &'tcx Expr<'_>,
-    body: &'tcx Expr<'_>,
-    expr: &'tcx Expr<'_>,
-) {
-    if let Some(higher::Range {
-        start: Some(start),
-        ref end,
-        limits,
-    }) = higher::range(arg)
-    {
-        // the var must be a single name
-        if let PatKind::Binding(_, canonical_id, ident, _) = pat.kind {
-            let mut visitor = VarVisitor {
-                cx,
-                var: canonical_id,
-                indexed_mut: FxHashSet::default(),
-                indexed_indirectly: FxHashMap::default(),
-                indexed_directly: FxHashMap::default(),
-                referenced: FxHashSet::default(),
-                nonindex: false,
-                prefer_mutable: false,
-            };
-            walk_expr(&mut visitor, body);
-
-            // linting condition: we only indexed one variable, and indexed it directly
-            if visitor.indexed_indirectly.is_empty() && visitor.indexed_directly.len() == 1 {
-                let (indexed, (indexed_extent, indexed_ty)) = visitor
-                    .indexed_directly
-                    .into_iter()
-                    .next()
-                    .expect("already checked that we have exactly 1 element");
-
-                // ensure that the indexed variable was declared before the loop, see #601
-                if let Some(indexed_extent) = indexed_extent {
-                    let parent_id = cx.tcx.hir().get_parent_item(expr.hir_id);
-                    let parent_def_id = cx.tcx.hir().local_def_id(parent_id);
-                    let region_scope_tree = cx.tcx.region_scope_tree(parent_def_id);
-                    let pat_extent = region_scope_tree.var_scope(pat.hir_id.local_id);
-                    if region_scope_tree.is_subscope_of(indexed_extent, pat_extent) {
-                        return;
-                    }
-                }
-
-                // don't lint if the container that is indexed does not have .iter() method
-                let has_iter = has_iter_method(cx, indexed_ty);
-                if has_iter.is_none() {
-                    return;
-                }
-
-                // don't lint if the container that is indexed into is also used without
-                // indexing
-                if visitor.referenced.contains(&indexed) {
-                    return;
-                }
-
-                let starts_at_zero = is_integer_const(cx, start, 0);
-
-                let skip = if starts_at_zero {
-                    String::new()
-                } else if visitor.indexed_mut.contains(&indexed) && contains_name(indexed, start) {
-                    return;
-                } else {
-                    format!(".skip({})", snippet(cx, start.span, ".."))
-                };
-
-                let mut end_is_start_plus_val = false;
-
-                let take = if let Some(end) = *end {
-                    let mut take_expr = end;
-
-                    if let ExprKind::Binary(ref op, ref left, ref right) = end.kind {
-                        if let BinOpKind::Add = op.node {
-                            let start_equal_left = SpanlessEq::new(cx).eq_expr(start, left);
-                            let start_equal_right = SpanlessEq::new(cx).eq_expr(start, right);
-
-                            if start_equal_left {
-                                take_expr = right;
-                            } else if start_equal_right {
-                                take_expr = left;
-                            }
-
-                            end_is_start_plus_val = start_equal_left | start_equal_right;
-                        }
-                    }
-
-                    if is_len_call(end, indexed) || is_end_eq_array_len(cx, end, limits, indexed_ty) {
-                        String::new()
-                    } else if visitor.indexed_mut.contains(&indexed) && contains_name(indexed, take_expr) {
-                        return;
-                    } else {
-                        match limits {
-                            ast::RangeLimits::Closed => {
-                                let take_expr = sugg::Sugg::hir(cx, take_expr, "<count>");
-                                format!(".take({})", take_expr + sugg::ONE)
-                            },
-                            ast::RangeLimits::HalfOpen => format!(".take({})", snippet(cx, take_expr.span, "..")),
-                        }
-                    }
-                } else {
-                    String::new()
-                };
-
-                let (ref_mut, method) = if visitor.indexed_mut.contains(&indexed) {
-                    ("mut ", "iter_mut")
-                } else {
-                    ("", "iter")
-                };
-
-                let take_is_empty = take.is_empty();
-                let mut method_1 = take;
-                let mut method_2 = skip;
-
-                if end_is_start_plus_val {
-                    mem::swap(&mut method_1, &mut method_2);
-                }
-
-                if visitor.nonindex {
-                    span_lint_and_then(
-                        cx,
-                        NEEDLESS_RANGE_LOOP,
-                        expr.span,
-                        &format!("the loop variable `{}` is used to index `{}`", ident.name, indexed),
-                        |diag| {
-                            multispan_sugg(
-                                diag,
-                                "consider using an iterator",
-                                vec![
-                                    (pat.span, format!("({}, <item>)", ident.name)),
-                                    (
-                                        arg.span,
-                                        format!("{}.{}().enumerate(){}{}", indexed, method, method_1, method_2),
-                                    ),
-                                ],
-                            );
-                        },
-                    );
-                } else {
-                    let repl = if starts_at_zero && take_is_empty {
-                        format!("&{}{}", ref_mut, indexed)
-                    } else {
-                        format!("{}.{}(){}{}", indexed, method, method_1, method_2)
-                    };
-
-                    span_lint_and_then(
-                        cx,
-                        NEEDLESS_RANGE_LOOP,
-                        expr.span,
-                        &format!("the loop variable `{}` is only used to index `{}`", ident.name, indexed),
-                        |diag| {
-                            multispan_sugg(
-                                diag,
-                                "consider using an iterator",
-                                vec![(pat.span, "<item>".to_string()), (arg.span, repl)],
-                            );
-                        },
-                    );
-                }
-            }
-        }
-    }
-}
-
-fn is_len_call(expr: &Expr<'_>, var: Symbol) -> bool {
-    if_chain! {
-        if let ExprKind::MethodCall(ref method, _, ref len_args, _) = expr.kind;
-        if len_args.len() == 1;
-        if method.ident.name == sym!(len);
-        if let ExprKind::Path(QPath::Resolved(_, ref path)) = len_args[0].kind;
-        if path.segments.len() == 1;
-        if path.segments[0].ident.name == var;
-        then {
-            return true;
-        }
-    }
-
-    false
-}
-
-fn is_end_eq_array_len<'tcx>(
-    cx: &LateContext<'tcx>,
-    end: &Expr<'_>,
-    limits: ast::RangeLimits,
-    indexed_ty: Ty<'tcx>,
-) -> bool {
-    if_chain! {
-        if let ExprKind::Lit(ref lit) = end.kind;
-        if let ast::LitKind::Int(end_int, _) = lit.node;
-        if let ty::Array(_, arr_len_const) = indexed_ty.kind();
-        if let Some(arr_len) = arr_len_const.try_eval_usize(cx.tcx, cx.param_env);
-        then {
-            return match limits {
-                ast::RangeLimits::Closed => end_int + 1 >= arr_len.into(),
-                ast::RangeLimits::HalfOpen => end_int >= arr_len.into(),
-            };
-        }
-    }
-
-    false
-}
-
-fn lint_iter_method(cx: &LateContext<'_>, args: &[Expr<'_>], arg: &Expr<'_>, method_name: &str) {
-    let mut applicability = Applicability::MachineApplicable;
-    let object = snippet_with_applicability(cx, args[0].span, "_", &mut applicability);
-    let muta = if method_name == "iter_mut" { "mut " } else { "" };
-    span_lint_and_sugg(
-        cx,
-        EXPLICIT_ITER_LOOP,
-        arg.span,
-        "it is more concise to loop over references to containers instead of using explicit \
-         iteration methods",
-        "to write this more concisely, try",
-        format!("&{}{}", muta, object),
-        applicability,
-    )
-}
-
-fn check_for_loop_arg(cx: &LateContext<'_>, pat: &Pat<'_>, arg: &Expr<'_>, expr: &Expr<'_>) {
-    let mut next_loop_linted = false; // whether or not ITER_NEXT_LOOP lint was used
-    if let ExprKind::MethodCall(ref method, _, ref args, _) = arg.kind {
-        // just the receiver, no arguments
-        if args.len() == 1 {
-            let method_name = &*method.ident.as_str();
-            // check for looping over x.iter() or x.iter_mut(), could use &x or &mut x
-            if method_name == "iter" || method_name == "iter_mut" {
-                if is_ref_iterable_type(cx, &args[0]) {
-                    lint_iter_method(cx, args, arg, method_name);
-                }
-            } else if method_name == "into_iter" && match_trait_method(cx, arg, &paths::INTO_ITERATOR) {
-                let receiver_ty = cx.typeck_results().expr_ty(&args[0]);
-                let receiver_ty_adjusted = cx.typeck_results().expr_ty_adjusted(&args[0]);
-                if TyS::same_type(receiver_ty, receiver_ty_adjusted) {
-                    let mut applicability = Applicability::MachineApplicable;
-                    let object = snippet_with_applicability(cx, args[0].span, "_", &mut applicability);
-                    span_lint_and_sugg(
-                        cx,
-                        EXPLICIT_INTO_ITER_LOOP,
-                        arg.span,
-                        "it is more concise to loop over containers instead of using explicit \
-                         iteration methods",
-                        "to write this more concisely, try",
-                        object.to_string(),
-                        applicability,
-                    );
-                } else {
-                    let ref_receiver_ty = cx.tcx.mk_ref(
-                        cx.tcx.lifetimes.re_erased,
-                        ty::TypeAndMut {
-                            ty: receiver_ty,
-                            mutbl: Mutability::Not,
-                        },
-                    );
-                    if TyS::same_type(receiver_ty_adjusted, ref_receiver_ty) {
-                        lint_iter_method(cx, args, arg, method_name)
-                    }
-                }
-            } else if method_name == "next" && match_trait_method(cx, arg, &paths::ITERATOR) {
-                span_lint(
-                    cx,
-                    ITER_NEXT_LOOP,
-                    expr.span,
-                    "you are iterating over `Iterator::next()` which is an Option; this will compile but is \
-                    probably not what you want",
-                );
-                next_loop_linted = true;
-            }
-        }
-    }
-    if !next_loop_linted {
-        check_arg_type(cx, pat, arg);
-    }
-}
-
-/// Checks for `for` loops over `Option`s and `Result`s.
-fn check_arg_type(cx: &LateContext<'_>, pat: &Pat<'_>, arg: &Expr<'_>) {
-    let ty = cx.typeck_results().expr_ty(arg);
-    if is_type_diagnostic_item(cx, ty, sym::option_type) {
-        span_lint_and_help(
-            cx,
-            FOR_LOOPS_OVER_FALLIBLES,
-            arg.span,
-            &format!(
-                "for loop over `{0}`, which is an `Option`. This is more readably written as an \
-                `if let` statement",
-                snippet(cx, arg.span, "_")
-            ),
-            None,
-            &format!(
-                "consider replacing `for {0} in {1}` with `if let Some({0}) = {1}`",
-                snippet(cx, pat.span, "_"),
-                snippet(cx, arg.span, "_")
-            ),
-        );
-    } else if is_type_diagnostic_item(cx, ty, sym::result_type) {
-        span_lint_and_help(
-            cx,
-            FOR_LOOPS_OVER_FALLIBLES,
-            arg.span,
-            &format!(
-                "for loop over `{0}`, which is a `Result`. This is more readably written as an \
-                `if let` statement",
-                snippet(cx, arg.span, "_")
-            ),
-            None,
-            &format!(
-                "consider replacing `for {0} in {1}` with `if let Ok({0}) = {1}`",
-                snippet(cx, pat.span, "_"),
-                snippet(cx, arg.span, "_")
-            ),
-        );
-    }
-}
-
-// To trigger the EXPLICIT_COUNTER_LOOP lint, a variable must be
-// incremented exactly once in the loop body, and initialized to zero
-// at the start of the loop.
-fn check_for_loop_explicit_counter<'tcx>(
-    cx: &LateContext<'tcx>,
-    pat: &'tcx Pat<'_>,
-    arg: &'tcx Expr<'_>,
-    body: &'tcx Expr<'_>,
-    expr: &'tcx Expr<'_>,
-) {
-    // Look for variables that are incremented once per loop iteration.
-    let mut increment_visitor = IncrementVisitor::new(cx);
-    walk_expr(&mut increment_visitor, body);
-
-    // For each candidate, check the parent block to see if
-    // it's initialized to zero at the start of the loop.
-    if let Some(block) = get_enclosing_block(&cx, expr.hir_id) {
-        for id in increment_visitor.into_results() {
-            let mut initialize_visitor = InitializeVisitor::new(cx, expr, id);
-            walk_block(&mut initialize_visitor, block);
-
-            if_chain! {
-                if let Some((name, initializer)) = initialize_visitor.get_result();
-                if is_integer_const(cx, initializer, 0);
-                then {
-                    let mut applicability = Applicability::MachineApplicable;
-
-                    let for_span = get_span_of_entire_for_loop(expr);
-
-                    span_lint_and_sugg(
-                        cx,
-                        EXPLICIT_COUNTER_LOOP,
-                        for_span.with_hi(arg.span.hi()),
-                        &format!("the variable `{}` is used as a loop counter", name),
-                        "consider using",
-                        format!(
-                            "for ({}, {}) in {}.enumerate()",
-                            name,
-                            snippet_with_applicability(cx, pat.span, "item", &mut applicability),
-                            make_iterator_snippet(cx, arg, &mut applicability),
-                        ),
-                        applicability,
-                    );
-                }
-            }
-        }
-    }
-}
-
-/// If `arg` was the argument to a `for` loop, return the "cleanest" way of writing the
-/// actual `Iterator` that the loop uses.
-fn make_iterator_snippet(cx: &LateContext<'_>, arg: &Expr<'_>, applic_ref: &mut Applicability) -> String {
-    let impls_iterator = get_trait_def_id(cx, &paths::ITERATOR).map_or(false, |id| {
-        implements_trait(cx, cx.typeck_results().expr_ty(arg), id, &[])
-    });
-    if impls_iterator {
-        format!(
-            "{}",
-            sugg::Sugg::hir_with_applicability(cx, arg, "_", applic_ref).maybe_par()
-        )
-    } else {
-        // (&x).into_iter() ==> x.iter()
-        // (&mut x).into_iter() ==> x.iter_mut()
-        match &arg.kind {
-            ExprKind::AddrOf(BorrowKind::Ref, mutability, arg_inner)
-                if has_iter_method(cx, cx.typeck_results().expr_ty(&arg_inner)).is_some() =>
-            {
-                let meth_name = match mutability {
-                    Mutability::Mut => "iter_mut",
-                    Mutability::Not => "iter",
-                };
-                format!(
-                    "{}.{}()",
-                    sugg::Sugg::hir_with_applicability(cx, &arg_inner, "_", applic_ref).maybe_par(),
-                    meth_name,
-                )
-            }
-            _ => format!(
-                "{}.into_iter()",
-                sugg::Sugg::hir_with_applicability(cx, arg, "_", applic_ref).maybe_par()
-            ),
-        }
-    }
-}
-
-/// Checks for the `FOR_KV_MAP` lint.
-fn check_for_loop_over_map_kv<'tcx>(
-    cx: &LateContext<'tcx>,
-    pat: &'tcx Pat<'_>,
-    arg: &'tcx Expr<'_>,
-    body: &'tcx Expr<'_>,
-    expr: &'tcx Expr<'_>,
-) {
-    let pat_span = pat.span;
-
-    if let PatKind::Tuple(ref pat, _) = pat.kind {
-        if pat.len() == 2 {
-            let arg_span = arg.span;
-            let (new_pat_span, kind, ty, mutbl) = match *cx.typeck_results().expr_ty(arg).kind() {
-                ty::Ref(_, ty, mutbl) => match (&pat[0].kind, &pat[1].kind) {
-                    (key, _) if pat_is_wild(cx, key, body) => (pat[1].span, "value", ty, mutbl),
-                    (_, value) if pat_is_wild(cx, value, body) => (pat[0].span, "key", ty, Mutability::Not),
-                    _ => return,
-                },
-                _ => return,
-            };
-            let mutbl = match mutbl {
-                Mutability::Not => "",
-                Mutability::Mut => "_mut",
-            };
-            let arg = match arg.kind {
-                ExprKind::AddrOf(BorrowKind::Ref, _, ref expr) => &**expr,
-                _ => arg,
-            };
-
-            if is_type_diagnostic_item(cx, ty, sym!(hashmap_type)) || match_type(cx, ty, &paths::BTREEMAP) {
-                span_lint_and_then(
-                    cx,
-                    FOR_KV_MAP,
-                    expr.span,
-                    &format!("you seem to want to iterate on a map's {}s", kind),
-                    |diag| {
-                        let map = sugg::Sugg::hir(cx, arg, "map");
-                        multispan_sugg(
-                            diag,
-                            "use the corresponding method",
-                            vec![
-                                (pat_span, snippet(cx, new_pat_span, kind).into_owned()),
-                                (arg_span, format!("{}.{}s{}()", map.maybe_par(), kind, mutbl)),
-                            ],
-                        );
-                    },
-                );
-            }
-        }
-    }
-}
-
-fn check_for_single_element_loop<'tcx>(
-    cx: &LateContext<'tcx>,
-    pat: &'tcx Pat<'_>,
-    arg: &'tcx Expr<'_>,
-    body: &'tcx Expr<'_>,
-    expr: &'tcx Expr<'_>,
-) {
-    if_chain! {
-        if let ExprKind::AddrOf(BorrowKind::Ref, _, ref arg_expr) = arg.kind;
-        if let PatKind::Binding(.., target, _) = pat.kind;
-        if let ExprKind::Array([arg_expression]) = arg_expr.kind;
-        if let ExprKind::Path(ref list_item) = arg_expression.kind;
-        if let Some(list_item_name) = single_segment_path(list_item).map(|ps| ps.ident.name);
-        if let ExprKind::Block(ref block, _) = body.kind;
-        if !block.stmts.is_empty();
-
-        then {
-            let for_span = get_span_of_entire_for_loop(expr);
-            let mut block_str = snippet(cx, block.span, "..").into_owned();
-            block_str.remove(0);
-            block_str.pop();
-
-
-            span_lint_and_sugg(
-                cx,
-                SINGLE_ELEMENT_LOOP,
-                for_span,
-                "for loop over a single element",
-                "try",
-                format!("{{\n{}let {} = &{};{}}}", " ".repeat(indent_of(cx, block.stmts[0].span).unwrap_or(0)), target.name, list_item_name, block_str),
-                Applicability::MachineApplicable
-            )
-        }
-    }
-}
-
-/// Check for unnecessary `if let` usage in a for loop where only the `Some` or `Ok` variant of the
-/// iterator element is used.
-fn check_manual_flatten<'tcx>(
-    cx: &LateContext<'tcx>,
-    pat: &'tcx Pat<'_>,
-    arg: &'tcx Expr<'_>,
-    body: &'tcx Expr<'_>,
-    span: Span,
-) {
-    if let ExprKind::Block(ref block, _) = body.kind {
-        // Ensure the `if let` statement is the only expression or statement in the for-loop
-        let inner_expr = if block.stmts.len() == 1 && block.expr.is_none() {
-            let match_stmt = &block.stmts[0];
-            if let StmtKind::Semi(inner_expr) = match_stmt.kind {
-                Some(inner_expr)
-            } else {
-                None
-            }
-        } else if block.stmts.is_empty() {
-            block.expr
-        } else {
-            None
-        };
-
-        if_chain! {
-            if let Some(inner_expr) = inner_expr;
-            if let ExprKind::Match(
-                ref match_expr, ref match_arms, MatchSource::IfLetDesugar{ contains_else_clause: false }
-            ) = inner_expr.kind;
-            // Ensure match_expr in `if let` statement is the same as the pat from the for-loop
-            if let PatKind::Binding(_, pat_hir_id, _, _) = pat.kind;
-            if path_to_local_id(match_expr, pat_hir_id);
-            // Ensure the `if let` statement is for the `Some` variant of `Option` or the `Ok` variant of `Result`
-            if let PatKind::TupleStruct(QPath::Resolved(None, path), _, _) = match_arms[0].pat.kind;
-            let some_ctor = is_some_ctor(cx, path.res);
-            let ok_ctor = is_ok_ctor(cx, path.res);
-            if some_ctor || ok_ctor;
-            let if_let_type = if some_ctor { "Some" } else { "Ok" };
-
-            then {
-                // Prepare the error message
-                let msg = format!("unnecessary `if let` since only the `{}` variant of the iterator element is used", if_let_type);
-
-                // Prepare the help message
-                let mut applicability = Applicability::MaybeIncorrect;
-                let arg_snippet = make_iterator_snippet(cx, arg, &mut applicability);
-
-                span_lint_and_then(
-                    cx,
-                    MANUAL_FLATTEN,
-                    span,
-                    &msg,
-                    |diag| {
-                        let sugg = format!("{}.flatten()", arg_snippet);
-                        diag.span_suggestion(
-                            arg.span,
-                            "try",
-                            sugg,
-                            Applicability::MaybeIncorrect,
-                        );
-                        diag.span_help(
-                            inner_expr.span,
-                            "...and remove the `if let` statement in the for loop",
-                        );
-                    }
-                );
-            }
-        }
-    }
-}
-
-struct MutatePairDelegate<'a, 'tcx> {
-    cx: &'a LateContext<'tcx>,
-    hir_id_low: Option<HirId>,
-    hir_id_high: Option<HirId>,
-    span_low: Option<Span>,
-    span_high: Option<Span>,
-}
-
-impl<'tcx> Delegate<'tcx> for MutatePairDelegate<'_, 'tcx> {
-    fn consume(&mut self, _: &PlaceWithHirId<'tcx>, _: HirId, _: ConsumeMode) {}
-
-    fn borrow(&mut self, cmt: &PlaceWithHirId<'tcx>, diag_expr_id: HirId, bk: ty::BorrowKind) {
-        if let ty::BorrowKind::MutBorrow = bk {
-            if let PlaceBase::Local(id) = cmt.place.base {
-                if Some(id) == self.hir_id_low {
-                    self.span_low = Some(self.cx.tcx.hir().span(diag_expr_id))
-                }
-                if Some(id) == self.hir_id_high {
-                    self.span_high = Some(self.cx.tcx.hir().span(diag_expr_id))
-                }
-            }
-        }
-    }
-
-    fn mutate(&mut self, cmt: &PlaceWithHirId<'tcx>, diag_expr_id: HirId) {
-        if let PlaceBase::Local(id) = cmt.place.base {
-            if Some(id) == self.hir_id_low {
-                self.span_low = Some(self.cx.tcx.hir().span(diag_expr_id))
-            }
-            if Some(id) == self.hir_id_high {
-                self.span_high = Some(self.cx.tcx.hir().span(diag_expr_id))
-            }
-        }
-    }
-}
-
-impl MutatePairDelegate<'_, '_> {
-    fn mutation_span(&self) -> (Option<Span>, Option<Span>) {
-        (self.span_low, self.span_high)
-    }
-}
-
-fn check_for_mut_range_bound(cx: &LateContext<'_>, arg: &Expr<'_>, body: &Expr<'_>) {
-    if let Some(higher::Range {
-        start: Some(start),
-        end: Some(end),
-        ..
-    }) = higher::range(arg)
-    {
-        let mut_ids = vec![check_for_mutability(cx, start), check_for_mutability(cx, end)];
-        if mut_ids[0].is_some() || mut_ids[1].is_some() {
-            let (span_low, span_high) = check_for_mutation(cx, body, &mut_ids);
-            mut_warn_with_span(cx, span_low);
-            mut_warn_with_span(cx, span_high);
-        }
-    }
-}
-
-fn mut_warn_with_span(cx: &LateContext<'_>, span: Option<Span>) {
-    if let Some(sp) = span {
-        span_lint(
-            cx,
-            MUT_RANGE_BOUND,
-            sp,
-            "attempt to mutate range bound within loop; note that the range of the loop is unchanged",
-        );
-    }
-}
-
-fn check_for_mutability(cx: &LateContext<'_>, bound: &Expr<'_>) -> Option<HirId> {
-    if_chain! {
-        if let Some(hir_id) = path_to_local(bound);
-        if let Node::Binding(pat) = cx.tcx.hir().get(hir_id);
-        if let PatKind::Binding(BindingAnnotation::Mutable, ..) = pat.kind;
-        then {
-            return Some(hir_id);
-        }
-    }
-    None
-}
-
-fn check_for_mutation<'tcx>(
-    cx: &LateContext<'tcx>,
-    body: &Expr<'_>,
-    bound_ids: &[Option<HirId>],
-) -> (Option<Span>, Option<Span>) {
-    let mut delegate = MutatePairDelegate {
-        cx,
-        hir_id_low: bound_ids[0],
-        hir_id_high: bound_ids[1],
-        span_low: None,
-        span_high: None,
-    };
-    cx.tcx.infer_ctxt().enter(|infcx| {
-        ExprUseVisitor::new(
-            &mut delegate,
-            &infcx,
-            body.hir_id.owner,
-            cx.param_env,
-            cx.typeck_results(),
-        )
-        .walk_expr(body);
-    });
-    delegate.mutation_span()
-}
-
-/// Returns `true` if the pattern is a `PatWild` or an ident prefixed with `_`.
-fn pat_is_wild<'tcx>(cx: &LateContext<'tcx>, pat: &'tcx PatKind<'_>, body: &'tcx Expr<'_>) -> bool {
-    match *pat {
-        PatKind::Wild => true,
-        PatKind::Binding(_, id, ident, None) if ident.as_str().starts_with('_') => {
-            !LocalUsedVisitor::new(cx, id).check_expr(body)
-        },
-        _ => false,
-    }
-}
-
-struct VarVisitor<'a, 'tcx> {
-    /// context reference
-    cx: &'a LateContext<'tcx>,
-    /// var name to look for as index
-    var: HirId,
-    /// indexed variables that are used mutably
-    indexed_mut: FxHashSet<Symbol>,
-    /// indirectly indexed variables (`v[(i + 4) % N]`), the extend is `None` for global
-    indexed_indirectly: FxHashMap<Symbol, Option<region::Scope>>,
-    /// subset of `indexed` of vars that are indexed directly: `v[i]`
-    /// this will not contain cases like `v[calc_index(i)]` or `v[(i + 4) % N]`
-    indexed_directly: FxHashMap<Symbol, (Option<region::Scope>, Ty<'tcx>)>,
-    /// Any names that are used outside an index operation.
-    /// Used to detect things like `&mut vec` used together with `vec[i]`
-    referenced: FxHashSet<Symbol>,
-    /// has the loop variable been used in expressions other than the index of
-    /// an index op?
-    nonindex: bool,
-    /// Whether we are inside the `$` in `&mut $` or `$ = foo` or `$.bar`, where bar
-    /// takes `&mut self`
-    prefer_mutable: bool,
-}
-
-impl<'a, 'tcx> VarVisitor<'a, 'tcx> {
-    fn check(&mut self, idx: &'tcx Expr<'_>, seqexpr: &'tcx Expr<'_>, expr: &'tcx Expr<'_>) -> bool {
-        if_chain! {
-            // the indexed container is referenced by a name
-            if let ExprKind::Path(ref seqpath) = seqexpr.kind;
-            if let QPath::Resolved(None, ref seqvar) = *seqpath;
-            if seqvar.segments.len() == 1;
-            then {
-                let index_used_directly = path_to_local_id(idx, self.var);
-                let indexed_indirectly = {
-                    let mut used_visitor = LocalUsedVisitor::new(self.cx, self.var);
-                    walk_expr(&mut used_visitor, idx);
-                    used_visitor.used
-                };
-
-                if indexed_indirectly || index_used_directly {
-                    if self.prefer_mutable {
-                        self.indexed_mut.insert(seqvar.segments[0].ident.name);
-                    }
-                    let res = self.cx.qpath_res(seqpath, seqexpr.hir_id);
-                    match res {
-                        Res::Local(hir_id) => {
-                            let parent_id = self.cx.tcx.hir().get_parent_item(expr.hir_id);
-                            let parent_def_id = self.cx.tcx.hir().local_def_id(parent_id);
-                            let extent = self.cx.tcx.region_scope_tree(parent_def_id).var_scope(hir_id.local_id);
-                            if indexed_indirectly {
-                                self.indexed_indirectly.insert(seqvar.segments[0].ident.name, Some(extent));
-                            }
-                            if index_used_directly {
-                                self.indexed_directly.insert(
-                                    seqvar.segments[0].ident.name,
-                                    (Some(extent), self.cx.typeck_results().node_type(seqexpr.hir_id)),
-                                );
-                            }
-                            return false;  // no need to walk further *on the variable*
-                        }
-                        Res::Def(DefKind::Static | DefKind::Const, ..) => {
-                            if indexed_indirectly {
-                                self.indexed_indirectly.insert(seqvar.segments[0].ident.name, None);
-                            }
-                            if index_used_directly {
-                                self.indexed_directly.insert(
-                                    seqvar.segments[0].ident.name,
-                                    (None, self.cx.typeck_results().node_type(seqexpr.hir_id)),
-                                );
-                            }
-                            return false;  // no need to walk further *on the variable*
-                        }
-                        _ => (),
-                    }
-                }
-            }
-        }
-        true
-    }
-}
-
-impl<'a, 'tcx> Visitor<'tcx> for VarVisitor<'a, 'tcx> {
-    type Map = Map<'tcx>;
-
-    fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
-        if_chain! {
-            // a range index op
-            if let ExprKind::MethodCall(ref meth, _, ref args, _) = expr.kind;
-            if (meth.ident.name == sym::index && match_trait_method(self.cx, expr, &paths::INDEX))
-                || (meth.ident.name == sym::index_mut && match_trait_method(self.cx, expr, &paths::INDEX_MUT));
-            if !self.check(&args[1], &args[0], expr);
-            then { return }
-        }
-
-        if_chain! {
-            // an index op
-            if let ExprKind::Index(ref seqexpr, ref idx) = expr.kind;
-            if !self.check(idx, seqexpr, expr);
-            then { return }
-        }
-
-        if_chain! {
-            // directly using a variable
-            if let ExprKind::Path(QPath::Resolved(None, path)) = expr.kind;
-            if let Res::Local(local_id) = path.res;
-            then {
-                if local_id == self.var {
-                    self.nonindex = true;
-                } else {
-                    // not the correct variable, but still a variable
-                    self.referenced.insert(path.segments[0].ident.name);
-                }
-            }
-        }
-
-        let old = self.prefer_mutable;
-        match expr.kind {
-            ExprKind::AssignOp(_, ref lhs, ref rhs) | ExprKind::Assign(ref lhs, ref rhs, _) => {
-                self.prefer_mutable = true;
-                self.visit_expr(lhs);
-                self.prefer_mutable = false;
-                self.visit_expr(rhs);
-            },
-            ExprKind::AddrOf(BorrowKind::Ref, mutbl, ref expr) => {
-                if mutbl == Mutability::Mut {
-                    self.prefer_mutable = true;
-                }
-                self.visit_expr(expr);
-            },
-            ExprKind::Call(ref f, args) => {
-                self.visit_expr(f);
-                for expr in args {
-                    let ty = self.cx.typeck_results().expr_ty_adjusted(expr);
-                    self.prefer_mutable = false;
-                    if let ty::Ref(_, _, mutbl) = *ty.kind() {
-                        if mutbl == Mutability::Mut {
-                            self.prefer_mutable = true;
-                        }
-                    }
-                    self.visit_expr(expr);
-                }
-            },
-            ExprKind::MethodCall(_, _, args, _) => {
-                let def_id = self.cx.typeck_results().type_dependent_def_id(expr.hir_id).unwrap();
-                for (ty, expr) in self.cx.tcx.fn_sig(def_id).inputs().skip_binder().iter().zip(args) {
-                    self.prefer_mutable = false;
-                    if let ty::Ref(_, _, mutbl) = *ty.kind() {
-                        if mutbl == Mutability::Mut {
-                            self.prefer_mutable = true;
-                        }
-                    }
-                    self.visit_expr(expr);
-                }
-            },
-            ExprKind::Closure(_, _, body_id, ..) => {
-                let body = self.cx.tcx.hir().body(body_id);
-                self.visit_expr(&body.value);
-            },
-            _ => walk_expr(self, expr),
-        }
-        self.prefer_mutable = old;
-    }
-    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
-        NestedVisitorMap::None
-    }
-}
-
-fn is_used_inside<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, container: &'tcx Expr<'_>) -> bool {
-    let def_id = match path_to_local(expr) {
-        Some(id) => id,
-        None => return false,
-    };
-    if let Some(used_mutably) = mutated_variables(container, cx) {
-        if used_mutably.contains(&def_id) {
-            return true;
-        }
-    }
-    false
-}
-
-fn is_iterator_used_after_while_let<'tcx>(cx: &LateContext<'tcx>, iter_expr: &'tcx Expr<'_>) -> bool {
-    let def_id = match path_to_local(iter_expr) {
-        Some(id) => id,
-        None => return false,
-    };
-    let mut visitor = VarUsedAfterLoopVisitor {
-        def_id,
-        iter_expr_id: iter_expr.hir_id,
-        past_while_let: false,
-        var_used_after_while_let: false,
-    };
-    if let Some(enclosing_block) = get_enclosing_block(cx, def_id) {
-        walk_block(&mut visitor, enclosing_block);
-    }
-    visitor.var_used_after_while_let
-}
-
-struct VarUsedAfterLoopVisitor {
-    def_id: HirId,
-    iter_expr_id: HirId,
-    past_while_let: bool,
-    var_used_after_while_let: bool,
-}
-
-impl<'tcx> Visitor<'tcx> for VarUsedAfterLoopVisitor {
-    type Map = Map<'tcx>;
-
-    fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
-        if self.past_while_let {
-            if path_to_local_id(expr, self.def_id) {
-                self.var_used_after_while_let = true;
-            }
-        } else if self.iter_expr_id == expr.hir_id {
-            self.past_while_let = true;
-        }
-        walk_expr(self, expr);
-    }
-    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
-        NestedVisitorMap::None
-    }
-}
-
-/// Returns `true` if the type of expr is one that provides `IntoIterator` impls
-/// for `&T` and `&mut T`, such as `Vec`.
-#[rustfmt::skip]
-fn is_ref_iterable_type(cx: &LateContext<'_>, e: &Expr<'_>) -> bool {
-    // no walk_ptrs_ty: calling iter() on a reference can make sense because it
-    // will allow further borrows afterwards
-    let ty = cx.typeck_results().expr_ty(e);
-    is_iterable_array(ty, cx) ||
-    is_type_diagnostic_item(cx, ty, sym::vec_type) ||
-    match_type(cx, ty, &paths::LINKED_LIST) ||
-    is_type_diagnostic_item(cx, ty, sym!(hashmap_type)) ||
-    is_type_diagnostic_item(cx, ty, sym!(hashset_type)) ||
-    is_type_diagnostic_item(cx, ty, sym!(vecdeque_type)) ||
-    match_type(cx, ty, &paths::BINARY_HEAP) ||
-    match_type(cx, ty, &paths::BTREEMAP) ||
-    match_type(cx, ty, &paths::BTREESET)
-}
-
-fn is_iterable_array<'tcx>(ty: Ty<'tcx>, cx: &LateContext<'tcx>) -> bool {
-    // IntoIterator is currently only implemented for array sizes <= 32 in rustc
-    match ty.kind() {
-        ty::Array(_, n) => n
-            .try_eval_usize(cx.tcx, cx.param_env)
-            .map_or(false, |val| (0..=32).contains(&val)),
-        _ => false,
-    }
-}
-
-/// If a block begins with a statement (possibly a `let` binding) and has an
-/// expression, return it.
-fn extract_expr_from_first_stmt<'tcx>(block: &Block<'tcx>) -> Option<&'tcx Expr<'tcx>> {
-    if block.stmts.is_empty() {
-        return None;
-    }
-    if let StmtKind::Local(ref local) = block.stmts[0].kind {
-        local.init //.map(|expr| expr)
-    } else {
-        None
-    }
-}
-
-/// If a block begins with an expression (with or without semicolon), return it.
-fn extract_first_expr<'tcx>(block: &Block<'tcx>) -> Option<&'tcx Expr<'tcx>> {
-    match block.expr {
-        Some(ref expr) if block.stmts.is_empty() => Some(expr),
-        None if !block.stmts.is_empty() => match block.stmts[0].kind {
-            StmtKind::Expr(ref expr) | StmtKind::Semi(ref expr) => Some(expr),
-            StmtKind::Local(..) | StmtKind::Item(..) => None,
-        },
-        _ => None,
-    }
-}
-
-/// Returns `true` if expr contains a single break expr without destination label
-/// and
-/// passed expression. The expression may be within a block.
-fn is_simple_break_expr(expr: &Expr<'_>) -> bool {
-    match expr.kind {
-        ExprKind::Break(dest, ref passed_expr) if dest.label.is_none() && passed_expr.is_none() => true,
-        ExprKind::Block(ref b, _) => extract_first_expr(b).map_or(false, |subexpr| is_simple_break_expr(subexpr)),
-        _ => false,
-    }
-}
-
-#[derive(Debug, PartialEq)]
-enum IncrementVisitorVarState {
-    Initial,  // Not examined yet
-    IncrOnce, // Incremented exactly once, may be a loop counter
-    DontWarn,
-}
-
-/// Scan a for loop for variables that are incremented exactly once and not used after that.
-struct IncrementVisitor<'a, 'tcx> {
-    cx: &'a LateContext<'tcx>,                          // context reference
-    states: FxHashMap<HirId, IncrementVisitorVarState>, // incremented variables
-    depth: u32,                                         // depth of conditional expressions
-    done: bool,
-}
-
-impl<'a, 'tcx> IncrementVisitor<'a, 'tcx> {
-    fn new(cx: &'a LateContext<'tcx>) -> Self {
-        Self {
-            cx,
-            states: FxHashMap::default(),
-            depth: 0,
-            done: false,
-        }
-    }
-
-    fn into_results(self) -> impl Iterator<Item = HirId> {
-        self.states.into_iter().filter_map(|(id, state)| {
-            if state == IncrementVisitorVarState::IncrOnce {
-                Some(id)
-            } else {
-                None
-            }
-        })
-    }
-}
-
-impl<'a, 'tcx> Visitor<'tcx> for IncrementVisitor<'a, 'tcx> {
-    type Map = Map<'tcx>;
-
-    fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
-        if self.done {
-            return;
-        }
-
-        // If node is a variable
-        if let Some(def_id) = path_to_local(expr) {
-            if let Some(parent) = get_parent_expr(self.cx, expr) {
-                let state = self.states.entry(def_id).or_insert(IncrementVisitorVarState::Initial);
-                if *state == IncrementVisitorVarState::IncrOnce {
-                    *state = IncrementVisitorVarState::DontWarn;
-                    return;
-                }
-
-                match parent.kind {
-                    ExprKind::AssignOp(op, ref lhs, ref rhs) => {
-                        if lhs.hir_id == expr.hir_id {
-                            *state = if op.node == BinOpKind::Add
-                                && is_integer_const(self.cx, rhs, 1)
-                                && *state == IncrementVisitorVarState::Initial
-                                && self.depth == 0
-                            {
-                                IncrementVisitorVarState::IncrOnce
-                            } else {
-                                // Assigned some other value or assigned multiple times
-                                IncrementVisitorVarState::DontWarn
-                            };
-                        }
-                    },
-                    ExprKind::Assign(ref lhs, _, _) if lhs.hir_id == expr.hir_id => {
-                        *state = IncrementVisitorVarState::DontWarn
-                    },
-                    ExprKind::AddrOf(BorrowKind::Ref, mutability, _) if mutability == Mutability::Mut => {
-                        *state = IncrementVisitorVarState::DontWarn
-                    },
-                    _ => (),
-                }
-            }
-
-            walk_expr(self, expr);
-        } else if is_loop(expr) || is_conditional(expr) {
-            self.depth += 1;
-            walk_expr(self, expr);
-            self.depth -= 1;
-        } else if let ExprKind::Continue(_) = expr.kind {
-            self.done = true;
-        } else {
-            walk_expr(self, expr);
-        }
-    }
-    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
-        NestedVisitorMap::None
-    }
-}
-
-enum InitializeVisitorState<'hir> {
-    Initial,          // Not examined yet
-    Declared(Symbol), // Declared but not (yet) initialized
-    Initialized {
-        name: Symbol,
-        initializer: &'hir Expr<'hir>,
-    },
-    DontWarn,
-}
-
-/// Checks whether a variable is initialized at the start of a loop and not modified
-/// and used after the loop.
-struct InitializeVisitor<'a, 'tcx> {
-    cx: &'a LateContext<'tcx>,  // context reference
-    end_expr: &'tcx Expr<'tcx>, // the for loop. Stop scanning here.
-    var_id: HirId,
-    state: InitializeVisitorState<'tcx>,
-    depth: u32, // depth of conditional expressions
-    past_loop: bool,
-}
-
-impl<'a, 'tcx> InitializeVisitor<'a, 'tcx> {
-    fn new(cx: &'a LateContext<'tcx>, end_expr: &'tcx Expr<'tcx>, var_id: HirId) -> Self {
-        Self {
-            cx,
-            end_expr,
-            var_id,
-            state: InitializeVisitorState::Initial,
-            depth: 0,
-            past_loop: false,
-        }
-    }
-
-    fn get_result(&self) -> Option<(Symbol, &'tcx Expr<'tcx>)> {
-        if let InitializeVisitorState::Initialized { name, initializer } = self.state {
-            Some((name, initializer))
-        } else {
-            None
-        }
-    }
-}
-
-impl<'a, 'tcx> Visitor<'tcx> for InitializeVisitor<'a, 'tcx> {
-    type Map = Map<'tcx>;
-
-    fn visit_stmt(&mut self, stmt: &'tcx Stmt<'_>) {
-        // Look for declarations of the variable
-        if_chain! {
-            if let StmtKind::Local(ref local) = stmt.kind;
-            if local.pat.hir_id == self.var_id;
-            if let PatKind::Binding(.., ident, _) = local.pat.kind;
-            then {
-                self.state = local.init.map_or(InitializeVisitorState::Declared(ident.name), |init| {
-                    InitializeVisitorState::Initialized {
-                        initializer: init,
-                        name: ident.name,
-                    }
-                })
-            }
-        }
-        walk_stmt(self, stmt);
-    }
-
-    fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
-        if matches!(self.state, InitializeVisitorState::DontWarn) {
-            return;
-        }
-        if expr.hir_id == self.end_expr.hir_id {
-            self.past_loop = true;
-            return;
-        }
-        // No need to visit expressions before the variable is
-        // declared
-        if matches!(self.state, InitializeVisitorState::Initial) {
-            return;
-        }
-
-        // If node is the desired variable, see how it's used
-        if path_to_local_id(expr, self.var_id) {
-            if self.past_loop {
-                self.state = InitializeVisitorState::DontWarn;
-                return;
-            }
-
-            if let Some(parent) = get_parent_expr(self.cx, expr) {
-                match parent.kind {
-                    ExprKind::AssignOp(_, ref lhs, _) if lhs.hir_id == expr.hir_id => {
-                        self.state = InitializeVisitorState::DontWarn;
-                    },
-                    ExprKind::Assign(ref lhs, ref rhs, _) if lhs.hir_id == expr.hir_id => {
-                        self.state = if_chain! {
-                            if self.depth == 0;
-                            if let InitializeVisitorState::Declared(name)
-                                | InitializeVisitorState::Initialized { name, ..} = self.state;
-                            then {
-                                InitializeVisitorState::Initialized { initializer: rhs, name }
-                            } else {
-                                InitializeVisitorState::DontWarn
-                            }
-                        }
-                    },
-                    ExprKind::AddrOf(BorrowKind::Ref, mutability, _) if mutability == Mutability::Mut => {
-                        self.state = InitializeVisitorState::DontWarn
-                    },
-                    _ => (),
-                }
-            }
-
-            walk_expr(self, expr);
-        } else if !self.past_loop && is_loop(expr) {
-            self.state = InitializeVisitorState::DontWarn;
-        } else if is_conditional(expr) {
-            self.depth += 1;
-            walk_expr(self, expr);
-            self.depth -= 1;
-        } else {
-            walk_expr(self, expr);
-        }
-    }
-
-    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
-        NestedVisitorMap::OnlyBodies(self.cx.tcx.hir())
-    }
-}
-
-fn is_loop(expr: &Expr<'_>) -> bool {
-    matches!(expr.kind, ExprKind::Loop(..))
-}
-
-fn is_conditional(expr: &Expr<'_>) -> bool {
-    matches!(expr.kind, ExprKind::If(..) | ExprKind::Match(..))
-}
-
-fn is_nested(cx: &LateContext<'_>, match_expr: &Expr<'_>, iter_expr: &Expr<'_>) -> bool {
-    if_chain! {
-        if let Some(loop_block) = get_enclosing_block(cx, match_expr.hir_id);
-        let parent_node = cx.tcx.hir().get_parent_node(loop_block.hir_id);
-        if let Some(Node::Expr(loop_expr)) = cx.tcx.hir().find(parent_node);
-        then {
-            return is_loop_nested(cx, loop_expr, iter_expr)
-        }
-    }
-    false
-}
-
-fn is_loop_nested(cx: &LateContext<'_>, loop_expr: &Expr<'_>, iter_expr: &Expr<'_>) -> bool {
-    let mut id = loop_expr.hir_id;
-    let iter_id = if let Some(id) = path_to_local(iter_expr) {
-        id
-    } else {
-        return true;
-    };
-    loop {
-        let parent = cx.tcx.hir().get_parent_node(id);
-        if parent == id {
-            return false;
-        }
-        match cx.tcx.hir().find(parent) {
-            Some(Node::Expr(expr)) => {
-                if let ExprKind::Loop(..) = expr.kind {
-                    return true;
-                };
-            },
-            Some(Node::Block(block)) => {
-                let mut block_visitor = LoopNestVisitor {
-                    hir_id: id,
-                    iterator: iter_id,
-                    nesting: Unknown,
-                };
-                walk_block(&mut block_visitor, block);
-                if block_visitor.nesting == RuledOut {
-                    return false;
-                }
-            },
-            Some(Node::Stmt(_)) => (),
-            _ => {
-                return false;
-            },
-        }
-        id = parent;
-    }
-}
-
-#[derive(PartialEq, Eq)]
-enum Nesting {
-    Unknown,     // no nesting detected yet
-    RuledOut,    // the iterator is initialized or assigned within scope
-    LookFurther, // no nesting detected, no further walk required
-}
-
-use self::Nesting::{LookFurther, RuledOut, Unknown};
-
-struct LoopNestVisitor {
-    hir_id: HirId,
-    iterator: HirId,
-    nesting: Nesting,
-}
-
-impl<'tcx> Visitor<'tcx> for LoopNestVisitor {
-    type Map = Map<'tcx>;
-
-    fn visit_stmt(&mut self, stmt: &'tcx Stmt<'_>) {
-        if stmt.hir_id == self.hir_id {
-            self.nesting = LookFurther;
-        } else if self.nesting == Unknown {
-            walk_stmt(self, stmt);
-        }
-    }
-
-    fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
-        if self.nesting != Unknown {
-            return;
-        }
-        if expr.hir_id == self.hir_id {
-            self.nesting = LookFurther;
-            return;
-        }
-        match expr.kind {
-            ExprKind::Assign(ref path, _, _) | ExprKind::AssignOp(_, ref path, _) => {
-                if path_to_local_id(path, self.iterator) {
-                    self.nesting = RuledOut;
-                }
-            },
-            _ => walk_expr(self, expr),
-        }
-    }
-
-    fn visit_pat(&mut self, pat: &'tcx Pat<'_>) {
-        if self.nesting != Unknown {
-            return;
-        }
-        if let PatKind::Binding(_, id, ..) = pat.kind {
-            if id == self.iterator {
-                self.nesting = RuledOut;
-                return;
-            }
-        }
-        walk_pat(self, pat)
-    }
-
-    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
-        NestedVisitorMap::None
-    }
-}
-
-fn check_infinite_loop<'tcx>(cx: &LateContext<'tcx>, cond: &'tcx Expr<'_>, expr: &'tcx Expr<'_>) {
-    if constant(cx, cx.typeck_results(), cond).is_some() {
-        // A pure constant condition (e.g., `while false`) is not linted.
-        return;
-    }
-
-    let mut var_visitor = VarCollectorVisitor {
-        cx,
-        ids: FxHashSet::default(),
-        def_ids: FxHashMap::default(),
-        skip: false,
-    };
-    var_visitor.visit_expr(cond);
-    if var_visitor.skip {
-        return;
-    }
-    let used_in_condition = &var_visitor.ids;
-    let no_cond_variable_mutated = if let Some(used_mutably) = mutated_variables(expr, cx) {
-        used_in_condition.is_disjoint(&used_mutably)
-    } else {
-        return;
-    };
-    let mutable_static_in_cond = var_visitor.def_ids.iter().any(|(_, v)| *v);
-
-    let mut has_break_or_return_visitor = HasBreakOrReturnVisitor {
-        has_break_or_return: false,
-    };
-    has_break_or_return_visitor.visit_expr(expr);
-    let has_break_or_return = has_break_or_return_visitor.has_break_or_return;
-
-    if no_cond_variable_mutated && !mutable_static_in_cond {
-        span_lint_and_then(
-            cx,
-            WHILE_IMMUTABLE_CONDITION,
-            cond.span,
-            "variables in the condition are not mutated in the loop body",
-            |diag| {
-                diag.note("this may lead to an infinite or to a never running loop");
-
-                if has_break_or_return {
-                    diag.note("this loop contains `return`s or `break`s");
-                    diag.help("rewrite it as `if cond { loop { } }`");
-                }
-            },
-        );
-    }
-}
-
-struct HasBreakOrReturnVisitor {
-    has_break_or_return: bool,
-}
-
-impl<'tcx> Visitor<'tcx> for HasBreakOrReturnVisitor {
-    type Map = Map<'tcx>;
-
-    fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
-        if self.has_break_or_return {
-            return;
-        }
-
-        match expr.kind {
-            ExprKind::Ret(_) | ExprKind::Break(_, _) => {
-                self.has_break_or_return = true;
-                return;
-            },
-            _ => {},
-        }
-
-        walk_expr(self, expr);
-    }
-
-    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
-        NestedVisitorMap::None
-    }
-}
-
-/// Collects the set of variables in an expression
-/// Stops analysis if a function call is found
-/// Note: In some cases such as `self`, there are no mutable annotation,
-/// All variables definition IDs are collected
-struct VarCollectorVisitor<'a, 'tcx> {
-    cx: &'a LateContext<'tcx>,
-    ids: FxHashSet<HirId>,
-    def_ids: FxHashMap<def_id::DefId, bool>,
-    skip: bool,
-}
-
-impl<'a, 'tcx> VarCollectorVisitor<'a, 'tcx> {
-    fn insert_def_id(&mut self, ex: &'tcx Expr<'_>) {
-        if_chain! {
-            if let ExprKind::Path(ref qpath) = ex.kind;
-            if let QPath::Resolved(None, _) = *qpath;
-            let res = self.cx.qpath_res(qpath, ex.hir_id);
-            then {
-                match res {
-                    Res::Local(hir_id) => {
-                        self.ids.insert(hir_id);
-                    },
-                    Res::Def(DefKind::Static, def_id) => {
-                        let mutable = self.cx.tcx.is_mutable_static(def_id);
-                        self.def_ids.insert(def_id, mutable);
-                    },
-                    _ => {},
-                }
-            }
-        }
-    }
-}
-
-impl<'a, 'tcx> Visitor<'tcx> for VarCollectorVisitor<'a, 'tcx> {
-    type Map = Map<'tcx>;
-
-    fn visit_expr(&mut self, ex: &'tcx Expr<'_>) {
-        match ex.kind {
-            ExprKind::Path(_) => self.insert_def_id(ex),
-            // If there is any function/method call… we just stop analysis
-            ExprKind::Call(..) | ExprKind::MethodCall(..) => self.skip = true,
-
-            _ => walk_expr(self, ex),
-        }
-    }
-
-    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
-        NestedVisitorMap::None
-    }
-}
-
-const NEEDLESS_COLLECT_MSG: &str = "avoid using `collect()` when not needed";
-
-fn check_needless_collect<'tcx>(expr: &'tcx Expr<'_>, cx: &LateContext<'tcx>) {
-    check_needless_collect_direct_usage(expr, cx);
-    check_needless_collect_indirect_usage(expr, cx);
-}
-fn check_needless_collect_direct_usage<'tcx>(expr: &'tcx Expr<'_>, cx: &LateContext<'tcx>) {
-    if_chain! {
-        if let ExprKind::MethodCall(ref method, _, ref args, _) = expr.kind;
-        if let ExprKind::MethodCall(ref chain_method, _, _, _) = args[0].kind;
-        if chain_method.ident.name == sym!(collect) && match_trait_method(cx, &args[0], &paths::ITERATOR);
-        if let Some(ref generic_args) = chain_method.args;
-        if let Some(GenericArg::Type(ref ty)) = generic_args.args.get(0);
-        then {
-            let ty = cx.typeck_results().node_type(ty.hir_id);
-            if is_type_diagnostic_item(cx, ty, sym::vec_type) ||
-                is_type_diagnostic_item(cx, ty, sym!(vecdeque_type)) ||
-                match_type(cx, ty, &paths::BTREEMAP) ||
-                is_type_diagnostic_item(cx, ty, sym!(hashmap_type)) {
-                if method.ident.name == sym!(len) {
-                    let span = shorten_needless_collect_span(expr);
-                    span_lint_and_sugg(
-                        cx,
-                        NEEDLESS_COLLECT,
-                        span,
-                        NEEDLESS_COLLECT_MSG,
-                        "replace with",
-                        "count()".to_string(),
-                        Applicability::MachineApplicable,
-                    );
-                }
-                if method.ident.name == sym!(is_empty) {
-                    let span = shorten_needless_collect_span(expr);
-                    span_lint_and_sugg(
-                        cx,
-                        NEEDLESS_COLLECT,
-                        span,
-                        NEEDLESS_COLLECT_MSG,
-                        "replace with",
-                        "next().is_none()".to_string(),
-                        Applicability::MachineApplicable,
-                    );
-                }
-                if method.ident.name == sym!(contains) {
-                    let contains_arg = snippet(cx, args[1].span, "??");
-                    let span = shorten_needless_collect_span(expr);
-                    span_lint_and_then(
-                        cx,
-                        NEEDLESS_COLLECT,
-                        span,
-                        NEEDLESS_COLLECT_MSG,
-                        |diag| {
-                            let (arg, pred) = contains_arg
-                                    .strip_prefix('&')
-                                    .map_or(("&x", &*contains_arg), |s| ("x", s));
-                            diag.span_suggestion(
-                                span,
-                                "replace with",
-                                format!(
-                                    "any(|{}| x == {})",
-                                    arg, pred
-                                ),
-                                Applicability::MachineApplicable,
-                            );
-                        }
-                    );
-                }
-            }
-        }
-    }
-}
-
-fn check_needless_collect_indirect_usage<'tcx>(expr: &'tcx Expr<'_>, cx: &LateContext<'tcx>) {
-    if let ExprKind::Block(ref block, _) = expr.kind {
-        for ref stmt in block.stmts {
-            if_chain! {
-                if let StmtKind::Local(
-                    Local { pat: Pat { hir_id: pat_id, kind: PatKind::Binding(_, _, ident, .. ), .. },
-                    init: Some(ref init_expr), .. }
-                ) = stmt.kind;
-                if let ExprKind::MethodCall(ref method_name, _, &[ref iter_source], ..) = init_expr.kind;
-                if method_name.ident.name == sym!(collect) && match_trait_method(cx, &init_expr, &paths::ITERATOR);
-                if let Some(ref generic_args) = method_name.args;
-                if let Some(GenericArg::Type(ref ty)) = generic_args.args.get(0);
-                if let ty = cx.typeck_results().node_type(ty.hir_id);
-                if is_type_diagnostic_item(cx, ty, sym::vec_type) ||
-                    is_type_diagnostic_item(cx, ty, sym!(vecdeque_type)) ||
-                    match_type(cx, ty, &paths::LINKED_LIST);
-                if let Some(iter_calls) = detect_iter_and_into_iters(block, *ident);
-                if iter_calls.len() == 1;
-                then {
-                    let mut used_count_visitor = UsedCountVisitor {
-                        cx,
-                        id: *pat_id,
-                        count: 0,
-                    };
-                    walk_block(&mut used_count_visitor, block);
-                    if used_count_visitor.count > 1 {
-                        return;
-                    }
-
-                    // Suggest replacing iter_call with iter_replacement, and removing stmt
-                    let iter_call = &iter_calls[0];
-                    span_lint_and_then(
-                        cx,
-                        NEEDLESS_COLLECT,
-                        stmt.span.until(iter_call.span),
-                        NEEDLESS_COLLECT_MSG,
-                        |diag| {
-                            let iter_replacement = format!("{}{}", Sugg::hir(cx, iter_source, ".."), iter_call.get_iter_method(cx));
-                            diag.multipart_suggestion(
-                                iter_call.get_suggestion_text(),
-                                vec![
-                                    (stmt.span, String::new()),
-                                    (iter_call.span, iter_replacement)
-                                ],
-                                Applicability::MachineApplicable,// MaybeIncorrect,
-                            ).emit();
-                        },
-                    );
-                }
-            }
-        }
-    }
-}
-
-struct IterFunction {
-    func: IterFunctionKind,
-    span: Span,
-}
-impl IterFunction {
-    fn get_iter_method(&self, cx: &LateContext<'_>) -> String {
-        match &self.func {
-            IterFunctionKind::IntoIter => String::new(),
-            IterFunctionKind::Len => String::from(".count()"),
-            IterFunctionKind::IsEmpty => String::from(".next().is_none()"),
-            IterFunctionKind::Contains(span) => {
-                let s = snippet(cx, *span, "..");
-                if let Some(stripped) = s.strip_prefix('&') {
-                    format!(".any(|x| x == {})", stripped)
-                } else {
-                    format!(".any(|x| x == *{})", s)
-                }
-            },
-        }
-    }
-    fn get_suggestion_text(&self) -> &'static str {
-        match &self.func {
-            IterFunctionKind::IntoIter => {
-                "use the original Iterator instead of collecting it and then producing a new one"
-            },
-            IterFunctionKind::Len => {
-                "take the original Iterator's count instead of collecting it and finding the length"
-            },
-            IterFunctionKind::IsEmpty => {
-                "check if the original Iterator has anything instead of collecting it and seeing if it's empty"
-            },
-            IterFunctionKind::Contains(_) => {
-                "check if the original Iterator contains an element instead of collecting then checking"
-            },
-        }
-    }
-}
-enum IterFunctionKind {
-    IntoIter,
-    Len,
-    IsEmpty,
-    Contains(Span),
-}
-
-struct IterFunctionVisitor {
-    uses: Vec<IterFunction>,
-    seen_other: bool,
-    target: Ident,
-}
-impl<'tcx> Visitor<'tcx> for IterFunctionVisitor {
-    fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
-        // Check function calls on our collection
-        if_chain! {
-            if let ExprKind::MethodCall(method_name, _, ref args, _) = &expr.kind;
-            if let Some(Expr { kind: ExprKind::Path(QPath::Resolved(_, ref path)), .. }) = args.get(0);
-            if let &[name] = &path.segments;
-            if name.ident == self.target;
-            then {
-                let len = sym!(len);
-                let is_empty = sym!(is_empty);
-                let contains = sym!(contains);
-                match method_name.ident.name {
-                    sym::into_iter => self.uses.push(
-                        IterFunction { func: IterFunctionKind::IntoIter, span: expr.span }
-                    ),
-                    name if name == len => self.uses.push(
-                        IterFunction { func: IterFunctionKind::Len, span: expr.span }
-                    ),
-                    name if name == is_empty => self.uses.push(
-                        IterFunction { func: IterFunctionKind::IsEmpty, span: expr.span }
-                    ),
-                    name if name == contains => self.uses.push(
-                        IterFunction { func: IterFunctionKind::Contains(args[1].span), span: expr.span }
-                    ),
-                    _ => self.seen_other = true,
-                }
-                return
-            }
-        }
-        // Check if the collection is used for anything else
-        if_chain! {
-            if let Expr { kind: ExprKind::Path(QPath::Resolved(_, ref path)), .. } = expr;
-            if let &[name] = &path.segments;
-            if name.ident == self.target;
-            then {
-                self.seen_other = true;
-            } else {
-                walk_expr(self, expr);
-            }
-        }
-    }
-
-    type Map = Map<'tcx>;
-    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
-        NestedVisitorMap::None
-    }
-}
-
-struct UsedCountVisitor<'a, 'tcx> {
-    cx: &'a LateContext<'tcx>,
-    id: HirId,
-    count: usize,
-}
-
-impl<'a, 'tcx> Visitor<'tcx> for UsedCountVisitor<'a, 'tcx> {
-    type Map = Map<'tcx>;
-
-    fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
-        if path_to_local_id(expr, self.id) {
-            self.count += 1;
-        } else {
-            walk_expr(self, expr);
-        }
-    }
-
-    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
-        NestedVisitorMap::OnlyBodies(self.cx.tcx.hir())
-    }
-}
-
-/// Detect the occurrences of calls to `iter` or `into_iter` for the
-/// given identifier
-fn detect_iter_and_into_iters<'tcx>(block: &'tcx Block<'tcx>, identifier: Ident) -> Option<Vec<IterFunction>> {
-    let mut visitor = IterFunctionVisitor {
-        uses: Vec::new(),
-        target: identifier,
-        seen_other: false,
-    };
-    visitor.visit_block(block);
-    if visitor.seen_other { None } else { Some(visitor.uses) }
-}
-
-fn shorten_needless_collect_span(expr: &Expr<'_>) -> Span {
-    if_chain! {
-        if let ExprKind::MethodCall(.., args, _) = &expr.kind;
-        if let ExprKind::MethodCall(_, span, ..) = &args[0].kind;
-        then {
-            return expr.span.with_lo(span.lo());
-        }
-    }
-    unreachable!();
-}
diff --git a/clippy_lints/src/loops/empty_loop.rs b/clippy_lints/src/loops/empty_loop.rs
new file mode 100644
index 00000000000..67d1ac89b97
--- /dev/null
+++ b/clippy_lints/src/loops/empty_loop.rs
@@ -0,0 +1,17 @@
+use super::EMPTY_LOOP;
+use crate::utils::{is_in_panic_handler, is_no_std_crate, span_lint_and_help};
+
+use rustc_hir::{Block, Expr};
+use rustc_lint::LateContext;
+
+pub(super) fn check(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, loop_block: &'tcx Block<'_>) {
+    if loop_block.stmts.is_empty() && loop_block.expr.is_none() && !is_in_panic_handler(cx, expr) {
+        let msg = "empty `loop {}` wastes CPU cycles";
+        let help = if is_no_std_crate(cx.tcx.hir().krate()) {
+            "you should either use `panic!()` or add a call pausing or sleeping the thread to the loop body"
+        } else {
+            "you should either use `panic!()` or add `std::thread::sleep(..);` to the loop body"
+        };
+        span_lint_and_help(cx, EMPTY_LOOP, expr.span, msg, None, help);
+    }
+}
diff --git a/clippy_lints/src/loops/explicit_counter_loop.rs b/clippy_lints/src/loops/explicit_counter_loop.rs
new file mode 100644
index 00000000000..8d98b940c66
--- /dev/null
+++ b/clippy_lints/src/loops/explicit_counter_loop.rs
@@ -0,0 +1,58 @@
+use super::{
+    get_span_of_entire_for_loop, make_iterator_snippet, IncrementVisitor, InitializeVisitor, EXPLICIT_COUNTER_LOOP,
+};
+use crate::utils::{get_enclosing_block, is_integer_const, snippet_with_applicability, span_lint_and_sugg};
+use if_chain::if_chain;
+use rustc_errors::Applicability;
+use rustc_hir::intravisit::{walk_block, walk_expr};
+use rustc_hir::{Expr, Pat};
+use rustc_lint::LateContext;
+
+// To trigger the EXPLICIT_COUNTER_LOOP lint, a variable must be
+// incremented exactly once in the loop body, and initialized to zero
+// at the start of the loop.
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    pat: &'tcx Pat<'_>,
+    arg: &'tcx Expr<'_>,
+    body: &'tcx Expr<'_>,
+    expr: &'tcx Expr<'_>,
+) {
+    // Look for variables that are incremented once per loop iteration.
+    let mut increment_visitor = IncrementVisitor::new(cx);
+    walk_expr(&mut increment_visitor, body);
+
+    // For each candidate, check the parent block to see if
+    // it's initialized to zero at the start of the loop.
+    if let Some(block) = get_enclosing_block(&cx, expr.hir_id) {
+        for id in increment_visitor.into_results() {
+            let mut initialize_visitor = InitializeVisitor::new(cx, expr, id);
+            walk_block(&mut initialize_visitor, block);
+
+            if_chain! {
+                if let Some((name, initializer)) = initialize_visitor.get_result();
+                if is_integer_const(cx, initializer, 0);
+                then {
+                    let mut applicability = Applicability::MachineApplicable;
+
+                    let for_span = get_span_of_entire_for_loop(expr);
+
+                    span_lint_and_sugg(
+                        cx,
+                        EXPLICIT_COUNTER_LOOP,
+                        for_span.with_hi(arg.span.hi()),
+                        &format!("the variable `{}` is used as a loop counter", name),
+                        "consider using",
+                        format!(
+                            "for ({}, {}) in {}.enumerate()",
+                            name,
+                            snippet_with_applicability(cx, pat.span, "item", &mut applicability),
+                            make_iterator_snippet(cx, arg, &mut applicability),
+                        ),
+                        applicability,
+                    );
+                }
+            }
+        }
+    }
+}
diff --git a/clippy_lints/src/loops/explicit_into_iter_loop.rs b/clippy_lints/src/loops/explicit_into_iter_loop.rs
new file mode 100644
index 00000000000..1d778205a2a
--- /dev/null
+++ b/clippy_lints/src/loops/explicit_into_iter_loop.rs
@@ -0,0 +1,27 @@
+use super::EXPLICIT_INTO_ITER_LOOP;
+use crate::utils::{snippet_with_applicability, span_lint_and_sugg};
+use rustc_errors::Applicability;
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+use rustc_middle::ty::TyS;
+
+pub(super) fn check(cx: &LateContext<'_>, args: &'hir [Expr<'hir>], arg: &Expr<'_>) {
+    let receiver_ty = cx.typeck_results().expr_ty(&args[0]);
+    let receiver_ty_adjusted = cx.typeck_results().expr_ty_adjusted(&args[0]);
+    if !TyS::same_type(receiver_ty, receiver_ty_adjusted) {
+        return;
+    }
+
+    let mut applicability = Applicability::MachineApplicable;
+    let object = snippet_with_applicability(cx, args[0].span, "_", &mut applicability);
+    span_lint_and_sugg(
+        cx,
+        EXPLICIT_INTO_ITER_LOOP,
+        arg.span,
+        "it is more concise to loop over containers instead of using explicit \
+            iteration methods",
+        "to write this more concisely, try",
+        object.to_string(),
+        applicability,
+    );
+}
diff --git a/clippy_lints/src/loops/explicit_iter_loop.rs b/clippy_lints/src/loops/explicit_iter_loop.rs
new file mode 100644
index 00000000000..44d08916891
--- /dev/null
+++ b/clippy_lints/src/loops/explicit_iter_loop.rs
@@ -0,0 +1,74 @@
+use super::EXPLICIT_ITER_LOOP;
+use crate::utils::{match_trait_method, snippet_with_applicability, span_lint_and_sugg};
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, Mutability};
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, Ty, TyS};
+use rustc_span::symbol::sym;
+
+use crate::utils::{is_type_diagnostic_item, match_type, paths};
+
+pub(super) fn check(cx: &LateContext<'_>, args: &[Expr<'_>], arg: &Expr<'_>, method_name: &str) {
+    let should_lint = match method_name {
+        "iter" | "iter_mut" => is_ref_iterable_type(cx, &args[0]),
+        "into_iter" if match_trait_method(cx, arg, &paths::INTO_ITERATOR) => {
+            let receiver_ty = cx.typeck_results().expr_ty(&args[0]);
+            let receiver_ty_adjusted = cx.typeck_results().expr_ty_adjusted(&args[0]);
+            let ref_receiver_ty = cx.tcx.mk_ref(
+                cx.tcx.lifetimes.re_erased,
+                ty::TypeAndMut {
+                    ty: receiver_ty,
+                    mutbl: Mutability::Not,
+                },
+            );
+            TyS::same_type(receiver_ty_adjusted, ref_receiver_ty)
+        },
+        _ => false,
+    };
+
+    if !should_lint {
+        return;
+    }
+
+    let mut applicability = Applicability::MachineApplicable;
+    let object = snippet_with_applicability(cx, args[0].span, "_", &mut applicability);
+    let muta = if method_name == "iter_mut" { "mut " } else { "" };
+    span_lint_and_sugg(
+        cx,
+        EXPLICIT_ITER_LOOP,
+        arg.span,
+        "it is more concise to loop over references to containers instead of using explicit \
+         iteration methods",
+        "to write this more concisely, try",
+        format!("&{}{}", muta, object),
+        applicability,
+    )
+}
+
+/// Returns `true` if the type of expr is one that provides `IntoIterator` impls
+/// for `&T` and `&mut T`, such as `Vec`.
+#[rustfmt::skip]
+fn is_ref_iterable_type(cx: &LateContext<'_>, e: &Expr<'_>) -> bool {
+    // no walk_ptrs_ty: calling iter() on a reference can make sense because it
+    // will allow further borrows afterwards
+    let ty = cx.typeck_results().expr_ty(e);
+    is_iterable_array(ty, cx) ||
+    is_type_diagnostic_item(cx, ty, sym::vec_type) ||
+    match_type(cx, ty, &paths::LINKED_LIST) ||
+    is_type_diagnostic_item(cx, ty, sym!(hashmap_type)) ||
+    is_type_diagnostic_item(cx, ty, sym!(hashset_type)) ||
+    is_type_diagnostic_item(cx, ty, sym!(vecdeque_type)) ||
+    match_type(cx, ty, &paths::BINARY_HEAP) ||
+    match_type(cx, ty, &paths::BTREEMAP) ||
+    match_type(cx, ty, &paths::BTREESET)
+}
+
+fn is_iterable_array<'tcx>(ty: Ty<'tcx>, cx: &LateContext<'tcx>) -> bool {
+    // IntoIterator is currently only implemented for array sizes <= 32 in rustc
+    match ty.kind() {
+        ty::Array(_, n) => n
+            .try_eval_usize(cx.tcx, cx.param_env)
+            .map_or(false, |val| (0..=32).contains(&val)),
+        _ => false,
+    }
+}
diff --git a/clippy_lints/src/loops/for_kv_map.rs b/clippy_lints/src/loops/for_kv_map.rs
new file mode 100644
index 00000000000..eb53c3179ca
--- /dev/null
+++ b/clippy_lints/src/loops/for_kv_map.rs
@@ -0,0 +1,70 @@
+use super::FOR_KV_MAP;
+use crate::utils::visitors::LocalUsedVisitor;
+use crate::utils::{is_type_diagnostic_item, match_type, multispan_sugg, paths, snippet, span_lint_and_then, sugg};
+use rustc_hir::{BorrowKind, Expr, ExprKind, Mutability, Pat, PatKind};
+use rustc_lint::LateContext;
+use rustc_middle::ty;
+
+/// Checks for the `FOR_KV_MAP` lint.
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    pat: &'tcx Pat<'_>,
+    arg: &'tcx Expr<'_>,
+    body: &'tcx Expr<'_>,
+    expr: &'tcx Expr<'_>,
+) {
+    let pat_span = pat.span;
+
+    if let PatKind::Tuple(ref pat, _) = pat.kind {
+        if pat.len() == 2 {
+            let arg_span = arg.span;
+            let (new_pat_span, kind, ty, mutbl) = match *cx.typeck_results().expr_ty(arg).kind() {
+                ty::Ref(_, ty, mutbl) => match (&pat[0].kind, &pat[1].kind) {
+                    (key, _) if pat_is_wild(cx, key, body) => (pat[1].span, "value", ty, mutbl),
+                    (_, value) if pat_is_wild(cx, value, body) => (pat[0].span, "key", ty, Mutability::Not),
+                    _ => return,
+                },
+                _ => return,
+            };
+            let mutbl = match mutbl {
+                Mutability::Not => "",
+                Mutability::Mut => "_mut",
+            };
+            let arg = match arg.kind {
+                ExprKind::AddrOf(BorrowKind::Ref, _, ref expr) => &**expr,
+                _ => arg,
+            };
+
+            if is_type_diagnostic_item(cx, ty, sym!(hashmap_type)) || match_type(cx, ty, &paths::BTREEMAP) {
+                span_lint_and_then(
+                    cx,
+                    FOR_KV_MAP,
+                    expr.span,
+                    &format!("you seem to want to iterate on a map's {}s", kind),
+                    |diag| {
+                        let map = sugg::Sugg::hir(cx, arg, "map");
+                        multispan_sugg(
+                            diag,
+                            "use the corresponding method",
+                            vec![
+                                (pat_span, snippet(cx, new_pat_span, kind).into_owned()),
+                                (arg_span, format!("{}.{}s{}()", map.maybe_par(), kind, mutbl)),
+                            ],
+                        );
+                    },
+                );
+            }
+        }
+    }
+}
+
+/// Returns `true` if the pattern is a `PatWild` or an ident prefixed with `_`.
+fn pat_is_wild<'tcx>(cx: &LateContext<'tcx>, pat: &'tcx PatKind<'_>, body: &'tcx Expr<'_>) -> bool {
+    match *pat {
+        PatKind::Wild => true,
+        PatKind::Binding(_, id, ident, None) if ident.as_str().starts_with('_') => {
+            !LocalUsedVisitor::new(cx, id).check_expr(body)
+        },
+        _ => false,
+    }
+}
diff --git a/clippy_lints/src/loops/for_loops_over_fallibles.rs b/clippy_lints/src/loops/for_loops_over_fallibles.rs
new file mode 100644
index 00000000000..db22d90a304
--- /dev/null
+++ b/clippy_lints/src/loops/for_loops_over_fallibles.rs
@@ -0,0 +1,45 @@
+use super::FOR_LOOPS_OVER_FALLIBLES;
+use crate::utils::{is_type_diagnostic_item, snippet, span_lint_and_help};
+use rustc_hir::{Expr, Pat};
+use rustc_lint::LateContext;
+use rustc_span::symbol::sym;
+
+/// Checks for `for` loops over `Option`s and `Result`s.
+pub(super) fn check(cx: &LateContext<'_>, pat: &Pat<'_>, arg: &Expr<'_>) {
+    let ty = cx.typeck_results().expr_ty(arg);
+    if is_type_diagnostic_item(cx, ty, sym::option_type) {
+        span_lint_and_help(
+            cx,
+            FOR_LOOPS_OVER_FALLIBLES,
+            arg.span,
+            &format!(
+                "for loop over `{0}`, which is an `Option`. This is more readably written as an \
+                `if let` statement",
+                snippet(cx, arg.span, "_")
+            ),
+            None,
+            &format!(
+                "consider replacing `for {0} in {1}` with `if let Some({0}) = {1}`",
+                snippet(cx, pat.span, "_"),
+                snippet(cx, arg.span, "_")
+            ),
+        );
+    } else if is_type_diagnostic_item(cx, ty, sym::result_type) {
+        span_lint_and_help(
+            cx,
+            FOR_LOOPS_OVER_FALLIBLES,
+            arg.span,
+            &format!(
+                "for loop over `{0}`, which is a `Result`. This is more readably written as an \
+                `if let` statement",
+                snippet(cx, arg.span, "_")
+            ),
+            None,
+            &format!(
+                "consider replacing `for {0} in {1}` with `if let Ok({0}) = {1}`",
+                snippet(cx, pat.span, "_"),
+                snippet(cx, arg.span, "_")
+            ),
+        );
+    }
+}
diff --git a/clippy_lints/src/loops/iter_next_loop.rs b/clippy_lints/src/loops/iter_next_loop.rs
new file mode 100644
index 00000000000..cf78bbc49a3
--- /dev/null
+++ b/clippy_lints/src/loops/iter_next_loop.rs
@@ -0,0 +1,19 @@
+use super::ITER_NEXT_LOOP;
+use crate::utils::{match_trait_method, paths, span_lint};
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+
+pub(super) fn check(cx: &LateContext<'_>, arg: &Expr<'_>, expr: &Expr<'_>) -> bool {
+    if match_trait_method(cx, arg, &paths::ITERATOR) {
+        span_lint(
+            cx,
+            ITER_NEXT_LOOP,
+            expr.span,
+            "you are iterating over `Iterator::next()` which is an Option; this will compile but is \
+            probably not what you want",
+        );
+        true
+    } else {
+        false
+    }
+}
diff --git a/clippy_lints/src/loops/manual_flatten.rs b/clippy_lints/src/loops/manual_flatten.rs
new file mode 100644
index 00000000000..3d3ae6f3152
--- /dev/null
+++ b/clippy_lints/src/loops/manual_flatten.rs
@@ -0,0 +1,79 @@
+use super::utils::make_iterator_snippet;
+use super::MANUAL_FLATTEN;
+use crate::utils::{is_ok_ctor, is_some_ctor, path_to_local_id, span_lint_and_then};
+use if_chain::if_chain;
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, ExprKind, MatchSource, Pat, PatKind, QPath, StmtKind};
+use rustc_lint::LateContext;
+use rustc_span::source_map::Span;
+
+/// Check for unnecessary `if let` usage in a for loop where only the `Some` or `Ok` variant of the
+/// iterator element is used.
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    pat: &'tcx Pat<'_>,
+    arg: &'tcx Expr<'_>,
+    body: &'tcx Expr<'_>,
+    span: Span,
+) {
+    if let ExprKind::Block(ref block, _) = body.kind {
+        // Ensure the `if let` statement is the only expression or statement in the for-loop
+        let inner_expr = if block.stmts.len() == 1 && block.expr.is_none() {
+            let match_stmt = &block.stmts[0];
+            if let StmtKind::Semi(inner_expr) = match_stmt.kind {
+                Some(inner_expr)
+            } else {
+                None
+            }
+        } else if block.stmts.is_empty() {
+            block.expr
+        } else {
+            None
+        };
+
+        if_chain! {
+            if let Some(inner_expr) = inner_expr;
+            if let ExprKind::Match(
+                ref match_expr, ref match_arms, MatchSource::IfLetDesugar{ contains_else_clause: false }
+            ) = inner_expr.kind;
+            // Ensure match_expr in `if let` statement is the same as the pat from the for-loop
+            if let PatKind::Binding(_, pat_hir_id, _, _) = pat.kind;
+            if path_to_local_id(match_expr, pat_hir_id);
+            // Ensure the `if let` statement is for the `Some` variant of `Option` or the `Ok` variant of `Result`
+            if let PatKind::TupleStruct(QPath::Resolved(None, path), _, _) = match_arms[0].pat.kind;
+            let some_ctor = is_some_ctor(cx, path.res);
+            let ok_ctor = is_ok_ctor(cx, path.res);
+            if some_ctor || ok_ctor;
+            let if_let_type = if some_ctor { "Some" } else { "Ok" };
+
+            then {
+                // Prepare the error message
+                let msg = format!("unnecessary `if let` since only the `{}` variant of the iterator element is used", if_let_type);
+
+                // Prepare the help message
+                let mut applicability = Applicability::MaybeIncorrect;
+                let arg_snippet = make_iterator_snippet(cx, arg, &mut applicability);
+
+                span_lint_and_then(
+                    cx,
+                    MANUAL_FLATTEN,
+                    span,
+                    &msg,
+                    |diag| {
+                        let sugg = format!("{}.flatten()", arg_snippet);
+                        diag.span_suggestion(
+                            arg.span,
+                            "try",
+                            sugg,
+                            Applicability::MaybeIncorrect,
+                        );
+                        diag.span_help(
+                            inner_expr.span,
+                            "...and remove the `if let` statement in the for loop",
+                        );
+                    }
+                );
+            }
+        }
+    }
+}
diff --git a/clippy_lints/src/loops/manual_memcpy.rs b/clippy_lints/src/loops/manual_memcpy.rs
new file mode 100644
index 00000000000..bf0b8cc2459
--- /dev/null
+++ b/clippy_lints/src/loops/manual_memcpy.rs
@@ -0,0 +1,451 @@
+use super::{get_span_of_entire_for_loop, IncrementVisitor, InitializeVisitor, MANUAL_MEMCPY};
+use crate::utils::sugg::Sugg;
+use crate::utils::{
+    get_enclosing_block, higher, is_type_diagnostic_item, path_to_local, snippet, span_lint_and_sugg, sugg,
+};
+use if_chain::if_chain;
+use rustc_ast::ast;
+use rustc_errors::Applicability;
+use rustc_hir::intravisit::walk_block;
+use rustc_hir::{BinOpKind, Block, Expr, ExprKind, HirId, Pat, PatKind, StmtKind};
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, Ty};
+use rustc_span::symbol::sym;
+use std::iter::Iterator;
+
+/// Checks for for loops that sequentially copy items from one slice-like
+/// object to another.
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    pat: &'tcx Pat<'_>,
+    arg: &'tcx Expr<'_>,
+    body: &'tcx Expr<'_>,
+    expr: &'tcx Expr<'_>,
+) -> bool {
+    if let Some(higher::Range {
+        start: Some(start),
+        end: Some(end),
+        limits,
+    }) = higher::range(arg)
+    {
+        // the var must be a single name
+        if let PatKind::Binding(_, canonical_id, _, _) = pat.kind {
+            let mut starts = vec![Start {
+                id: canonical_id,
+                kind: StartKind::Range,
+            }];
+
+            // This is one of few ways to return different iterators
+            // derived from: https://stackoverflow.com/questions/29760668/conditionally-iterate-over-one-of-several-possible-iterators/52064434#52064434
+            let mut iter_a = None;
+            let mut iter_b = None;
+
+            if let ExprKind::Block(block, _) = body.kind {
+                if let Some(loop_counters) = get_loop_counters(cx, block, expr) {
+                    starts.extend(loop_counters);
+                }
+                iter_a = Some(get_assignments(block, &starts));
+            } else {
+                iter_b = Some(get_assignment(body));
+            }
+
+            let assignments = iter_a.into_iter().flatten().chain(iter_b.into_iter());
+
+            let big_sugg = assignments
+                // The only statements in the for loops can be indexed assignments from
+                // indexed retrievals (except increments of loop counters).
+                .map(|o| {
+                    o.and_then(|(lhs, rhs)| {
+                        let rhs = fetch_cloned_expr(rhs);
+                        if_chain! {
+                            if let ExprKind::Index(base_left, idx_left) = lhs.kind;
+                            if let ExprKind::Index(base_right, idx_right) = rhs.kind;
+                            if is_slice_like(cx, cx.typeck_results().expr_ty(base_left))
+                                && is_slice_like(cx, cx.typeck_results().expr_ty(base_right));
+                            if let Some((start_left, offset_left)) = get_details_from_idx(cx, &idx_left, &starts);
+                            if let Some((start_right, offset_right)) = get_details_from_idx(cx, &idx_right, &starts);
+
+                            // Source and destination must be different
+                            if path_to_local(base_left) != path_to_local(base_right);
+                            then {
+                                Some((IndexExpr { base: base_left, idx: start_left, idx_offset: offset_left },
+                                    IndexExpr { base: base_right, idx: start_right, idx_offset: offset_right }))
+                            } else {
+                                None
+                            }
+                        }
+                    })
+                })
+                .map(|o| o.map(|(dst, src)| build_manual_memcpy_suggestion(cx, start, end, limits, &dst, &src)))
+                .collect::<Option<Vec<_>>>()
+                .filter(|v| !v.is_empty())
+                .map(|v| v.join("\n    "));
+
+            if let Some(big_sugg) = big_sugg {
+                span_lint_and_sugg(
+                    cx,
+                    MANUAL_MEMCPY,
+                    get_span_of_entire_for_loop(expr),
+                    "it looks like you're manually copying between slices",
+                    "try replacing the loop by",
+                    big_sugg,
+                    Applicability::Unspecified,
+                );
+                return true;
+            }
+        }
+    }
+    false
+}
+
+fn build_manual_memcpy_suggestion<'tcx>(
+    cx: &LateContext<'tcx>,
+    start: &Expr<'_>,
+    end: &Expr<'_>,
+    limits: ast::RangeLimits,
+    dst: &IndexExpr<'_>,
+    src: &IndexExpr<'_>,
+) -> String {
+    fn print_offset(offset: MinifyingSugg<'static>) -> MinifyingSugg<'static> {
+        if offset.as_str() == "0" {
+            sugg::EMPTY.into()
+        } else {
+            offset
+        }
+    }
+
+    let print_limit = |end: &Expr<'_>, end_str: &str, base: &Expr<'_>, sugg: MinifyingSugg<'static>| {
+        if_chain! {
+            if let ExprKind::MethodCall(method, _, len_args, _) = end.kind;
+            if method.ident.name == sym!(len);
+            if len_args.len() == 1;
+            if let Some(arg) = len_args.get(0);
+            if path_to_local(arg) == path_to_local(base);
+            then {
+                if sugg.as_str() == end_str {
+                    sugg::EMPTY.into()
+                } else {
+                    sugg
+                }
+            } else {
+                match limits {
+                    ast::RangeLimits::Closed => {
+                        sugg + &sugg::ONE.into()
+                    },
+                    ast::RangeLimits::HalfOpen => sugg,
+                }
+            }
+        }
+    };
+
+    let start_str = Sugg::hir(cx, start, "").into();
+    let end_str: MinifyingSugg<'_> = Sugg::hir(cx, end, "").into();
+
+    let print_offset_and_limit = |idx_expr: &IndexExpr<'_>| match idx_expr.idx {
+        StartKind::Range => (
+            print_offset(apply_offset(&start_str, &idx_expr.idx_offset)).into_sugg(),
+            print_limit(
+                end,
+                end_str.as_str(),
+                idx_expr.base,
+                apply_offset(&end_str, &idx_expr.idx_offset),
+            )
+            .into_sugg(),
+        ),
+        StartKind::Counter { initializer } => {
+            let counter_start = Sugg::hir(cx, initializer, "").into();
+            (
+                print_offset(apply_offset(&counter_start, &idx_expr.idx_offset)).into_sugg(),
+                print_limit(
+                    end,
+                    end_str.as_str(),
+                    idx_expr.base,
+                    apply_offset(&end_str, &idx_expr.idx_offset) + &counter_start - &start_str,
+                )
+                .into_sugg(),
+            )
+        },
+    };
+
+    let (dst_offset, dst_limit) = print_offset_and_limit(&dst);
+    let (src_offset, src_limit) = print_offset_and_limit(&src);
+
+    let dst_base_str = snippet(cx, dst.base.span, "???");
+    let src_base_str = snippet(cx, src.base.span, "???");
+
+    let dst = if dst_offset == sugg::EMPTY && dst_limit == sugg::EMPTY {
+        dst_base_str
+    } else {
+        format!(
+            "{}[{}..{}]",
+            dst_base_str,
+            dst_offset.maybe_par(),
+            dst_limit.maybe_par()
+        )
+        .into()
+    };
+
+    format!(
+        "{}.clone_from_slice(&{}[{}..{}]);",
+        dst,
+        src_base_str,
+        src_offset.maybe_par(),
+        src_limit.maybe_par()
+    )
+}
+
+/// a wrapper of `Sugg`. Besides what `Sugg` do, this removes unnecessary `0`;
+/// and also, it avoids subtracting a variable from the same one by replacing it with `0`.
+/// it exists for the convenience of the overloaded operators while normal functions can do the
+/// same.
+#[derive(Clone)]
+struct MinifyingSugg<'a>(Sugg<'a>);
+
+impl<'a> MinifyingSugg<'a> {
+    fn as_str(&self) -> &str {
+        let Sugg::NonParen(s) | Sugg::MaybeParen(s) | Sugg::BinOp(_, s) = &self.0;
+        s.as_ref()
+    }
+
+    fn into_sugg(self) -> Sugg<'a> {
+        self.0
+    }
+}
+
+impl<'a> From<Sugg<'a>> for MinifyingSugg<'a> {
+    fn from(sugg: Sugg<'a>) -> Self {
+        Self(sugg)
+    }
+}
+
+impl std::ops::Add for &MinifyingSugg<'static> {
+    type Output = MinifyingSugg<'static>;
+    fn add(self, rhs: &MinifyingSugg<'static>) -> MinifyingSugg<'static> {
+        match (self.as_str(), rhs.as_str()) {
+            ("0", _) => rhs.clone(),
+            (_, "0") => self.clone(),
+            (_, _) => (&self.0 + &rhs.0).into(),
+        }
+    }
+}
+
+impl std::ops::Sub for &MinifyingSugg<'static> {
+    type Output = MinifyingSugg<'static>;
+    fn sub(self, rhs: &MinifyingSugg<'static>) -> MinifyingSugg<'static> {
+        match (self.as_str(), rhs.as_str()) {
+            (_, "0") => self.clone(),
+            ("0", _) => (-rhs.0.clone()).into(),
+            (x, y) if x == y => sugg::ZERO.into(),
+            (_, _) => (&self.0 - &rhs.0).into(),
+        }
+    }
+}
+
+impl std::ops::Add<&MinifyingSugg<'static>> for MinifyingSugg<'static> {
+    type Output = MinifyingSugg<'static>;
+    fn add(self, rhs: &MinifyingSugg<'static>) -> MinifyingSugg<'static> {
+        match (self.as_str(), rhs.as_str()) {
+            ("0", _) => rhs.clone(),
+            (_, "0") => self,
+            (_, _) => (self.0 + &rhs.0).into(),
+        }
+    }
+}
+
+impl std::ops::Sub<&MinifyingSugg<'static>> for MinifyingSugg<'static> {
+    type Output = MinifyingSugg<'static>;
+    fn sub(self, rhs: &MinifyingSugg<'static>) -> MinifyingSugg<'static> {
+        match (self.as_str(), rhs.as_str()) {
+            (_, "0") => self,
+            ("0", _) => (-rhs.0.clone()).into(),
+            (x, y) if x == y => sugg::ZERO.into(),
+            (_, _) => (self.0 - &rhs.0).into(),
+        }
+    }
+}
+
+/// a wrapper around `MinifyingSugg`, which carries a operator like currying
+/// so that the suggested code become more efficient (e.g. `foo + -bar` `foo - bar`).
+struct Offset {
+    value: MinifyingSugg<'static>,
+    sign: OffsetSign,
+}
+
+#[derive(Clone, Copy)]
+enum OffsetSign {
+    Positive,
+    Negative,
+}
+
+impl Offset {
+    fn negative(value: Sugg<'static>) -> Self {
+        Self {
+            value: value.into(),
+            sign: OffsetSign::Negative,
+        }
+    }
+
+    fn positive(value: Sugg<'static>) -> Self {
+        Self {
+            value: value.into(),
+            sign: OffsetSign::Positive,
+        }
+    }
+
+    fn empty() -> Self {
+        Self::positive(sugg::ZERO)
+    }
+}
+
+fn apply_offset(lhs: &MinifyingSugg<'static>, rhs: &Offset) -> MinifyingSugg<'static> {
+    match rhs.sign {
+        OffsetSign::Positive => lhs + &rhs.value,
+        OffsetSign::Negative => lhs - &rhs.value,
+    }
+}
+
+#[derive(Debug, Clone, Copy)]
+enum StartKind<'hir> {
+    Range,
+    Counter { initializer: &'hir Expr<'hir> },
+}
+
+struct IndexExpr<'hir> {
+    base: &'hir Expr<'hir>,
+    idx: StartKind<'hir>,
+    idx_offset: Offset,
+}
+
+struct Start<'hir> {
+    id: HirId,
+    kind: StartKind<'hir>,
+}
+
+fn is_slice_like<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'_>) -> bool {
+    let is_slice = match ty.kind() {
+        ty::Ref(_, subty, _) => is_slice_like(cx, subty),
+        ty::Slice(..) | ty::Array(..) => true,
+        _ => false,
+    };
+
+    is_slice || is_type_diagnostic_item(cx, ty, sym::vec_type) || is_type_diagnostic_item(cx, ty, sym!(vecdeque_type))
+}
+
+fn fetch_cloned_expr<'tcx>(expr: &'tcx Expr<'tcx>) -> &'tcx Expr<'tcx> {
+    if_chain! {
+        if let ExprKind::MethodCall(method, _, args, _) = expr.kind;
+        if method.ident.name == sym::clone;
+        if args.len() == 1;
+        if let Some(arg) = args.get(0);
+        then { arg } else { expr }
+    }
+}
+
+fn get_details_from_idx<'tcx>(
+    cx: &LateContext<'tcx>,
+    idx: &Expr<'_>,
+    starts: &[Start<'tcx>],
+) -> Option<(StartKind<'tcx>, Offset)> {
+    fn get_start<'tcx>(e: &Expr<'_>, starts: &[Start<'tcx>]) -> Option<StartKind<'tcx>> {
+        let id = path_to_local(e)?;
+        starts.iter().find(|start| start.id == id).map(|start| start.kind)
+    }
+
+    fn get_offset<'tcx>(cx: &LateContext<'tcx>, e: &Expr<'_>, starts: &[Start<'tcx>]) -> Option<Sugg<'static>> {
+        match &e.kind {
+            ExprKind::Lit(l) => match l.node {
+                ast::LitKind::Int(x, _ty) => Some(Sugg::NonParen(x.to_string().into())),
+                _ => None,
+            },
+            ExprKind::Path(..) if get_start(e, starts).is_none() => Some(Sugg::hir(cx, e, "???")),
+            _ => None,
+        }
+    }
+
+    match idx.kind {
+        ExprKind::Binary(op, lhs, rhs) => match op.node {
+            BinOpKind::Add => {
+                let offset_opt = get_start(lhs, starts)
+                    .and_then(|s| get_offset(cx, rhs, starts).map(|o| (s, o)))
+                    .or_else(|| get_start(rhs, starts).and_then(|s| get_offset(cx, lhs, starts).map(|o| (s, o))));
+
+                offset_opt.map(|(s, o)| (s, Offset::positive(o)))
+            },
+            BinOpKind::Sub => {
+                get_start(lhs, starts).and_then(|s| get_offset(cx, rhs, starts).map(|o| (s, Offset::negative(o))))
+            },
+            _ => None,
+        },
+        ExprKind::Path(..) => get_start(idx, starts).map(|s| (s, Offset::empty())),
+        _ => None,
+    }
+}
+
+fn get_assignment<'tcx>(e: &'tcx Expr<'tcx>) -> Option<(&'tcx Expr<'tcx>, &'tcx Expr<'tcx>)> {
+    if let ExprKind::Assign(lhs, rhs, _) = e.kind {
+        Some((lhs, rhs))
+    } else {
+        None
+    }
+}
+
+/// Get assignments from the given block.
+/// The returned iterator yields `None` if no assignment expressions are there,
+/// filtering out the increments of the given whitelisted loop counters;
+/// because its job is to make sure there's nothing other than assignments and the increments.
+fn get_assignments<'a, 'tcx>(
+    Block { stmts, expr, .. }: &'tcx Block<'tcx>,
+    loop_counters: &'a [Start<'tcx>],
+) -> impl Iterator<Item = Option<(&'tcx Expr<'tcx>, &'tcx Expr<'tcx>)>> + 'a {
+    // As the `filter` and `map` below do different things, I think putting together
+    // just increases complexity. (cc #3188 and #4193)
+    stmts
+        .iter()
+        .filter_map(move |stmt| match stmt.kind {
+            StmtKind::Local(..) | StmtKind::Item(..) => None,
+            StmtKind::Expr(e) | StmtKind::Semi(e) => Some(e),
+        })
+        .chain((*expr).into_iter())
+        .filter(move |e| {
+            if let ExprKind::AssignOp(_, place, _) = e.kind {
+                path_to_local(place).map_or(false, |id| {
+                    !loop_counters
+                        .iter()
+                        // skip the first item which should be `StartKind::Range`
+                        // this makes it possible to use the slice with `StartKind::Range` in the same iterator loop.
+                        .skip(1)
+                        .any(|counter| counter.id == id)
+                })
+            } else {
+                true
+            }
+        })
+        .map(get_assignment)
+}
+
+fn get_loop_counters<'a, 'tcx>(
+    cx: &'a LateContext<'tcx>,
+    body: &'tcx Block<'tcx>,
+    expr: &'tcx Expr<'_>,
+) -> Option<impl Iterator<Item = Start<'tcx>> + 'a> {
+    // Look for variables that are incremented once per loop iteration.
+    let mut increment_visitor = IncrementVisitor::new(cx);
+    walk_block(&mut increment_visitor, body);
+
+    // For each candidate, check the parent block to see if
+    // it's initialized to zero at the start of the loop.
+    get_enclosing_block(&cx, expr.hir_id).and_then(|block| {
+        increment_visitor
+            .into_results()
+            .filter_map(move |var_id| {
+                let mut initialize_visitor = InitializeVisitor::new(cx, expr, var_id);
+                walk_block(&mut initialize_visitor, block);
+
+                initialize_visitor.get_result().map(|(_, initializer)| Start {
+                    id: var_id,
+                    kind: StartKind::Counter { initializer },
+                })
+            })
+            .into()
+    })
+}
diff --git a/clippy_lints/src/loops/mod.rs b/clippy_lints/src/loops/mod.rs
new file mode 100644
index 00000000000..2a372c6307e
--- /dev/null
+++ b/clippy_lints/src/loops/mod.rs
@@ -0,0 +1,627 @@
+mod empty_loop;
+mod explicit_counter_loop;
+mod explicit_into_iter_loop;
+mod explicit_iter_loop;
+mod for_kv_map;
+mod for_loops_over_fallibles;
+mod iter_next_loop;
+mod manual_flatten;
+mod manual_memcpy;
+mod mut_range_bound;
+mod needless_collect;
+mod needless_range_loop;
+mod never_loop;
+mod same_item_push;
+mod single_element_loop;
+mod utils;
+mod while_immutable_condition;
+mod while_let_loop;
+mod while_let_on_iterator;
+
+use crate::utils::higher;
+use rustc_hir::{Expr, ExprKind, LoopSource, Pat};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::source_map::Span;
+use utils::{get_span_of_entire_for_loop, make_iterator_snippet, IncrementVisitor, InitializeVisitor};
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for for-loops that manually copy items between
+    /// slices that could be optimized by having a memcpy.
+    ///
+    /// **Why is this bad?** It is not as fast as a memcpy.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// # let src = vec![1];
+    /// # let mut dst = vec![0; 65];
+    /// for i in 0..src.len() {
+    ///     dst[i + 64] = src[i];
+    /// }
+    /// ```
+    /// Could be written as:
+    /// ```rust
+    /// # let src = vec![1];
+    /// # let mut dst = vec![0; 65];
+    /// dst[64..(src.len() + 64)].clone_from_slice(&src[..]);
+    /// ```
+    pub MANUAL_MEMCPY,
+    perf,
+    "manually copying items between slices"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for looping over the range of `0..len` of some
+    /// collection just to get the values by index.
+    ///
+    /// **Why is this bad?** Just iterating the collection itself makes the intent
+    /// more clear and is probably faster.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// let vec = vec!['a', 'b', 'c'];
+    /// for i in 0..vec.len() {
+    ///     println!("{}", vec[i]);
+    /// }
+    /// ```
+    /// Could be written as:
+    /// ```rust
+    /// let vec = vec!['a', 'b', 'c'];
+    /// for i in vec {
+    ///     println!("{}", i);
+    /// }
+    /// ```
+    pub NEEDLESS_RANGE_LOOP,
+    style,
+    "for-looping over a range of indices where an iterator over items would do"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for loops on `x.iter()` where `&x` will do, and
+    /// suggests the latter.
+    ///
+    /// **Why is this bad?** Readability.
+    ///
+    /// **Known problems:** False negatives. We currently only warn on some known
+    /// types.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// // with `y` a `Vec` or slice:
+    /// # let y = vec![1];
+    /// for x in y.iter() {
+    ///     // ..
+    /// }
+    /// ```
+    /// can be rewritten to
+    /// ```rust
+    /// # let y = vec![1];
+    /// for x in &y {
+    ///     // ..
+    /// }
+    /// ```
+    pub EXPLICIT_ITER_LOOP,
+    pedantic,
+    "for-looping over `_.iter()` or `_.iter_mut()` when `&_` or `&mut _` would do"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for loops on `y.into_iter()` where `y` will do, and
+    /// suggests the latter.
+    ///
+    /// **Why is this bad?** Readability.
+    ///
+    /// **Known problems:** None
+    ///
+    /// **Example:**
+    /// ```rust
+    /// # let y = vec![1];
+    /// // with `y` a `Vec` or slice:
+    /// for x in y.into_iter() {
+    ///     // ..
+    /// }
+    /// ```
+    /// can be rewritten to
+    /// ```rust
+    /// # let y = vec![1];
+    /// for x in y {
+    ///     // ..
+    /// }
+    /// ```
+    pub EXPLICIT_INTO_ITER_LOOP,
+    pedantic,
+    "for-looping over `_.into_iter()` when `_` would do"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for loops on `x.next()`.
+    ///
+    /// **Why is this bad?** `next()` returns either `Some(value)` if there was a
+    /// value, or `None` otherwise. The insidious thing is that `Option<_>`
+    /// implements `IntoIterator`, so that possibly one value will be iterated,
+    /// leading to some hard to find bugs. No one will want to write such code
+    /// [except to win an Underhanded Rust
+    /// Contest](https://www.reddit.com/r/rust/comments/3hb0wm/underhanded_rust_contest/cu5yuhr).
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```ignore
+    /// for x in y.next() {
+    ///     ..
+    /// }
+    /// ```
+    pub ITER_NEXT_LOOP,
+    correctness,
+    "for-looping over `_.next()` which is probably not intended"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for `for` loops over `Option` or `Result` values.
+    ///
+    /// **Why is this bad?** Readability. This is more clearly expressed as an `if
+    /// let`.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// # let opt = Some(1);
+    ///
+    /// // Bad
+    /// for x in opt {
+    ///     // ..
+    /// }
+    ///
+    /// // Good
+    /// if let Some(x) = opt {
+    ///     // ..
+    /// }
+    /// ```
+    ///
+    /// // or
+    ///
+    /// ```rust
+    /// # let res: Result<i32, std::io::Error> = Ok(1);
+    ///
+    /// // Bad
+    /// for x in &res {
+    ///     // ..
+    /// }
+    ///
+    /// // Good
+    /// if let Ok(x) = res {
+    ///     // ..
+    /// }
+    /// ```
+    pub FOR_LOOPS_OVER_FALLIBLES,
+    correctness,
+    "for-looping over an `Option` or a `Result`, which is more clearly expressed as an `if let`"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Detects `loop + match` combinations that are easier
+    /// written as a `while let` loop.
+    ///
+    /// **Why is this bad?** The `while let` loop is usually shorter and more
+    /// readable.
+    ///
+    /// **Known problems:** Sometimes the wrong binding is displayed ([#383](https://github.com/rust-lang/rust-clippy/issues/383)).
+    ///
+    /// **Example:**
+    /// ```rust,no_run
+    /// # let y = Some(1);
+    /// loop {
+    ///     let x = match y {
+    ///         Some(x) => x,
+    ///         None => break,
+    ///     };
+    ///     // .. do something with x
+    /// }
+    /// // is easier written as
+    /// while let Some(x) = y {
+    ///     // .. do something with x
+    /// };
+    /// ```
+    pub WHILE_LET_LOOP,
+    complexity,
+    "`loop { if let { ... } else break }`, which can be written as a `while let` loop"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for functions collecting an iterator when collect
+    /// is not needed.
+    ///
+    /// **Why is this bad?** `collect` causes the allocation of a new data structure,
+    /// when this allocation may not be needed.
+    ///
+    /// **Known problems:**
+    /// None
+    ///
+    /// **Example:**
+    /// ```rust
+    /// # let iterator = vec![1].into_iter();
+    /// let len = iterator.clone().collect::<Vec<_>>().len();
+    /// // should be
+    /// let len = iterator.count();
+    /// ```
+    pub NEEDLESS_COLLECT,
+    perf,
+    "collecting an iterator when collect is not needed"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks `for` loops over slices with an explicit counter
+    /// and suggests the use of `.enumerate()`.
+    ///
+    /// **Why is it bad?** Using `.enumerate()` makes the intent more clear,
+    /// declutters the code and may be faster in some instances.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// # let v = vec![1];
+    /// # fn bar(bar: usize, baz: usize) {}
+    /// let mut i = 0;
+    /// for item in &v {
+    ///     bar(i, *item);
+    ///     i += 1;
+    /// }
+    /// ```
+    /// Could be written as
+    /// ```rust
+    /// # let v = vec![1];
+    /// # fn bar(bar: usize, baz: usize) {}
+    /// for (i, item) in v.iter().enumerate() { bar(i, *item); }
+    /// ```
+    pub EXPLICIT_COUNTER_LOOP,
+    complexity,
+    "for-looping with an explicit counter when `_.enumerate()` would do"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for empty `loop` expressions.
+    ///
+    /// **Why is this bad?** These busy loops burn CPU cycles without doing
+    /// anything. It is _almost always_ a better idea to `panic!` than to have
+    /// a busy loop.
+    ///
+    /// If panicking isn't possible, think of the environment and either:
+    ///   - block on something
+    ///   - sleep the thread for some microseconds
+    ///   - yield or pause the thread
+    ///
+    /// For `std` targets, this can be done with
+    /// [`std::thread::sleep`](https://doc.rust-lang.org/std/thread/fn.sleep.html)
+    /// or [`std::thread::yield_now`](https://doc.rust-lang.org/std/thread/fn.yield_now.html).
+    ///
+    /// For `no_std` targets, doing this is more complicated, especially because
+    /// `#[panic_handler]`s can't panic. To stop/pause the thread, you will
+    /// probably need to invoke some target-specific intrinsic. Examples include:
+    ///   - [`x86_64::instructions::hlt`](https://docs.rs/x86_64/0.12.2/x86_64/instructions/fn.hlt.html)
+    ///   - [`cortex_m::asm::wfi`](https://docs.rs/cortex-m/0.6.3/cortex_m/asm/fn.wfi.html)
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```no_run
+    /// loop {}
+    /// ```
+    pub EMPTY_LOOP,
+    style,
+    "empty `loop {}`, which should block or sleep"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for `while let` expressions on iterators.
+    ///
+    /// **Why is this bad?** Readability. A simple `for` loop is shorter and conveys
+    /// the intent better.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```ignore
+    /// while let Some(val) = iter() {
+    ///     ..
+    /// }
+    /// ```
+    pub WHILE_LET_ON_ITERATOR,
+    style,
+    "using a `while let` loop instead of a for loop on an iterator"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for iterating a map (`HashMap` or `BTreeMap`) and
+    /// ignoring either the keys or values.
+    ///
+    /// **Why is this bad?** Readability. There are `keys` and `values` methods that
+    /// can be used to express that don't need the values or keys.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```ignore
+    /// for (k, _) in &map {
+    ///     ..
+    /// }
+    /// ```
+    ///
+    /// could be replaced by
+    ///
+    /// ```ignore
+    /// for k in map.keys() {
+    ///     ..
+    /// }
+    /// ```
+    pub FOR_KV_MAP,
+    style,
+    "looping on a map using `iter` when `keys` or `values` would do"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for loops that will always `break`, `return` or
+    /// `continue` an outer loop.
+    ///
+    /// **Why is this bad?** This loop never loops, all it does is obfuscating the
+    /// code.
+    ///
+    /// **Known problems:** None
+    ///
+    /// **Example:**
+    /// ```rust
+    /// loop {
+    ///     ..;
+    ///     break;
+    /// }
+    /// ```
+    pub NEVER_LOOP,
+    correctness,
+    "any loop that will always `break` or `return`"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for loops which have a range bound that is a mutable variable
+    ///
+    /// **Why is this bad?** One might think that modifying the mutable variable changes the loop bounds
+    ///
+    /// **Known problems:** None
+    ///
+    /// **Example:**
+    /// ```rust
+    /// let mut foo = 42;
+    /// for i in 0..foo {
+    ///     foo -= 1;
+    ///     println!("{}", i); // prints numbers from 0 to 42, not 0 to 21
+    /// }
+    /// ```
+    pub MUT_RANGE_BOUND,
+    complexity,
+    "for loop over a range where one of the bounds is a mutable variable"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks whether variables used within while loop condition
+    /// can be (and are) mutated in the body.
+    ///
+    /// **Why is this bad?** If the condition is unchanged, entering the body of the loop
+    /// will lead to an infinite loop.
+    ///
+    /// **Known problems:** If the `while`-loop is in a closure, the check for mutation of the
+    /// condition variables in the body can cause false negatives. For example when only `Upvar` `a` is
+    /// in the condition and only `Upvar` `b` gets mutated in the body, the lint will not trigger.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// let i = 0;
+    /// while i > 10 {
+    ///     println!("let me loop forever!");
+    /// }
+    /// ```
+    pub WHILE_IMMUTABLE_CONDITION,
+    correctness,
+    "variables used within while expression are not mutated in the body"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks whether a for loop is being used to push a constant
+    /// value into a Vec.
+    ///
+    /// **Why is this bad?** This kind of operation can be expressed more succinctly with
+    /// `vec![item;SIZE]` or `vec.resize(NEW_SIZE, item)` and using these alternatives may also
+    /// have better performance.
+    /// **Known problems:** None
+    ///
+    /// **Example:**
+    /// ```rust
+    /// let item1 = 2;
+    /// let item2 = 3;
+    /// let mut vec: Vec<u8> = Vec::new();
+    /// for _ in 0..20 {
+    ///    vec.push(item1);
+    /// }
+    /// for _ in 0..30 {
+    ///     vec.push(item2);
+    /// }
+    /// ```
+    /// could be written as
+    /// ```rust
+    /// let item1 = 2;
+    /// let item2 = 3;
+    /// let mut vec: Vec<u8> = vec![item1; 20];
+    /// vec.resize(20 + 30, item2);
+    /// ```
+    pub SAME_ITEM_PUSH,
+    style,
+    "the same item is pushed inside of a for loop"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks whether a for loop has a single element.
+    ///
+    /// **Why is this bad?** There is no reason to have a loop of a
+    /// single element.
+    /// **Known problems:** None
+    ///
+    /// **Example:**
+    /// ```rust
+    /// let item1 = 2;
+    /// for item in &[item1] {
+    ///     println!("{}", item);
+    /// }
+    /// ```
+    /// could be written as
+    /// ```rust
+    /// let item1 = 2;
+    /// let item = &item1;
+    /// println!("{}", item);
+    /// ```
+    pub SINGLE_ELEMENT_LOOP,
+    complexity,
+    "there is no reason to have a single element loop"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Check for unnecessary `if let` usage in a for loop
+    /// where only the `Some` or `Ok` variant of the iterator element is used.
+    ///
+    /// **Why is this bad?** It is verbose and can be simplified
+    /// by first calling the `flatten` method on the `Iterator`.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    ///
+    /// ```rust
+    /// let x = vec![Some(1), Some(2), Some(3)];
+    /// for n in x {
+    ///     if let Some(n) = n {
+    ///         println!("{}", n);
+    ///     }
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// let x = vec![Some(1), Some(2), Some(3)];
+    /// for n in x.into_iter().flatten() {
+    ///     println!("{}", n);
+    /// }
+    /// ```
+    pub MANUAL_FLATTEN,
+    complexity,
+    "for loops over `Option`s or `Result`s with a single expression can be simplified"
+}
+
+declare_lint_pass!(Loops => [
+    MANUAL_MEMCPY,
+    MANUAL_FLATTEN,
+    NEEDLESS_RANGE_LOOP,
+    EXPLICIT_ITER_LOOP,
+    EXPLICIT_INTO_ITER_LOOP,
+    ITER_NEXT_LOOP,
+    FOR_LOOPS_OVER_FALLIBLES,
+    WHILE_LET_LOOP,
+    NEEDLESS_COLLECT,
+    EXPLICIT_COUNTER_LOOP,
+    EMPTY_LOOP,
+    WHILE_LET_ON_ITERATOR,
+    FOR_KV_MAP,
+    NEVER_LOOP,
+    MUT_RANGE_BOUND,
+    WHILE_IMMUTABLE_CONDITION,
+    SAME_ITEM_PUSH,
+    SINGLE_ELEMENT_LOOP,
+]);
+
+impl<'tcx> LateLintPass<'tcx> for Loops {
+    #[allow(clippy::too_many_lines)]
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+        if let Some((pat, arg, body, span)) = higher::for_loop(expr) {
+            // we don't want to check expanded macros
+            // this check is not at the top of the function
+            // since higher::for_loop expressions are marked as expansions
+            if body.span.from_expansion() {
+                return;
+            }
+            check_for_loop(cx, pat, arg, body, expr, span);
+        }
+
+        // we don't want to check expanded macros
+        if expr.span.from_expansion() {
+            return;
+        }
+
+        // check for never_loop
+        never_loop::check(cx, expr);
+
+        // check for `loop { if let {} else break }` that could be `while let`
+        // (also matches an explicit "match" instead of "if let")
+        // (even if the "match" or "if let" is used for declaration)
+        if let ExprKind::Loop(ref block, _, LoopSource::Loop, _) = expr.kind {
+            // also check for empty `loop {}` statements, skipping those in #[panic_handler]
+            empty_loop::check(cx, expr, block);
+            while_let_loop::check(cx, expr, block);
+        }
+
+        while_let_on_iterator::check(cx, expr);
+
+        if let Some((cond, body)) = higher::while_loop(&expr) {
+            while_immutable_condition::check(cx, cond, body);
+        }
+
+        needless_collect::check(expr, cx);
+    }
+}
+
+fn check_for_loop<'tcx>(
+    cx: &LateContext<'tcx>,
+    pat: &'tcx Pat<'_>,
+    arg: &'tcx Expr<'_>,
+    body: &'tcx Expr<'_>,
+    expr: &'tcx Expr<'_>,
+    span: Span,
+) {
+    let is_manual_memcpy_triggered = manual_memcpy::check(cx, pat, arg, body, expr);
+    if !is_manual_memcpy_triggered {
+        needless_range_loop::check(cx, pat, arg, body, expr);
+        explicit_counter_loop::check(cx, pat, arg, body, expr);
+    }
+    check_for_loop_arg(cx, pat, arg, expr);
+    for_kv_map::check(cx, pat, arg, body, expr);
+    mut_range_bound::check(cx, arg, body);
+    single_element_loop::check(cx, pat, arg, body, expr);
+    same_item_push::check(cx, pat, arg, body, expr);
+    manual_flatten::check(cx, pat, arg, body, span);
+}
+
+fn check_for_loop_arg(cx: &LateContext<'_>, pat: &Pat<'_>, arg: &Expr<'_>, expr: &Expr<'_>) {
+    let mut next_loop_linted = false; // whether or not ITER_NEXT_LOOP lint was used
+
+    if let ExprKind::MethodCall(ref method, _, ref args, _) = arg.kind {
+        // just the receiver, no arguments
+        if args.len() == 1 {
+            let method_name = &*method.ident.as_str();
+            // check for looping over x.iter() or x.iter_mut(), could use &x or &mut x
+            match method_name {
+                "iter" | "iter_mut" => explicit_iter_loop::check(cx, args, arg, method_name),
+                "into_iter" => {
+                    explicit_iter_loop::check(cx, args, arg, method_name);
+                    explicit_into_iter_loop::check(cx, args, arg);
+                },
+                "next" => {
+                    next_loop_linted = iter_next_loop::check(cx, arg, expr);
+                },
+                _ => {},
+            }
+        }
+    }
+
+    if !next_loop_linted {
+        for_loops_over_fallibles::check(cx, pat, arg);
+    }
+}
diff --git a/clippy_lints/src/loops/mut_range_bound.rs b/clippy_lints/src/loops/mut_range_bound.rs
new file mode 100644
index 00000000000..3ae592950f1
--- /dev/null
+++ b/clippy_lints/src/loops/mut_range_bound.rs
@@ -0,0 +1,115 @@
+use super::MUT_RANGE_BOUND;
+use crate::utils::{higher, path_to_local, span_lint};
+use if_chain::if_chain;
+use rustc_hir::{BindingAnnotation, Expr, HirId, Node, PatKind};
+use rustc_infer::infer::TyCtxtInferExt;
+use rustc_lint::LateContext;
+use rustc_middle::ty;
+use rustc_span::source_map::Span;
+use rustc_typeck::expr_use_visitor::{ConsumeMode, Delegate, ExprUseVisitor, PlaceBase, PlaceWithHirId};
+
+pub(super) fn check(cx: &LateContext<'_>, arg: &Expr<'_>, body: &Expr<'_>) {
+    if let Some(higher::Range {
+        start: Some(start),
+        end: Some(end),
+        ..
+    }) = higher::range(arg)
+    {
+        let mut_ids = vec![check_for_mutability(cx, start), check_for_mutability(cx, end)];
+        if mut_ids[0].is_some() || mut_ids[1].is_some() {
+            let (span_low, span_high) = check_for_mutation(cx, body, &mut_ids);
+            mut_warn_with_span(cx, span_low);
+            mut_warn_with_span(cx, span_high);
+        }
+    }
+}
+
+fn mut_warn_with_span(cx: &LateContext<'_>, span: Option<Span>) {
+    if let Some(sp) = span {
+        span_lint(
+            cx,
+            MUT_RANGE_BOUND,
+            sp,
+            "attempt to mutate range bound within loop; note that the range of the loop is unchanged",
+        );
+    }
+}
+
+fn check_for_mutability(cx: &LateContext<'_>, bound: &Expr<'_>) -> Option<HirId> {
+    if_chain! {
+        if let Some(hir_id) = path_to_local(bound);
+        if let Node::Binding(pat) = cx.tcx.hir().get(hir_id);
+        if let PatKind::Binding(BindingAnnotation::Mutable, ..) = pat.kind;
+        then {
+            return Some(hir_id);
+        }
+    }
+    None
+}
+
+fn check_for_mutation<'tcx>(
+    cx: &LateContext<'tcx>,
+    body: &Expr<'_>,
+    bound_ids: &[Option<HirId>],
+) -> (Option<Span>, Option<Span>) {
+    let mut delegate = MutatePairDelegate {
+        cx,
+        hir_id_low: bound_ids[0],
+        hir_id_high: bound_ids[1],
+        span_low: None,
+        span_high: None,
+    };
+    cx.tcx.infer_ctxt().enter(|infcx| {
+        ExprUseVisitor::new(
+            &mut delegate,
+            &infcx,
+            body.hir_id.owner,
+            cx.param_env,
+            cx.typeck_results(),
+        )
+        .walk_expr(body);
+    });
+    delegate.mutation_span()
+}
+
+struct MutatePairDelegate<'a, 'tcx> {
+    cx: &'a LateContext<'tcx>,
+    hir_id_low: Option<HirId>,
+    hir_id_high: Option<HirId>,
+    span_low: Option<Span>,
+    span_high: Option<Span>,
+}
+
+impl<'tcx> Delegate<'tcx> for MutatePairDelegate<'_, 'tcx> {
+    fn consume(&mut self, _: &PlaceWithHirId<'tcx>, _: HirId, _: ConsumeMode) {}
+
+    fn borrow(&mut self, cmt: &PlaceWithHirId<'tcx>, diag_expr_id: HirId, bk: ty::BorrowKind) {
+        if let ty::BorrowKind::MutBorrow = bk {
+            if let PlaceBase::Local(id) = cmt.place.base {
+                if Some(id) == self.hir_id_low {
+                    self.span_low = Some(self.cx.tcx.hir().span(diag_expr_id))
+                }
+                if Some(id) == self.hir_id_high {
+                    self.span_high = Some(self.cx.tcx.hir().span(diag_expr_id))
+                }
+            }
+        }
+    }
+
+    fn mutate(&mut self, cmt: &PlaceWithHirId<'tcx>, diag_expr_id: HirId) {
+        if let PlaceBase::Local(id) = cmt.place.base {
+            if Some(id) == self.hir_id_low {
+                self.span_low = Some(self.cx.tcx.hir().span(diag_expr_id))
+            }
+            if Some(id) == self.hir_id_high {
+                self.span_high = Some(self.cx.tcx.hir().span(diag_expr_id))
+            }
+        }
+    }
+}
+
+impl MutatePairDelegate<'_, '_> {
+    fn mutation_span(&self) -> (Option<Span>, Option<Span>) {
+        (self.span_low, self.span_high)
+    }
+}
diff --git a/clippy_lints/src/loops/needless_collect.rs b/clippy_lints/src/loops/needless_collect.rs
new file mode 100644
index 00000000000..6f271309105
--- /dev/null
+++ b/clippy_lints/src/loops/needless_collect.rs
@@ -0,0 +1,283 @@
+use super::NEEDLESS_COLLECT;
+use crate::utils::sugg::Sugg;
+use crate::utils::{
+    is_type_diagnostic_item, match_trait_method, match_type, path_to_local_id, paths, snippet, span_lint_and_sugg,
+    span_lint_and_then,
+};
+use if_chain::if_chain;
+use rustc_errors::Applicability;
+use rustc_hir::intravisit::{walk_block, walk_expr, NestedVisitorMap, Visitor};
+use rustc_hir::{Block, Expr, ExprKind, GenericArg, HirId, Local, Pat, PatKind, QPath, StmtKind};
+use rustc_lint::LateContext;
+use rustc_middle::hir::map::Map;
+use rustc_span::source_map::Span;
+use rustc_span::symbol::{sym, Ident};
+
+const NEEDLESS_COLLECT_MSG: &str = "avoid using `collect()` when not needed";
+
+pub(super) fn check<'tcx>(expr: &'tcx Expr<'_>, cx: &LateContext<'tcx>) {
+    check_needless_collect_direct_usage(expr, cx);
+    check_needless_collect_indirect_usage(expr, cx);
+}
+fn check_needless_collect_direct_usage<'tcx>(expr: &'tcx Expr<'_>, cx: &LateContext<'tcx>) {
+    if_chain! {
+        if let ExprKind::MethodCall(ref method, _, ref args, _) = expr.kind;
+        if let ExprKind::MethodCall(ref chain_method, _, _, _) = args[0].kind;
+        if chain_method.ident.name == sym!(collect) && match_trait_method(cx, &args[0], &paths::ITERATOR);
+        if let Some(ref generic_args) = chain_method.args;
+        if let Some(GenericArg::Type(ref ty)) = generic_args.args.get(0);
+        then {
+            let ty = cx.typeck_results().node_type(ty.hir_id);
+            if is_type_diagnostic_item(cx, ty, sym::vec_type) ||
+                is_type_diagnostic_item(cx, ty, sym!(vecdeque_type)) ||
+                match_type(cx, ty, &paths::BTREEMAP) ||
+                is_type_diagnostic_item(cx, ty, sym!(hashmap_type)) {
+                if method.ident.name == sym!(len) {
+                    let span = shorten_needless_collect_span(expr);
+                    span_lint_and_sugg(
+                        cx,
+                        NEEDLESS_COLLECT,
+                        span,
+                        NEEDLESS_COLLECT_MSG,
+                        "replace with",
+                        "count()".to_string(),
+                        Applicability::MachineApplicable,
+                    );
+                }
+                if method.ident.name == sym!(is_empty) {
+                    let span = shorten_needless_collect_span(expr);
+                    span_lint_and_sugg(
+                        cx,
+                        NEEDLESS_COLLECT,
+                        span,
+                        NEEDLESS_COLLECT_MSG,
+                        "replace with",
+                        "next().is_none()".to_string(),
+                        Applicability::MachineApplicable,
+                    );
+                }
+                if method.ident.name == sym!(contains) {
+                    let contains_arg = snippet(cx, args[1].span, "??");
+                    let span = shorten_needless_collect_span(expr);
+                    span_lint_and_then(
+                        cx,
+                        NEEDLESS_COLLECT,
+                        span,
+                        NEEDLESS_COLLECT_MSG,
+                        |diag| {
+                            let (arg, pred) = contains_arg
+                                    .strip_prefix('&')
+                                    .map_or(("&x", &*contains_arg), |s| ("x", s));
+                            diag.span_suggestion(
+                                span,
+                                "replace with",
+                                format!(
+                                    "any(|{}| x == {})",
+                                    arg, pred
+                                ),
+                                Applicability::MachineApplicable,
+                            );
+                        }
+                    );
+                }
+            }
+        }
+    }
+}
+
+fn check_needless_collect_indirect_usage<'tcx>(expr: &'tcx Expr<'_>, cx: &LateContext<'tcx>) {
+    if let ExprKind::Block(ref block, _) = expr.kind {
+        for ref stmt in block.stmts {
+            if_chain! {
+                if let StmtKind::Local(
+                    Local { pat: Pat { hir_id: pat_id, kind: PatKind::Binding(_, _, ident, .. ), .. },
+                    init: Some(ref init_expr), .. }
+                ) = stmt.kind;
+                if let ExprKind::MethodCall(ref method_name, _, &[ref iter_source], ..) = init_expr.kind;
+                if method_name.ident.name == sym!(collect) && match_trait_method(cx, &init_expr, &paths::ITERATOR);
+                if let Some(ref generic_args) = method_name.args;
+                if let Some(GenericArg::Type(ref ty)) = generic_args.args.get(0);
+                if let ty = cx.typeck_results().node_type(ty.hir_id);
+                if is_type_diagnostic_item(cx, ty, sym::vec_type) ||
+                    is_type_diagnostic_item(cx, ty, sym!(vecdeque_type)) ||
+                    match_type(cx, ty, &paths::LINKED_LIST);
+                if let Some(iter_calls) = detect_iter_and_into_iters(block, *ident);
+                if iter_calls.len() == 1;
+                then {
+                    let mut used_count_visitor = UsedCountVisitor {
+                        cx,
+                        id: *pat_id,
+                        count: 0,
+                    };
+                    walk_block(&mut used_count_visitor, block);
+                    if used_count_visitor.count > 1 {
+                        return;
+                    }
+
+                    // Suggest replacing iter_call with iter_replacement, and removing stmt
+                    let iter_call = &iter_calls[0];
+                    span_lint_and_then(
+                        cx,
+                        super::NEEDLESS_COLLECT,
+                        stmt.span.until(iter_call.span),
+                        NEEDLESS_COLLECT_MSG,
+                        |diag| {
+                            let iter_replacement = format!("{}{}", Sugg::hir(cx, iter_source, ".."), iter_call.get_iter_method(cx));
+                            diag.multipart_suggestion(
+                                iter_call.get_suggestion_text(),
+                                vec![
+                                    (stmt.span, String::new()),
+                                    (iter_call.span, iter_replacement)
+                                ],
+                                Applicability::MachineApplicable,// MaybeIncorrect,
+                            ).emit();
+                        },
+                    );
+                }
+            }
+        }
+    }
+}
+
+struct IterFunction {
+    func: IterFunctionKind,
+    span: Span,
+}
+impl IterFunction {
+    fn get_iter_method(&self, cx: &LateContext<'_>) -> String {
+        match &self.func {
+            IterFunctionKind::IntoIter => String::new(),
+            IterFunctionKind::Len => String::from(".count()"),
+            IterFunctionKind::IsEmpty => String::from(".next().is_none()"),
+            IterFunctionKind::Contains(span) => {
+                let s = snippet(cx, *span, "..");
+                if let Some(stripped) = s.strip_prefix('&') {
+                    format!(".any(|x| x == {})", stripped)
+                } else {
+                    format!(".any(|x| x == *{})", s)
+                }
+            },
+        }
+    }
+    fn get_suggestion_text(&self) -> &'static str {
+        match &self.func {
+            IterFunctionKind::IntoIter => {
+                "use the original Iterator instead of collecting it and then producing a new one"
+            },
+            IterFunctionKind::Len => {
+                "take the original Iterator's count instead of collecting it and finding the length"
+            },
+            IterFunctionKind::IsEmpty => {
+                "check if the original Iterator has anything instead of collecting it and seeing if it's empty"
+            },
+            IterFunctionKind::Contains(_) => {
+                "check if the original Iterator contains an element instead of collecting then checking"
+            },
+        }
+    }
+}
+enum IterFunctionKind {
+    IntoIter,
+    Len,
+    IsEmpty,
+    Contains(Span),
+}
+
+struct IterFunctionVisitor {
+    uses: Vec<IterFunction>,
+    seen_other: bool,
+    target: Ident,
+}
+impl<'tcx> Visitor<'tcx> for IterFunctionVisitor {
+    fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
+        // Check function calls on our collection
+        if_chain! {
+            if let ExprKind::MethodCall(method_name, _, ref args, _) = &expr.kind;
+            if let Some(Expr { kind: ExprKind::Path(QPath::Resolved(_, ref path)), .. }) = args.get(0);
+            if let &[name] = &path.segments;
+            if name.ident == self.target;
+            then {
+                let len = sym!(len);
+                let is_empty = sym!(is_empty);
+                let contains = sym!(contains);
+                match method_name.ident.name {
+                    sym::into_iter => self.uses.push(
+                        IterFunction { func: IterFunctionKind::IntoIter, span: expr.span }
+                    ),
+                    name if name == len => self.uses.push(
+                        IterFunction { func: IterFunctionKind::Len, span: expr.span }
+                    ),
+                    name if name == is_empty => self.uses.push(
+                        IterFunction { func: IterFunctionKind::IsEmpty, span: expr.span }
+                    ),
+                    name if name == contains => self.uses.push(
+                        IterFunction { func: IterFunctionKind::Contains(args[1].span), span: expr.span }
+                    ),
+                    _ => self.seen_other = true,
+                }
+                return
+            }
+        }
+        // Check if the collection is used for anything else
+        if_chain! {
+            if let Expr { kind: ExprKind::Path(QPath::Resolved(_, ref path)), .. } = expr;
+            if let &[name] = &path.segments;
+            if name.ident == self.target;
+            then {
+                self.seen_other = true;
+            } else {
+                walk_expr(self, expr);
+            }
+        }
+    }
+
+    type Map = Map<'tcx>;
+    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
+        NestedVisitorMap::None
+    }
+}
+
+struct UsedCountVisitor<'a, 'tcx> {
+    cx: &'a LateContext<'tcx>,
+    id: HirId,
+    count: usize,
+}
+
+impl<'a, 'tcx> Visitor<'tcx> for UsedCountVisitor<'a, 'tcx> {
+    type Map = Map<'tcx>;
+
+    fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
+        if path_to_local_id(expr, self.id) {
+            self.count += 1;
+        } else {
+            walk_expr(self, expr);
+        }
+    }
+
+    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
+        NestedVisitorMap::OnlyBodies(self.cx.tcx.hir())
+    }
+}
+
+/// Detect the occurrences of calls to `iter` or `into_iter` for the
+/// given identifier
+fn detect_iter_and_into_iters<'tcx>(block: &'tcx Block<'tcx>, identifier: Ident) -> Option<Vec<IterFunction>> {
+    let mut visitor = IterFunctionVisitor {
+        uses: Vec::new(),
+        target: identifier,
+        seen_other: false,
+    };
+    visitor.visit_block(block);
+    if visitor.seen_other { None } else { Some(visitor.uses) }
+}
+
+fn shorten_needless_collect_span(expr: &Expr<'_>) -> Span {
+    if_chain! {
+        if let ExprKind::MethodCall(.., args, _) = &expr.kind;
+        if let ExprKind::MethodCall(_, span, ..) = &args[0].kind;
+        then {
+            return expr.span.with_lo(span.lo());
+        }
+    }
+    unreachable!();
+}
diff --git a/clippy_lints/src/loops/needless_range_loop.rs b/clippy_lints/src/loops/needless_range_loop.rs
new file mode 100644
index 00000000000..5f02e4b9d87
--- /dev/null
+++ b/clippy_lints/src/loops/needless_range_loop.rs
@@ -0,0 +1,391 @@
+use super::NEEDLESS_RANGE_LOOP;
+use crate::utils::visitors::LocalUsedVisitor;
+use crate::utils::{
+    contains_name, has_iter_method, higher, is_integer_const, match_trait_method, multispan_sugg, path_to_local_id,
+    paths, snippet, span_lint_and_then, sugg, SpanlessEq,
+};
+use if_chain::if_chain;
+use rustc_ast::ast;
+use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_hir::def::{DefKind, Res};
+use rustc_hir::intravisit::{walk_expr, NestedVisitorMap, Visitor};
+use rustc_hir::{BinOpKind, BorrowKind, Expr, ExprKind, HirId, Mutability, Pat, PatKind, QPath};
+use rustc_lint::LateContext;
+use rustc_middle::hir::map::Map;
+use rustc_middle::middle::region;
+use rustc_middle::ty::{self, Ty};
+use rustc_span::symbol::{sym, Symbol};
+use std::iter::Iterator;
+use std::mem;
+
+/// Checks for looping over a range and then indexing a sequence with it.
+/// The iteratee must be a range literal.
+#[allow(clippy::too_many_lines)]
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    pat: &'tcx Pat<'_>,
+    arg: &'tcx Expr<'_>,
+    body: &'tcx Expr<'_>,
+    expr: &'tcx Expr<'_>,
+) {
+    if let Some(higher::Range {
+        start: Some(start),
+        ref end,
+        limits,
+    }) = higher::range(arg)
+    {
+        // the var must be a single name
+        if let PatKind::Binding(_, canonical_id, ident, _) = pat.kind {
+            let mut visitor = VarVisitor {
+                cx,
+                var: canonical_id,
+                indexed_mut: FxHashSet::default(),
+                indexed_indirectly: FxHashMap::default(),
+                indexed_directly: FxHashMap::default(),
+                referenced: FxHashSet::default(),
+                nonindex: false,
+                prefer_mutable: false,
+            };
+            walk_expr(&mut visitor, body);
+
+            // linting condition: we only indexed one variable, and indexed it directly
+            if visitor.indexed_indirectly.is_empty() && visitor.indexed_directly.len() == 1 {
+                let (indexed, (indexed_extent, indexed_ty)) = visitor
+                    .indexed_directly
+                    .into_iter()
+                    .next()
+                    .expect("already checked that we have exactly 1 element");
+
+                // ensure that the indexed variable was declared before the loop, see #601
+                if let Some(indexed_extent) = indexed_extent {
+                    let parent_id = cx.tcx.hir().get_parent_item(expr.hir_id);
+                    let parent_def_id = cx.tcx.hir().local_def_id(parent_id);
+                    let region_scope_tree = cx.tcx.region_scope_tree(parent_def_id);
+                    let pat_extent = region_scope_tree.var_scope(pat.hir_id.local_id);
+                    if region_scope_tree.is_subscope_of(indexed_extent, pat_extent) {
+                        return;
+                    }
+                }
+
+                // don't lint if the container that is indexed does not have .iter() method
+                let has_iter = has_iter_method(cx, indexed_ty);
+                if has_iter.is_none() {
+                    return;
+                }
+
+                // don't lint if the container that is indexed into is also used without
+                // indexing
+                if visitor.referenced.contains(&indexed) {
+                    return;
+                }
+
+                let starts_at_zero = is_integer_const(cx, start, 0);
+
+                let skip = if starts_at_zero {
+                    String::new()
+                } else if visitor.indexed_mut.contains(&indexed) && contains_name(indexed, start) {
+                    return;
+                } else {
+                    format!(".skip({})", snippet(cx, start.span, ".."))
+                };
+
+                let mut end_is_start_plus_val = false;
+
+                let take = if let Some(end) = *end {
+                    let mut take_expr = end;
+
+                    if let ExprKind::Binary(ref op, ref left, ref right) = end.kind {
+                        if let BinOpKind::Add = op.node {
+                            let start_equal_left = SpanlessEq::new(cx).eq_expr(start, left);
+                            let start_equal_right = SpanlessEq::new(cx).eq_expr(start, right);
+
+                            if start_equal_left {
+                                take_expr = right;
+                            } else if start_equal_right {
+                                take_expr = left;
+                            }
+
+                            end_is_start_plus_val = start_equal_left | start_equal_right;
+                        }
+                    }
+
+                    if is_len_call(end, indexed) || is_end_eq_array_len(cx, end, limits, indexed_ty) {
+                        String::new()
+                    } else if visitor.indexed_mut.contains(&indexed) && contains_name(indexed, take_expr) {
+                        return;
+                    } else {
+                        match limits {
+                            ast::RangeLimits::Closed => {
+                                let take_expr = sugg::Sugg::hir(cx, take_expr, "<count>");
+                                format!(".take({})", take_expr + sugg::ONE)
+                            },
+                            ast::RangeLimits::HalfOpen => format!(".take({})", snippet(cx, take_expr.span, "..")),
+                        }
+                    }
+                } else {
+                    String::new()
+                };
+
+                let (ref_mut, method) = if visitor.indexed_mut.contains(&indexed) {
+                    ("mut ", "iter_mut")
+                } else {
+                    ("", "iter")
+                };
+
+                let take_is_empty = take.is_empty();
+                let mut method_1 = take;
+                let mut method_2 = skip;
+
+                if end_is_start_plus_val {
+                    mem::swap(&mut method_1, &mut method_2);
+                }
+
+                if visitor.nonindex {
+                    span_lint_and_then(
+                        cx,
+                        NEEDLESS_RANGE_LOOP,
+                        expr.span,
+                        &format!("the loop variable `{}` is used to index `{}`", ident.name, indexed),
+                        |diag| {
+                            multispan_sugg(
+                                diag,
+                                "consider using an iterator",
+                                vec![
+                                    (pat.span, format!("({}, <item>)", ident.name)),
+                                    (
+                                        arg.span,
+                                        format!("{}.{}().enumerate(){}{}", indexed, method, method_1, method_2),
+                                    ),
+                                ],
+                            );
+                        },
+                    );
+                } else {
+                    let repl = if starts_at_zero && take_is_empty {
+                        format!("&{}{}", ref_mut, indexed)
+                    } else {
+                        format!("{}.{}(){}{}", indexed, method, method_1, method_2)
+                    };
+
+                    span_lint_and_then(
+                        cx,
+                        NEEDLESS_RANGE_LOOP,
+                        expr.span,
+                        &format!("the loop variable `{}` is only used to index `{}`", ident.name, indexed),
+                        |diag| {
+                            multispan_sugg(
+                                diag,
+                                "consider using an iterator",
+                                vec![(pat.span, "<item>".to_string()), (arg.span, repl)],
+                            );
+                        },
+                    );
+                }
+            }
+        }
+    }
+}
+
+fn is_len_call(expr: &Expr<'_>, var: Symbol) -> bool {
+    if_chain! {
+        if let ExprKind::MethodCall(ref method, _, ref len_args, _) = expr.kind;
+        if len_args.len() == 1;
+        if method.ident.name == sym!(len);
+        if let ExprKind::Path(QPath::Resolved(_, ref path)) = len_args[0].kind;
+        if path.segments.len() == 1;
+        if path.segments[0].ident.name == var;
+        then {
+            return true;
+        }
+    }
+
+    false
+}
+
+fn is_end_eq_array_len<'tcx>(
+    cx: &LateContext<'tcx>,
+    end: &Expr<'_>,
+    limits: ast::RangeLimits,
+    indexed_ty: Ty<'tcx>,
+) -> bool {
+    if_chain! {
+        if let ExprKind::Lit(ref lit) = end.kind;
+        if let ast::LitKind::Int(end_int, _) = lit.node;
+        if let ty::Array(_, arr_len_const) = indexed_ty.kind();
+        if let Some(arr_len) = arr_len_const.try_eval_usize(cx.tcx, cx.param_env);
+        then {
+            return match limits {
+                ast::RangeLimits::Closed => end_int + 1 >= arr_len.into(),
+                ast::RangeLimits::HalfOpen => end_int >= arr_len.into(),
+            };
+        }
+    }
+
+    false
+}
+
+struct VarVisitor<'a, 'tcx> {
+    /// context reference
+    cx: &'a LateContext<'tcx>,
+    /// var name to look for as index
+    var: HirId,
+    /// indexed variables that are used mutably
+    indexed_mut: FxHashSet<Symbol>,
+    /// indirectly indexed variables (`v[(i + 4) % N]`), the extend is `None` for global
+    indexed_indirectly: FxHashMap<Symbol, Option<region::Scope>>,
+    /// subset of `indexed` of vars that are indexed directly: `v[i]`
+    /// this will not contain cases like `v[calc_index(i)]` or `v[(i + 4) % N]`
+    indexed_directly: FxHashMap<Symbol, (Option<region::Scope>, Ty<'tcx>)>,
+    /// Any names that are used outside an index operation.
+    /// Used to detect things like `&mut vec` used together with `vec[i]`
+    referenced: FxHashSet<Symbol>,
+    /// has the loop variable been used in expressions other than the index of
+    /// an index op?
+    nonindex: bool,
+    /// Whether we are inside the `$` in `&mut $` or `$ = foo` or `$.bar`, where bar
+    /// takes `&mut self`
+    prefer_mutable: bool,
+}
+
+impl<'a, 'tcx> VarVisitor<'a, 'tcx> {
+    fn check(&mut self, idx: &'tcx Expr<'_>, seqexpr: &'tcx Expr<'_>, expr: &'tcx Expr<'_>) -> bool {
+        if_chain! {
+            // the indexed container is referenced by a name
+            if let ExprKind::Path(ref seqpath) = seqexpr.kind;
+            if let QPath::Resolved(None, ref seqvar) = *seqpath;
+            if seqvar.segments.len() == 1;
+            then {
+                let index_used_directly = path_to_local_id(idx, self.var);
+                let indexed_indirectly = {
+                    let mut used_visitor = LocalUsedVisitor::new(self.cx, self.var);
+                    walk_expr(&mut used_visitor, idx);
+                    used_visitor.used
+                };
+
+                if indexed_indirectly || index_used_directly {
+                    if self.prefer_mutable {
+                        self.indexed_mut.insert(seqvar.segments[0].ident.name);
+                    }
+                    let res = self.cx.qpath_res(seqpath, seqexpr.hir_id);
+                    match res {
+                        Res::Local(hir_id) => {
+                            let parent_id = self.cx.tcx.hir().get_parent_item(expr.hir_id);
+                            let parent_def_id = self.cx.tcx.hir().local_def_id(parent_id);
+                            let extent = self.cx.tcx.region_scope_tree(parent_def_id).var_scope(hir_id.local_id);
+                            if indexed_indirectly {
+                                self.indexed_indirectly.insert(seqvar.segments[0].ident.name, Some(extent));
+                            }
+                            if index_used_directly {
+                                self.indexed_directly.insert(
+                                    seqvar.segments[0].ident.name,
+                                    (Some(extent), self.cx.typeck_results().node_type(seqexpr.hir_id)),
+                                );
+                            }
+                            return false;  // no need to walk further *on the variable*
+                        }
+                        Res::Def(DefKind::Static | DefKind::Const, ..) => {
+                            if indexed_indirectly {
+                                self.indexed_indirectly.insert(seqvar.segments[0].ident.name, None);
+                            }
+                            if index_used_directly {
+                                self.indexed_directly.insert(
+                                    seqvar.segments[0].ident.name,
+                                    (None, self.cx.typeck_results().node_type(seqexpr.hir_id)),
+                                );
+                            }
+                            return false;  // no need to walk further *on the variable*
+                        }
+                        _ => (),
+                    }
+                }
+            }
+        }
+        true
+    }
+}
+
+impl<'a, 'tcx> Visitor<'tcx> for VarVisitor<'a, 'tcx> {
+    type Map = Map<'tcx>;
+
+    fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
+        if_chain! {
+            // a range index op
+            if let ExprKind::MethodCall(ref meth, _, ref args, _) = expr.kind;
+            if (meth.ident.name == sym::index && match_trait_method(self.cx, expr, &paths::INDEX))
+                || (meth.ident.name == sym::index_mut && match_trait_method(self.cx, expr, &paths::INDEX_MUT));
+            if !self.check(&args[1], &args[0], expr);
+            then { return }
+        }
+
+        if_chain! {
+            // an index op
+            if let ExprKind::Index(ref seqexpr, ref idx) = expr.kind;
+            if !self.check(idx, seqexpr, expr);
+            then { return }
+        }
+
+        if_chain! {
+            // directly using a variable
+            if let ExprKind::Path(QPath::Resolved(None, path)) = expr.kind;
+            if let Res::Local(local_id) = path.res;
+            then {
+                if local_id == self.var {
+                    self.nonindex = true;
+                } else {
+                    // not the correct variable, but still a variable
+                    self.referenced.insert(path.segments[0].ident.name);
+                }
+            }
+        }
+
+        let old = self.prefer_mutable;
+        match expr.kind {
+            ExprKind::AssignOp(_, ref lhs, ref rhs) | ExprKind::Assign(ref lhs, ref rhs, _) => {
+                self.prefer_mutable = true;
+                self.visit_expr(lhs);
+                self.prefer_mutable = false;
+                self.visit_expr(rhs);
+            },
+            ExprKind::AddrOf(BorrowKind::Ref, mutbl, ref expr) => {
+                if mutbl == Mutability::Mut {
+                    self.prefer_mutable = true;
+                }
+                self.visit_expr(expr);
+            },
+            ExprKind::Call(ref f, args) => {
+                self.visit_expr(f);
+                for expr in args {
+                    let ty = self.cx.typeck_results().expr_ty_adjusted(expr);
+                    self.prefer_mutable = false;
+                    if let ty::Ref(_, _, mutbl) = *ty.kind() {
+                        if mutbl == Mutability::Mut {
+                            self.prefer_mutable = true;
+                        }
+                    }
+                    self.visit_expr(expr);
+                }
+            },
+            ExprKind::MethodCall(_, _, args, _) => {
+                let def_id = self.cx.typeck_results().type_dependent_def_id(expr.hir_id).unwrap();
+                for (ty, expr) in self.cx.tcx.fn_sig(def_id).inputs().skip_binder().iter().zip(args) {
+                    self.prefer_mutable = false;
+                    if let ty::Ref(_, _, mutbl) = *ty.kind() {
+                        if mutbl == Mutability::Mut {
+                            self.prefer_mutable = true;
+                        }
+                    }
+                    self.visit_expr(expr);
+                }
+            },
+            ExprKind::Closure(_, _, body_id, ..) => {
+                let body = self.cx.tcx.hir().body(body_id);
+                self.visit_expr(&body.value);
+            },
+            _ => walk_expr(self, expr),
+        }
+        self.prefer_mutable = old;
+    }
+    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
+        NestedVisitorMap::None
+    }
+}
diff --git a/clippy_lints/src/loops/never_loop.rs b/clippy_lints/src/loops/never_loop.rs
new file mode 100644
index 00000000000..45e1001d755
--- /dev/null
+++ b/clippy_lints/src/loops/never_loop.rs
@@ -0,0 +1,172 @@
+use super::NEVER_LOOP;
+use crate::utils::span_lint;
+use rustc_hir::{Block, Expr, ExprKind, HirId, InlineAsmOperand, Stmt, StmtKind};
+use rustc_lint::LateContext;
+use std::iter::{once, Iterator};
+
+pub(super) fn check(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+    if let ExprKind::Loop(ref block, _, _, _) = expr.kind {
+        match never_loop_block(block, expr.hir_id) {
+            NeverLoopResult::AlwaysBreak => span_lint(cx, NEVER_LOOP, expr.span, "this loop never actually loops"),
+            NeverLoopResult::MayContinueMainLoop | NeverLoopResult::Otherwise => (),
+        }
+    }
+}
+
+enum NeverLoopResult {
+    // A break/return always get triggered but not necessarily for the main loop.
+    AlwaysBreak,
+    // A continue may occur for the main loop.
+    MayContinueMainLoop,
+    Otherwise,
+}
+
+#[must_use]
+fn absorb_break(arg: &NeverLoopResult) -> NeverLoopResult {
+    match *arg {
+        NeverLoopResult::AlwaysBreak | NeverLoopResult::Otherwise => NeverLoopResult::Otherwise,
+        NeverLoopResult::MayContinueMainLoop => NeverLoopResult::MayContinueMainLoop,
+    }
+}
+
+// Combine two results for parts that are called in order.
+#[must_use]
+fn combine_seq(first: NeverLoopResult, second: NeverLoopResult) -> NeverLoopResult {
+    match first {
+        NeverLoopResult::AlwaysBreak | NeverLoopResult::MayContinueMainLoop => first,
+        NeverLoopResult::Otherwise => second,
+    }
+}
+
+// Combine two results where both parts are called but not necessarily in order.
+#[must_use]
+fn combine_both(left: NeverLoopResult, right: NeverLoopResult) -> NeverLoopResult {
+    match (left, right) {
+        (NeverLoopResult::MayContinueMainLoop, _) | (_, NeverLoopResult::MayContinueMainLoop) => {
+            NeverLoopResult::MayContinueMainLoop
+        },
+        (NeverLoopResult::AlwaysBreak, _) | (_, NeverLoopResult::AlwaysBreak) => NeverLoopResult::AlwaysBreak,
+        (NeverLoopResult::Otherwise, NeverLoopResult::Otherwise) => NeverLoopResult::Otherwise,
+    }
+}
+
+// Combine two results where only one of the part may have been executed.
+#[must_use]
+fn combine_branches(b1: NeverLoopResult, b2: NeverLoopResult) -> NeverLoopResult {
+    match (b1, b2) {
+        (NeverLoopResult::AlwaysBreak, NeverLoopResult::AlwaysBreak) => NeverLoopResult::AlwaysBreak,
+        (NeverLoopResult::MayContinueMainLoop, _) | (_, NeverLoopResult::MayContinueMainLoop) => {
+            NeverLoopResult::MayContinueMainLoop
+        },
+        (NeverLoopResult::Otherwise, _) | (_, NeverLoopResult::Otherwise) => NeverLoopResult::Otherwise,
+    }
+}
+
+fn never_loop_block(block: &Block<'_>, main_loop_id: HirId) -> NeverLoopResult {
+    let stmts = block.stmts.iter().map(stmt_to_expr);
+    let expr = once(block.expr.as_deref());
+    let mut iter = stmts.chain(expr).flatten();
+    never_loop_expr_seq(&mut iter, main_loop_id)
+}
+
+fn never_loop_expr_seq<'a, T: Iterator<Item = &'a Expr<'a>>>(es: &mut T, main_loop_id: HirId) -> NeverLoopResult {
+    es.map(|e| never_loop_expr(e, main_loop_id))
+        .fold(NeverLoopResult::Otherwise, combine_seq)
+}
+
+fn stmt_to_expr<'tcx>(stmt: &Stmt<'tcx>) -> Option<&'tcx Expr<'tcx>> {
+    match stmt.kind {
+        StmtKind::Semi(ref e, ..) | StmtKind::Expr(ref e, ..) => Some(e),
+        StmtKind::Local(ref local) => local.init.as_deref(),
+        _ => None,
+    }
+}
+
+fn never_loop_expr(expr: &Expr<'_>, main_loop_id: HirId) -> NeverLoopResult {
+    match expr.kind {
+        ExprKind::Box(ref e)
+        | ExprKind::Unary(_, ref e)
+        | ExprKind::Cast(ref e, _)
+        | ExprKind::Type(ref e, _)
+        | ExprKind::Field(ref e, _)
+        | ExprKind::AddrOf(_, _, ref e)
+        | ExprKind::Struct(_, _, Some(ref e))
+        | ExprKind::Repeat(ref e, _)
+        | ExprKind::DropTemps(ref e) => never_loop_expr(e, main_loop_id),
+        ExprKind::Array(ref es) | ExprKind::MethodCall(_, _, ref es, _) | ExprKind::Tup(ref es) => {
+            never_loop_expr_all(&mut es.iter(), main_loop_id)
+        },
+        ExprKind::Call(ref e, ref es) => never_loop_expr_all(&mut once(&**e).chain(es.iter()), main_loop_id),
+        ExprKind::Binary(_, ref e1, ref e2)
+        | ExprKind::Assign(ref e1, ref e2, _)
+        | ExprKind::AssignOp(_, ref e1, ref e2)
+        | ExprKind::Index(ref e1, ref e2) => never_loop_expr_all(&mut [&**e1, &**e2].iter().cloned(), main_loop_id),
+        ExprKind::Loop(ref b, _, _, _) => {
+            // Break can come from the inner loop so remove them.
+            absorb_break(&never_loop_block(b, main_loop_id))
+        },
+        ExprKind::If(ref e, ref e2, ref e3) => {
+            let e1 = never_loop_expr(e, main_loop_id);
+            let e2 = never_loop_expr(e2, main_loop_id);
+            let e3 = e3
+                .as_ref()
+                .map_or(NeverLoopResult::Otherwise, |e| never_loop_expr(e, main_loop_id));
+            combine_seq(e1, combine_branches(e2, e3))
+        },
+        ExprKind::Match(ref e, ref arms, _) => {
+            let e = never_loop_expr(e, main_loop_id);
+            if arms.is_empty() {
+                e
+            } else {
+                let arms = never_loop_expr_branch(&mut arms.iter().map(|a| &*a.body), main_loop_id);
+                combine_seq(e, arms)
+            }
+        },
+        ExprKind::Block(ref b, _) => never_loop_block(b, main_loop_id),
+        ExprKind::Continue(d) => {
+            let id = d
+                .target_id
+                .expect("target ID can only be missing in the presence of compilation errors");
+            if id == main_loop_id {
+                NeverLoopResult::MayContinueMainLoop
+            } else {
+                NeverLoopResult::AlwaysBreak
+            }
+        },
+        ExprKind::Break(_, ref e) | ExprKind::Ret(ref e) => e.as_ref().map_or(NeverLoopResult::AlwaysBreak, |e| {
+            combine_seq(never_loop_expr(e, main_loop_id), NeverLoopResult::AlwaysBreak)
+        }),
+        ExprKind::InlineAsm(ref asm) => asm
+            .operands
+            .iter()
+            .map(|(o, _)| match o {
+                InlineAsmOperand::In { expr, .. }
+                | InlineAsmOperand::InOut { expr, .. }
+                | InlineAsmOperand::Const { expr }
+                | InlineAsmOperand::Sym { expr } => never_loop_expr(expr, main_loop_id),
+                InlineAsmOperand::Out { expr, .. } => never_loop_expr_all(&mut expr.iter(), main_loop_id),
+                InlineAsmOperand::SplitInOut { in_expr, out_expr, .. } => {
+                    never_loop_expr_all(&mut once(in_expr).chain(out_expr.iter()), main_loop_id)
+                },
+            })
+            .fold(NeverLoopResult::Otherwise, combine_both),
+        ExprKind::Struct(_, _, None)
+        | ExprKind::Yield(_, _)
+        | ExprKind::Closure(_, _, _, _, _)
+        | ExprKind::LlvmInlineAsm(_)
+        | ExprKind::Path(_)
+        | ExprKind::ConstBlock(_)
+        | ExprKind::Lit(_)
+        | ExprKind::Err => NeverLoopResult::Otherwise,
+    }
+}
+
+fn never_loop_expr_all<'a, T: Iterator<Item = &'a Expr<'a>>>(es: &mut T, main_loop_id: HirId) -> NeverLoopResult {
+    es.map(|e| never_loop_expr(e, main_loop_id))
+        .fold(NeverLoopResult::Otherwise, combine_both)
+}
+
+fn never_loop_expr_branch<'a, T: Iterator<Item = &'a Expr<'a>>>(e: &mut T, main_loop_id: HirId) -> NeverLoopResult {
+    e.map(|e| never_loop_expr(e, main_loop_id))
+        .fold(NeverLoopResult::AlwaysBreak, combine_branches)
+}
diff --git a/clippy_lints/src/loops/same_item_push.rs b/clippy_lints/src/loops/same_item_push.rs
new file mode 100644
index 00000000000..f3585830e4a
--- /dev/null
+++ b/clippy_lints/src/loops/same_item_push.rs
@@ -0,0 +1,169 @@
+use super::SAME_ITEM_PUSH;
+use crate::utils::{implements_trait, is_type_diagnostic_item, snippet_with_macro_callsite, span_lint_and_help};
+use if_chain::if_chain;
+use rustc_hir::def::{DefKind, Res};
+use rustc_hir::intravisit::{walk_expr, NestedVisitorMap, Visitor};
+use rustc_hir::{BindingAnnotation, Block, Expr, ExprKind, Node, Pat, PatKind, Stmt, StmtKind};
+use rustc_lint::LateContext;
+use rustc_middle::hir::map::Map;
+use rustc_span::symbol::sym;
+use std::iter::Iterator;
+
+/// Detects for loop pushing the same item into a Vec
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    pat: &'tcx Pat<'_>,
+    _: &'tcx Expr<'_>,
+    body: &'tcx Expr<'_>,
+    _: &'tcx Expr<'_>,
+) {
+    fn emit_lint(cx: &LateContext<'_>, vec: &Expr<'_>, pushed_item: &Expr<'_>) {
+        let vec_str = snippet_with_macro_callsite(cx, vec.span, "");
+        let item_str = snippet_with_macro_callsite(cx, pushed_item.span, "");
+
+        span_lint_and_help(
+            cx,
+            SAME_ITEM_PUSH,
+            vec.span,
+            "it looks like the same item is being pushed into this Vec",
+            None,
+            &format!(
+                "try using vec![{};SIZE] or {}.resize(NEW_SIZE, {})",
+                item_str, vec_str, item_str
+            ),
+        )
+    }
+
+    if !matches!(pat.kind, PatKind::Wild) {
+        return;
+    }
+
+    // Determine whether it is safe to lint the body
+    let mut same_item_push_visitor = SameItemPushVisitor {
+        should_lint: true,
+        vec_push: None,
+        cx,
+    };
+    walk_expr(&mut same_item_push_visitor, body);
+    if same_item_push_visitor.should_lint {
+        if let Some((vec, pushed_item)) = same_item_push_visitor.vec_push {
+            let vec_ty = cx.typeck_results().expr_ty(vec);
+            let ty = vec_ty.walk().nth(1).unwrap().expect_ty();
+            if cx
+                .tcx
+                .lang_items()
+                .clone_trait()
+                .map_or(false, |id| implements_trait(cx, ty, id, &[]))
+            {
+                // Make sure that the push does not involve possibly mutating values
+                match pushed_item.kind {
+                    ExprKind::Path(ref qpath) => {
+                        match cx.qpath_res(qpath, pushed_item.hir_id) {
+                            // immutable bindings that are initialized with literal or constant
+                            Res::Local(hir_id) => {
+                                if_chain! {
+                                    let node = cx.tcx.hir().get(hir_id);
+                                    if let Node::Binding(pat) = node;
+                                    if let PatKind::Binding(bind_ann, ..) = pat.kind;
+                                    if !matches!(bind_ann, BindingAnnotation::RefMut | BindingAnnotation::Mutable);
+                                    let parent_node = cx.tcx.hir().get_parent_node(hir_id);
+                                    if let Some(Node::Local(parent_let_expr)) = cx.tcx.hir().find(parent_node);
+                                    if let Some(init) = parent_let_expr.init;
+                                    then {
+                                        match init.kind {
+                                            // immutable bindings that are initialized with literal
+                                            ExprKind::Lit(..) => emit_lint(cx, vec, pushed_item),
+                                            // immutable bindings that are initialized with constant
+                                            ExprKind::Path(ref path) => {
+                                                if let Res::Def(DefKind::Const, ..) = cx.qpath_res(path, init.hir_id) {
+                                                    emit_lint(cx, vec, pushed_item);
+                                                }
+                                            }
+                                            _ => {},
+                                        }
+                                    }
+                                }
+                            },
+                            // constant
+                            Res::Def(DefKind::Const, ..) => emit_lint(cx, vec, pushed_item),
+                            _ => {},
+                        }
+                    },
+                    ExprKind::Lit(..) => emit_lint(cx, vec, pushed_item),
+                    _ => {},
+                }
+            }
+        }
+    }
+}
+
+// Scans the body of the for loop and determines whether lint should be given
+struct SameItemPushVisitor<'a, 'tcx> {
+    should_lint: bool,
+    // this field holds the last vec push operation visited, which should be the only push seen
+    vec_push: Option<(&'tcx Expr<'tcx>, &'tcx Expr<'tcx>)>,
+    cx: &'a LateContext<'tcx>,
+}
+
+impl<'a, 'tcx> Visitor<'tcx> for SameItemPushVisitor<'a, 'tcx> {
+    type Map = Map<'tcx>;
+
+    fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
+        match &expr.kind {
+            // Non-determinism may occur ... don't give a lint
+            ExprKind::Loop(..) | ExprKind::Match(..) => self.should_lint = false,
+            ExprKind::Block(block, _) => self.visit_block(block),
+            _ => {},
+        }
+    }
+
+    fn visit_block(&mut self, b: &'tcx Block<'_>) {
+        for stmt in b.stmts.iter() {
+            self.visit_stmt(stmt);
+        }
+    }
+
+    fn visit_stmt(&mut self, s: &'tcx Stmt<'_>) {
+        let vec_push_option = get_vec_push(self.cx, s);
+        if vec_push_option.is_none() {
+            // Current statement is not a push so visit inside
+            match &s.kind {
+                StmtKind::Expr(expr) | StmtKind::Semi(expr) => self.visit_expr(&expr),
+                _ => {},
+            }
+        } else {
+            // Current statement is a push ...check whether another
+            // push had been previously done
+            if self.vec_push.is_none() {
+                self.vec_push = vec_push_option;
+            } else {
+                // There are multiple pushes ... don't lint
+                self.should_lint = false;
+            }
+        }
+    }
+
+    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
+        NestedVisitorMap::None
+    }
+}
+
+// Given some statement, determine if that statement is a push on a Vec. If it is, return
+// the Vec being pushed into and the item being pushed
+fn get_vec_push<'tcx>(cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'_>) -> Option<(&'tcx Expr<'tcx>, &'tcx Expr<'tcx>)> {
+    if_chain! {
+            // Extract method being called
+            if let StmtKind::Semi(semi_stmt) = &stmt.kind;
+            if let ExprKind::MethodCall(path, _, args, _) = &semi_stmt.kind;
+            // Figure out the parameters for the method call
+            if let Some(self_expr) = args.get(0);
+            if let Some(pushed_item) = args.get(1);
+            // Check that the method being called is push() on a Vec
+            if is_type_diagnostic_item(cx, cx.typeck_results().expr_ty(self_expr), sym::vec_type);
+            if path.ident.name.as_str() == "push";
+            then {
+                return Some((self_expr, pushed_item))
+            }
+    }
+    None
+}
diff --git a/clippy_lints/src/loops/single_element_loop.rs b/clippy_lints/src/loops/single_element_loop.rs
new file mode 100644
index 00000000000..38400c93c9a
--- /dev/null
+++ b/clippy_lints/src/loops/single_element_loop.rs
@@ -0,0 +1,42 @@
+use super::{get_span_of_entire_for_loop, SINGLE_ELEMENT_LOOP};
+use crate::utils::{indent_of, single_segment_path, snippet, span_lint_and_sugg};
+use if_chain::if_chain;
+use rustc_errors::Applicability;
+use rustc_hir::{BorrowKind, Expr, ExprKind, Pat, PatKind};
+use rustc_lint::LateContext;
+
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    pat: &'tcx Pat<'_>,
+    arg: &'tcx Expr<'_>,
+    body: &'tcx Expr<'_>,
+    expr: &'tcx Expr<'_>,
+) {
+    if_chain! {
+        if let ExprKind::AddrOf(BorrowKind::Ref, _, ref arg_expr) = arg.kind;
+        if let PatKind::Binding(.., target, _) = pat.kind;
+        if let ExprKind::Array([arg_expression]) = arg_expr.kind;
+        if let ExprKind::Path(ref list_item) = arg_expression.kind;
+        if let Some(list_item_name) = single_segment_path(list_item).map(|ps| ps.ident.name);
+        if let ExprKind::Block(ref block, _) = body.kind;
+        if !block.stmts.is_empty();
+
+        then {
+            let for_span = get_span_of_entire_for_loop(expr);
+            let mut block_str = snippet(cx, block.span, "..").into_owned();
+            block_str.remove(0);
+            block_str.pop();
+
+
+            span_lint_and_sugg(
+                cx,
+                SINGLE_ELEMENT_LOOP,
+                for_span,
+                "for loop over a single element",
+                "try",
+                format!("{{\n{}let {} = &{};{}}}", " ".repeat(indent_of(cx, block.stmts[0].span).unwrap_or(0)), target.name, list_item_name, block_str),
+                Applicability::MachineApplicable
+            )
+        }
+    }
+}
diff --git a/clippy_lints/src/loops/utils.rs b/clippy_lints/src/loops/utils.rs
new file mode 100644
index 00000000000..9e38e17719a
--- /dev/null
+++ b/clippy_lints/src/loops/utils.rs
@@ -0,0 +1,350 @@
+use crate::utils::{
+    get_parent_expr, get_trait_def_id, has_iter_method, implements_trait, is_integer_const, path_to_local,
+    path_to_local_id, paths, sugg,
+};
+use if_chain::if_chain;
+use rustc_data_structures::fx::FxHashMap;
+use rustc_errors::Applicability;
+use rustc_hir::intravisit::{walk_expr, walk_pat, walk_stmt, NestedVisitorMap, Visitor};
+use rustc_hir::{BinOpKind, BorrowKind, Expr, ExprKind, HirId, Mutability, Pat, PatKind, Stmt, StmtKind};
+use rustc_lint::LateContext;
+use rustc_middle::hir::map::Map;
+use rustc_span::source_map::Span;
+use rustc_span::symbol::Symbol;
+use std::iter::Iterator;
+
+#[derive(Debug, PartialEq)]
+enum IncrementVisitorVarState {
+    Initial,  // Not examined yet
+    IncrOnce, // Incremented exactly once, may be a loop counter
+    DontWarn,
+}
+
+/// Scan a for loop for variables that are incremented exactly once and not used after that.
+pub(super) struct IncrementVisitor<'a, 'tcx> {
+    cx: &'a LateContext<'tcx>,                          // context reference
+    states: FxHashMap<HirId, IncrementVisitorVarState>, // incremented variables
+    depth: u32,                                         // depth of conditional expressions
+    done: bool,
+}
+
+impl<'a, 'tcx> IncrementVisitor<'a, 'tcx> {
+    pub(super) fn new(cx: &'a LateContext<'tcx>) -> Self {
+        Self {
+            cx,
+            states: FxHashMap::default(),
+            depth: 0,
+            done: false,
+        }
+    }
+
+    pub(super) fn into_results(self) -> impl Iterator<Item = HirId> {
+        self.states.into_iter().filter_map(|(id, state)| {
+            if state == IncrementVisitorVarState::IncrOnce {
+                Some(id)
+            } else {
+                None
+            }
+        })
+    }
+}
+
+impl<'a, 'tcx> Visitor<'tcx> for IncrementVisitor<'a, 'tcx> {
+    type Map = Map<'tcx>;
+
+    fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
+        if self.done {
+            return;
+        }
+
+        // If node is a variable
+        if let Some(def_id) = path_to_local(expr) {
+            if let Some(parent) = get_parent_expr(self.cx, expr) {
+                let state = self.states.entry(def_id).or_insert(IncrementVisitorVarState::Initial);
+                if *state == IncrementVisitorVarState::IncrOnce {
+                    *state = IncrementVisitorVarState::DontWarn;
+                    return;
+                }
+
+                match parent.kind {
+                    ExprKind::AssignOp(op, ref lhs, ref rhs) => {
+                        if lhs.hir_id == expr.hir_id {
+                            *state = if op.node == BinOpKind::Add
+                                && is_integer_const(self.cx, rhs, 1)
+                                && *state == IncrementVisitorVarState::Initial
+                                && self.depth == 0
+                            {
+                                IncrementVisitorVarState::IncrOnce
+                            } else {
+                                // Assigned some other value or assigned multiple times
+                                IncrementVisitorVarState::DontWarn
+                            };
+                        }
+                    },
+                    ExprKind::Assign(ref lhs, _, _) if lhs.hir_id == expr.hir_id => {
+                        *state = IncrementVisitorVarState::DontWarn
+                    },
+                    ExprKind::AddrOf(BorrowKind::Ref, mutability, _) if mutability == Mutability::Mut => {
+                        *state = IncrementVisitorVarState::DontWarn
+                    },
+                    _ => (),
+                }
+            }
+
+            walk_expr(self, expr);
+        } else if is_loop(expr) || is_conditional(expr) {
+            self.depth += 1;
+            walk_expr(self, expr);
+            self.depth -= 1;
+        } else if let ExprKind::Continue(_) = expr.kind {
+            self.done = true;
+        } else {
+            walk_expr(self, expr);
+        }
+    }
+    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
+        NestedVisitorMap::None
+    }
+}
+
+enum InitializeVisitorState<'hir> {
+    Initial,          // Not examined yet
+    Declared(Symbol), // Declared but not (yet) initialized
+    Initialized {
+        name: Symbol,
+        initializer: &'hir Expr<'hir>,
+    },
+    DontWarn,
+}
+
+/// Checks whether a variable is initialized at the start of a loop and not modified
+/// and used after the loop.
+pub(super) struct InitializeVisitor<'a, 'tcx> {
+    cx: &'a LateContext<'tcx>,  // context reference
+    end_expr: &'tcx Expr<'tcx>, // the for loop. Stop scanning here.
+    var_id: HirId,
+    state: InitializeVisitorState<'tcx>,
+    depth: u32, // depth of conditional expressions
+    past_loop: bool,
+}
+
+impl<'a, 'tcx> InitializeVisitor<'a, 'tcx> {
+    pub(super) fn new(cx: &'a LateContext<'tcx>, end_expr: &'tcx Expr<'tcx>, var_id: HirId) -> Self {
+        Self {
+            cx,
+            end_expr,
+            var_id,
+            state: InitializeVisitorState::Initial,
+            depth: 0,
+            past_loop: false,
+        }
+    }
+
+    pub(super) fn get_result(&self) -> Option<(Symbol, &'tcx Expr<'tcx>)> {
+        if let InitializeVisitorState::Initialized { name, initializer } = self.state {
+            Some((name, initializer))
+        } else {
+            None
+        }
+    }
+}
+
+impl<'a, 'tcx> Visitor<'tcx> for InitializeVisitor<'a, 'tcx> {
+    type Map = Map<'tcx>;
+
+    fn visit_stmt(&mut self, stmt: &'tcx Stmt<'_>) {
+        // Look for declarations of the variable
+        if_chain! {
+            if let StmtKind::Local(ref local) = stmt.kind;
+            if local.pat.hir_id == self.var_id;
+            if let PatKind::Binding(.., ident, _) = local.pat.kind;
+            then {
+                self.state = local.init.map_or(InitializeVisitorState::Declared(ident.name), |init| {
+                    InitializeVisitorState::Initialized {
+                        initializer: init,
+                        name: ident.name,
+                    }
+                })
+            }
+        }
+        walk_stmt(self, stmt);
+    }
+
+    fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
+        if matches!(self.state, InitializeVisitorState::DontWarn) {
+            return;
+        }
+        if expr.hir_id == self.end_expr.hir_id {
+            self.past_loop = true;
+            return;
+        }
+        // No need to visit expressions before the variable is
+        // declared
+        if matches!(self.state, InitializeVisitorState::Initial) {
+            return;
+        }
+
+        // If node is the desired variable, see how it's used
+        if path_to_local_id(expr, self.var_id) {
+            if self.past_loop {
+                self.state = InitializeVisitorState::DontWarn;
+                return;
+            }
+
+            if let Some(parent) = get_parent_expr(self.cx, expr) {
+                match parent.kind {
+                    ExprKind::AssignOp(_, ref lhs, _) if lhs.hir_id == expr.hir_id => {
+                        self.state = InitializeVisitorState::DontWarn;
+                    },
+                    ExprKind::Assign(ref lhs, ref rhs, _) if lhs.hir_id == expr.hir_id => {
+                        self.state = if_chain! {
+                            if self.depth == 0;
+                            if let InitializeVisitorState::Declared(name)
+                                | InitializeVisitorState::Initialized { name, ..} = self.state;
+                            then {
+                                InitializeVisitorState::Initialized { initializer: rhs, name }
+                            } else {
+                                InitializeVisitorState::DontWarn
+                            }
+                        }
+                    },
+                    ExprKind::AddrOf(BorrowKind::Ref, mutability, _) if mutability == Mutability::Mut => {
+                        self.state = InitializeVisitorState::DontWarn
+                    },
+                    _ => (),
+                }
+            }
+
+            walk_expr(self, expr);
+        } else if !self.past_loop && is_loop(expr) {
+            self.state = InitializeVisitorState::DontWarn;
+        } else if is_conditional(expr) {
+            self.depth += 1;
+            walk_expr(self, expr);
+            self.depth -= 1;
+        } else {
+            walk_expr(self, expr);
+        }
+    }
+
+    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
+        NestedVisitorMap::OnlyBodies(self.cx.tcx.hir())
+    }
+}
+
+fn is_loop(expr: &Expr<'_>) -> bool {
+    matches!(expr.kind, ExprKind::Loop(..))
+}
+
+fn is_conditional(expr: &Expr<'_>) -> bool {
+    matches!(expr.kind, ExprKind::If(..) | ExprKind::Match(..))
+}
+
+#[derive(PartialEq, Eq)]
+pub(super) enum Nesting {
+    Unknown,     // no nesting detected yet
+    RuledOut,    // the iterator is initialized or assigned within scope
+    LookFurther, // no nesting detected, no further walk required
+}
+
+use self::Nesting::{LookFurther, RuledOut, Unknown};
+
+pub(super) struct LoopNestVisitor {
+    pub(super) hir_id: HirId,
+    pub(super) iterator: HirId,
+    pub(super) nesting: Nesting,
+}
+
+impl<'tcx> Visitor<'tcx> for LoopNestVisitor {
+    type Map = Map<'tcx>;
+
+    fn visit_stmt(&mut self, stmt: &'tcx Stmt<'_>) {
+        if stmt.hir_id == self.hir_id {
+            self.nesting = LookFurther;
+        } else if self.nesting == Unknown {
+            walk_stmt(self, stmt);
+        }
+    }
+
+    fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
+        if self.nesting != Unknown {
+            return;
+        }
+        if expr.hir_id == self.hir_id {
+            self.nesting = LookFurther;
+            return;
+        }
+        match expr.kind {
+            ExprKind::Assign(ref path, _, _) | ExprKind::AssignOp(_, ref path, _) => {
+                if path_to_local_id(path, self.iterator) {
+                    self.nesting = RuledOut;
+                }
+            },
+            _ => walk_expr(self, expr),
+        }
+    }
+
+    fn visit_pat(&mut self, pat: &'tcx Pat<'_>) {
+        if self.nesting != Unknown {
+            return;
+        }
+        if let PatKind::Binding(_, id, ..) = pat.kind {
+            if id == self.iterator {
+                self.nesting = RuledOut;
+                return;
+            }
+        }
+        walk_pat(self, pat)
+    }
+
+    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
+        NestedVisitorMap::None
+    }
+}
+
+// this function assumes the given expression is a `for` loop.
+pub(super) fn get_span_of_entire_for_loop(expr: &Expr<'_>) -> Span {
+    // for some reason this is the only way to get the `Span`
+    // of the entire `for` loop
+    if let ExprKind::Match(_, arms, _) = &expr.kind {
+        arms[0].body.span
+    } else {
+        unreachable!()
+    }
+}
+
+/// If `arg` was the argument to a `for` loop, return the "cleanest" way of writing the
+/// actual `Iterator` that the loop uses.
+pub(super) fn make_iterator_snippet(cx: &LateContext<'_>, arg: &Expr<'_>, applic_ref: &mut Applicability) -> String {
+    let impls_iterator = get_trait_def_id(cx, &paths::ITERATOR).map_or(false, |id| {
+        implements_trait(cx, cx.typeck_results().expr_ty(arg), id, &[])
+    });
+    if impls_iterator {
+        format!(
+            "{}",
+            sugg::Sugg::hir_with_applicability(cx, arg, "_", applic_ref).maybe_par()
+        )
+    } else {
+        // (&x).into_iter() ==> x.iter()
+        // (&mut x).into_iter() ==> x.iter_mut()
+        match &arg.kind {
+            ExprKind::AddrOf(BorrowKind::Ref, mutability, arg_inner)
+                if has_iter_method(cx, cx.typeck_results().expr_ty(&arg_inner)).is_some() =>
+            {
+                let meth_name = match mutability {
+                    Mutability::Mut => "iter_mut",
+                    Mutability::Not => "iter",
+                };
+                format!(
+                    "{}.{}()",
+                    sugg::Sugg::hir_with_applicability(cx, &arg_inner, "_", applic_ref).maybe_par(),
+                    meth_name,
+                )
+            }
+            _ => format!(
+                "{}.into_iter()",
+                sugg::Sugg::hir_with_applicability(cx, arg, "_", applic_ref).maybe_par()
+            ),
+        }
+    }
+}
diff --git a/clippy_lints/src/loops/while_immutable_condition.rs b/clippy_lints/src/loops/while_immutable_condition.rs
new file mode 100644
index 00000000000..05e0a722563
--- /dev/null
+++ b/clippy_lints/src/loops/while_immutable_condition.rs
@@ -0,0 +1,139 @@
+use super::WHILE_IMMUTABLE_CONDITION;
+use crate::consts::constant;
+use crate::utils::span_lint_and_then;
+use crate::utils::usage::mutated_variables;
+use if_chain::if_chain;
+use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_hir::def::{DefKind, Res};
+use rustc_hir::intravisit::{walk_expr, NestedVisitorMap, Visitor};
+use rustc_hir::{def_id, Expr, ExprKind, HirId, QPath};
+use rustc_lint::LateContext;
+use rustc_middle::hir::map::Map;
+use std::iter::Iterator;
+
+pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, cond: &'tcx Expr<'_>, expr: &'tcx Expr<'_>) {
+    if constant(cx, cx.typeck_results(), cond).is_some() {
+        // A pure constant condition (e.g., `while false`) is not linted.
+        return;
+    }
+
+    let mut var_visitor = VarCollectorVisitor {
+        cx,
+        ids: FxHashSet::default(),
+        def_ids: FxHashMap::default(),
+        skip: false,
+    };
+    var_visitor.visit_expr(cond);
+    if var_visitor.skip {
+        return;
+    }
+    let used_in_condition = &var_visitor.ids;
+    let no_cond_variable_mutated = if let Some(used_mutably) = mutated_variables(expr, cx) {
+        used_in_condition.is_disjoint(&used_mutably)
+    } else {
+        return;
+    };
+    let mutable_static_in_cond = var_visitor.def_ids.iter().any(|(_, v)| *v);
+
+    let mut has_break_or_return_visitor = HasBreakOrReturnVisitor {
+        has_break_or_return: false,
+    };
+    has_break_or_return_visitor.visit_expr(expr);
+    let has_break_or_return = has_break_or_return_visitor.has_break_or_return;
+
+    if no_cond_variable_mutated && !mutable_static_in_cond {
+        span_lint_and_then(
+            cx,
+            WHILE_IMMUTABLE_CONDITION,
+            cond.span,
+            "variables in the condition are not mutated in the loop body",
+            |diag| {
+                diag.note("this may lead to an infinite or to a never running loop");
+
+                if has_break_or_return {
+                    diag.note("this loop contains `return`s or `break`s");
+                    diag.help("rewrite it as `if cond { loop { } }`");
+                }
+            },
+        );
+    }
+}
+
+struct HasBreakOrReturnVisitor {
+    has_break_or_return: bool,
+}
+
+impl<'tcx> Visitor<'tcx> for HasBreakOrReturnVisitor {
+    type Map = Map<'tcx>;
+
+    fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
+        if self.has_break_or_return {
+            return;
+        }
+
+        match expr.kind {
+            ExprKind::Ret(_) | ExprKind::Break(_, _) => {
+                self.has_break_or_return = true;
+                return;
+            },
+            _ => {},
+        }
+
+        walk_expr(self, expr);
+    }
+
+    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
+        NestedVisitorMap::None
+    }
+}
+
+/// Collects the set of variables in an expression
+/// Stops analysis if a function call is found
+/// Note: In some cases such as `self`, there are no mutable annotation,
+/// All variables definition IDs are collected
+struct VarCollectorVisitor<'a, 'tcx> {
+    cx: &'a LateContext<'tcx>,
+    ids: FxHashSet<HirId>,
+    def_ids: FxHashMap<def_id::DefId, bool>,
+    skip: bool,
+}
+
+impl<'a, 'tcx> VarCollectorVisitor<'a, 'tcx> {
+    fn insert_def_id(&mut self, ex: &'tcx Expr<'_>) {
+        if_chain! {
+            if let ExprKind::Path(ref qpath) = ex.kind;
+            if let QPath::Resolved(None, _) = *qpath;
+            let res = self.cx.qpath_res(qpath, ex.hir_id);
+            then {
+                match res {
+                    Res::Local(hir_id) => {
+                        self.ids.insert(hir_id);
+                    },
+                    Res::Def(DefKind::Static, def_id) => {
+                        let mutable = self.cx.tcx.is_mutable_static(def_id);
+                        self.def_ids.insert(def_id, mutable);
+                    },
+                    _ => {},
+                }
+            }
+        }
+    }
+}
+
+impl<'a, 'tcx> Visitor<'tcx> for VarCollectorVisitor<'a, 'tcx> {
+    type Map = Map<'tcx>;
+
+    fn visit_expr(&mut self, ex: &'tcx Expr<'_>) {
+        match ex.kind {
+            ExprKind::Path(_) => self.insert_def_id(ex),
+            // If there is any function/method call… we just stop analysis
+            ExprKind::Call(..) | ExprKind::MethodCall(..) => self.skip = true,
+
+            _ => walk_expr(self, ex),
+        }
+    }
+
+    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
+        NestedVisitorMap::None
+    }
+}
diff --git a/clippy_lints/src/loops/while_let_loop.rs b/clippy_lints/src/loops/while_let_loop.rs
new file mode 100644
index 00000000000..65d8f2f1111
--- /dev/null
+++ b/clippy_lints/src/loops/while_let_loop.rs
@@ -0,0 +1,87 @@
+use super::WHILE_LET_LOOP;
+use crate::utils::{snippet_with_applicability, span_lint_and_sugg};
+use rustc_errors::Applicability;
+use rustc_hir::{Block, Expr, ExprKind, MatchSource, StmtKind};
+use rustc_lint::{LateContext, LintContext};
+use rustc_middle::lint::in_external_macro;
+
+pub(super) fn check(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, loop_block: &'tcx Block<'_>) {
+    // extract the expression from the first statement (if any) in a block
+    let inner_stmt_expr = extract_expr_from_first_stmt(loop_block);
+    // or extract the first expression (if any) from the block
+    if let Some(inner) = inner_stmt_expr.or_else(|| extract_first_expr(loop_block)) {
+        if let ExprKind::Match(ref matchexpr, ref arms, ref source) = inner.kind {
+            // ensure "if let" compatible match structure
+            match *source {
+                MatchSource::Normal | MatchSource::IfLetDesugar { .. } => {
+                    if arms.len() == 2
+                        && arms[0].guard.is_none()
+                        && arms[1].guard.is_none()
+                        && is_simple_break_expr(&arms[1].body)
+                    {
+                        if in_external_macro(cx.sess(), expr.span) {
+                            return;
+                        }
+
+                        // NOTE: we used to build a body here instead of using
+                        // ellipsis, this was removed because:
+                        // 1) it was ugly with big bodies;
+                        // 2) it was not indented properly;
+                        // 3) it wasn’t very smart (see #675).
+                        let mut applicability = Applicability::HasPlaceholders;
+                        span_lint_and_sugg(
+                            cx,
+                            WHILE_LET_LOOP,
+                            expr.span,
+                            "this loop could be written as a `while let` loop",
+                            "try",
+                            format!(
+                                "while let {} = {} {{ .. }}",
+                                snippet_with_applicability(cx, arms[0].pat.span, "..", &mut applicability),
+                                snippet_with_applicability(cx, matchexpr.span, "..", &mut applicability),
+                            ),
+                            applicability,
+                        );
+                    }
+                },
+                _ => (),
+            }
+        }
+    }
+}
+
+/// If a block begins with a statement (possibly a `let` binding) and has an
+/// expression, return it.
+fn extract_expr_from_first_stmt<'tcx>(block: &Block<'tcx>) -> Option<&'tcx Expr<'tcx>> {
+    if block.stmts.is_empty() {
+        return None;
+    }
+    if let StmtKind::Local(ref local) = block.stmts[0].kind {
+        local.init //.map(|expr| expr)
+    } else {
+        None
+    }
+}
+
+/// If a block begins with an expression (with or without semicolon), return it.
+fn extract_first_expr<'tcx>(block: &Block<'tcx>) -> Option<&'tcx Expr<'tcx>> {
+    match block.expr {
+        Some(ref expr) if block.stmts.is_empty() => Some(expr),
+        None if !block.stmts.is_empty() => match block.stmts[0].kind {
+            StmtKind::Expr(ref expr) | StmtKind::Semi(ref expr) => Some(expr),
+            StmtKind::Local(..) | StmtKind::Item(..) => None,
+        },
+        _ => None,
+    }
+}
+
+/// Returns `true` if expr contains a single break expr without destination label
+/// and
+/// passed expression. The expression may be within a block.
+fn is_simple_break_expr(expr: &Expr<'_>) -> bool {
+    match expr.kind {
+        ExprKind::Break(dest, ref passed_expr) if dest.label.is_none() && passed_expr.is_none() => true,
+        ExprKind::Block(ref b, _) => extract_first_expr(b).map_or(false, |subexpr| is_simple_break_expr(subexpr)),
+        _ => false,
+    }
+}
diff --git a/clippy_lints/src/loops/while_let_on_iterator.rs b/clippy_lints/src/loops/while_let_on_iterator.rs
new file mode 100644
index 00000000000..e5a47694faa
--- /dev/null
+++ b/clippy_lints/src/loops/while_let_on_iterator.rs
@@ -0,0 +1,171 @@
+use super::utils::{LoopNestVisitor, Nesting};
+use super::WHILE_LET_ON_ITERATOR;
+use crate::utils::usage::mutated_variables;
+use crate::utils::{
+    get_enclosing_block, get_trait_def_id, implements_trait, is_refutable, last_path_segment, match_trait_method,
+    path_to_local, path_to_local_id, paths, snippet_with_applicability, span_lint_and_sugg,
+};
+use if_chain::if_chain;
+use rustc_errors::Applicability;
+use rustc_hir::intravisit::{walk_block, walk_expr, NestedVisitorMap, Visitor};
+use rustc_hir::{Expr, ExprKind, HirId, MatchSource, Node, PatKind};
+use rustc_lint::LateContext;
+use rustc_middle::hir::map::Map;
+
+use rustc_span::symbol::sym;
+
+pub(super) fn check(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+    if let ExprKind::Match(ref match_expr, ref arms, MatchSource::WhileLetDesugar) = expr.kind {
+        let pat = &arms[0].pat.kind;
+        if let (
+            &PatKind::TupleStruct(ref qpath, ref pat_args, _),
+            &ExprKind::MethodCall(ref method_path, _, ref method_args, _),
+        ) = (pat, &match_expr.kind)
+        {
+            let iter_expr = &method_args[0];
+
+            // Don't lint when the iterator is recreated on every iteration
+            if_chain! {
+                if let ExprKind::MethodCall(..) | ExprKind::Call(..) = iter_expr.kind;
+                if let Some(iter_def_id) = get_trait_def_id(cx, &paths::ITERATOR);
+                if implements_trait(cx, cx.typeck_results().expr_ty(iter_expr), iter_def_id, &[]);
+                then {
+                    return;
+                }
+            }
+
+            let lhs_constructor = last_path_segment(qpath);
+            if method_path.ident.name == sym::next
+                && match_trait_method(cx, match_expr, &paths::ITERATOR)
+                && lhs_constructor.ident.name == sym::Some
+                && (pat_args.is_empty()
+                    || !is_refutable(cx, &pat_args[0])
+                        && !is_used_inside(cx, iter_expr, &arms[0].body)
+                        && !is_iterator_used_after_while_let(cx, iter_expr)
+                        && !is_nested(cx, expr, &method_args[0]))
+            {
+                let mut applicability = Applicability::MachineApplicable;
+                let iterator = snippet_with_applicability(cx, method_args[0].span, "_", &mut applicability);
+                let loop_var = if pat_args.is_empty() {
+                    "_".to_string()
+                } else {
+                    snippet_with_applicability(cx, pat_args[0].span, "_", &mut applicability).into_owned()
+                };
+                span_lint_and_sugg(
+                    cx,
+                    WHILE_LET_ON_ITERATOR,
+                    expr.span.with_hi(match_expr.span.hi()),
+                    "this loop could be written as a `for` loop",
+                    "try",
+                    format!("for {} in {}", loop_var, iterator),
+                    applicability,
+                );
+            }
+        }
+    }
+}
+
+fn is_used_inside<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, container: &'tcx Expr<'_>) -> bool {
+    let def_id = match path_to_local(expr) {
+        Some(id) => id,
+        None => return false,
+    };
+    if let Some(used_mutably) = mutated_variables(container, cx) {
+        if used_mutably.contains(&def_id) {
+            return true;
+        }
+    }
+    false
+}
+
+fn is_iterator_used_after_while_let<'tcx>(cx: &LateContext<'tcx>, iter_expr: &'tcx Expr<'_>) -> bool {
+    let def_id = match path_to_local(iter_expr) {
+        Some(id) => id,
+        None => return false,
+    };
+    let mut visitor = VarUsedAfterLoopVisitor {
+        def_id,
+        iter_expr_id: iter_expr.hir_id,
+        past_while_let: false,
+        var_used_after_while_let: false,
+    };
+    if let Some(enclosing_block) = get_enclosing_block(cx, def_id) {
+        walk_block(&mut visitor, enclosing_block);
+    }
+    visitor.var_used_after_while_let
+}
+
+fn is_nested(cx: &LateContext<'_>, match_expr: &Expr<'_>, iter_expr: &Expr<'_>) -> bool {
+    if_chain! {
+        if let Some(loop_block) = get_enclosing_block(cx, match_expr.hir_id);
+        let parent_node = cx.tcx.hir().get_parent_node(loop_block.hir_id);
+        if let Some(Node::Expr(loop_expr)) = cx.tcx.hir().find(parent_node);
+        then {
+            return is_loop_nested(cx, loop_expr, iter_expr)
+        }
+    }
+    false
+}
+
+fn is_loop_nested(cx: &LateContext<'_>, loop_expr: &Expr<'_>, iter_expr: &Expr<'_>) -> bool {
+    let mut id = loop_expr.hir_id;
+    let iter_id = if let Some(id) = path_to_local(iter_expr) {
+        id
+    } else {
+        return true;
+    };
+    loop {
+        let parent = cx.tcx.hir().get_parent_node(id);
+        if parent == id {
+            return false;
+        }
+        match cx.tcx.hir().find(parent) {
+            Some(Node::Expr(expr)) => {
+                if let ExprKind::Loop(..) = expr.kind {
+                    return true;
+                };
+            },
+            Some(Node::Block(block)) => {
+                let mut block_visitor = LoopNestVisitor {
+                    hir_id: id,
+                    iterator: iter_id,
+                    nesting: Nesting::Unknown,
+                };
+                walk_block(&mut block_visitor, block);
+                if block_visitor.nesting == Nesting::RuledOut {
+                    return false;
+                }
+            },
+            Some(Node::Stmt(_)) => (),
+            _ => {
+                return false;
+            },
+        }
+        id = parent;
+    }
+}
+
+struct VarUsedAfterLoopVisitor {
+    def_id: HirId,
+    iter_expr_id: HirId,
+    past_while_let: bool,
+    var_used_after_while_let: bool,
+}
+
+impl<'tcx> Visitor<'tcx> for VarUsedAfterLoopVisitor {
+    type Map = Map<'tcx>;
+
+    fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
+        if self.past_while_let {
+            if path_to_local_id(expr, self.def_id) {
+                self.var_used_after_while_let = true;
+            }
+        } else if self.iter_expr_id == expr.hir_id {
+            self.past_while_let = true;
+        }
+        walk_expr(self, expr);
+    }
+    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
+        NestedVisitorMap::None
+    }
+}