Auto merge of #7995 - Alexendoo:needless_late_init, r=giraffate

Add `needless_late_init` lint

examples:

```rust
let a;
a = 1;
// to
let a = 1;
```
```rust
let b;
match 3 {
    0 => b = "zero",
    1 => b = "one",
    _ => b = "many",
}
// to
let b = match 3 {
    0 => "zero",
    1 => "one",
    _ => "many",
};
```
```rust
let c;
if true {
    c = 1;
} else {
    c = -1;
}
// to
let c = if true {
    1
} else {
    -1
};
```

changelog: Add [`needless_late_init`]

19 files changed, 882 insertions, 31 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 401557b3eac..157ea0c963a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3033,6 +3033,7 @@ Released 2018-09-13
 [`needless_continue`]: https://rust-lang.github.io/rust-clippy/master/index.html#needless_continue
 [`needless_doctest_main`]: https://rust-lang.github.io/rust-clippy/master/index.html#needless_doctest_main
 [`needless_for_each`]: https://rust-lang.github.io/rust-clippy/master/index.html#needless_for_each
+[`needless_late_init`]: https://rust-lang.github.io/rust-clippy/master/index.html#needless_late_init
 [`needless_lifetimes`]: https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes
 [`needless_option_as_deref`]: https://rust-lang.github.io/rust-clippy/master/index.html#needless_option_as_deref
 [`needless_pass_by_value`]: https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_value
diff --git a/clippy_lints/src/lib.register_all.rs b/clippy_lints/src/lib.register_all.rs
index 612240135ac..04912120e27 100644
--- a/clippy_lints/src/lib.register_all.rs
+++ b/clippy_lints/src/lib.register_all.rs
@@ -206,6 +206,7 @@ store.register_group(true, "clippy::all", Some("clippy_all"), vec![
     LintId::of(needless_bool::BOOL_COMPARISON),
     LintId::of(needless_bool::NEEDLESS_BOOL),
     LintId::of(needless_borrowed_ref::NEEDLESS_BORROWED_REFERENCE),
+    LintId::of(needless_late_init::NEEDLESS_LATE_INIT),
     LintId::of(needless_option_as_deref::NEEDLESS_OPTION_AS_DEREF),
     LintId::of(needless_question_mark::NEEDLESS_QUESTION_MARK),
     LintId::of(needless_update::NEEDLESS_UPDATE),
diff --git a/clippy_lints/src/lib.register_lints.rs b/clippy_lints/src/lib.register_lints.rs
index 19c35a5e5f4..bb159e50373 100644
--- a/clippy_lints/src/lib.register_lints.rs
+++ b/clippy_lints/src/lib.register_lints.rs
@@ -362,6 +362,7 @@ store.register_lints(&[
     needless_borrowed_ref::NEEDLESS_BORROWED_REFERENCE,
     needless_continue::NEEDLESS_CONTINUE,
     needless_for_each::NEEDLESS_FOR_EACH,
+    needless_late_init::NEEDLESS_LATE_INIT,
     needless_option_as_deref::NEEDLESS_OPTION_AS_DEREF,
     needless_pass_by_value::NEEDLESS_PASS_BY_VALUE,
     needless_question_mark::NEEDLESS_QUESTION_MARK,
diff --git a/clippy_lints/src/lib.register_style.rs b/clippy_lints/src/lib.register_style.rs
index f336441ea84..ea87e7e7a73 100644
--- a/clippy_lints/src/lib.register_style.rs
+++ b/clippy_lints/src/lib.register_style.rs
@@ -82,6 +82,7 @@ store.register_group(true, "clippy::style", Some("clippy_style"), vec![
     LintId::of(misc_early::REDUNDANT_PATTERN),
     LintId::of(mut_mutex_lock::MUT_MUTEX_LOCK),
     LintId::of(mut_reference::UNNECESSARY_MUT_PASSED),
+    LintId::of(needless_late_init::NEEDLESS_LATE_INIT),
     LintId::of(neg_multiply::NEG_MULTIPLY),
     LintId::of(new_without_default::NEW_WITHOUT_DEFAULT),
     LintId::of(non_copy_const::BORROW_INTERIOR_MUTABLE_CONST),
diff --git a/clippy_lints/src/lib.rs b/clippy_lints/src/lib.rs
index 3dafdf8f0d5..bd9710ec407 100644
--- a/clippy_lints/src/lib.rs
+++ b/clippy_lints/src/lib.rs
@@ -299,6 +299,7 @@ mod needless_bool;
 mod needless_borrowed_ref;
 mod needless_continue;
 mod needless_for_each;
+mod needless_late_init;
 mod needless_option_as_deref;
 mod needless_pass_by_value;
 mod needless_question_mark;
@@ -851,6 +852,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(move || Box::new(format_args::FormatArgs));
     store.register_late_pass(|| Box::new(trailing_empty_array::TrailingEmptyArray));
     store.register_early_pass(|| Box::new(octal_escapes::OctalEscapes));
+    store.register_late_pass(|| Box::new(needless_late_init::NeedlessLateInit));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 
diff --git a/clippy_lints/src/needless_late_init.rs b/clippy_lints/src/needless_late_init.rs
new file mode 100644
index 00000000000..e0522f3fe0b
--- /dev/null
+++ b/clippy_lints/src/needless_late_init.rs
@@ -0,0 +1,349 @@
+use clippy_utils::diagnostics::span_lint_and_then;
+use clippy_utils::path_to_local;
+use clippy_utils::source::snippet_opt;
+use clippy_utils::visitors::{expr_visitor, is_local_used};
+use rustc_errors::Applicability;
+use rustc_hir::intravisit::Visitor;
+use rustc_hir::{Block, Expr, ExprKind, HirId, Local, LocalSource, MatchSource, Node, Pat, PatKind, Stmt, StmtKind};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::Span;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for late initializations that can be replaced by a `let` statement
+    /// with an initializer.
+    ///
+    /// ### Why is this bad?
+    /// Assigning in the `let` statement is less repetitive.
+    ///
+    /// ### Example
+    /// ```rust
+    /// let a;
+    /// a = 1;
+    ///
+    /// let b;
+    /// match 3 {
+    ///     0 => b = "zero",
+    ///     1 => b = "one",
+    ///     _ => b = "many",
+    /// }
+    ///
+    /// let c;
+    /// if true {
+    ///     c = 1;
+    /// } else {
+    ///     c = -1;
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// let a = 1;
+    ///
+    /// let b = match 3 {
+    ///     0 => "zero",
+    ///     1 => "one",
+    ///     _ => "many",
+    /// };
+    ///
+    /// let c = if true {
+    ///     1
+    /// } else {
+    ///     -1
+    /// };
+    /// ```
+    #[clippy::version = "1.58.0"]
+    pub NEEDLESS_LATE_INIT,
+    style,
+    "late initializations that can be replaced by a `let` statement with an initializer"
+}
+declare_lint_pass!(NeedlessLateInit => [NEEDLESS_LATE_INIT]);
+
+fn contains_assign_expr<'tcx>(cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'tcx>) -> bool {
+    let mut seen = false;
+    expr_visitor(cx, |expr| {
+        if let ExprKind::Assign(..) = expr.kind {
+            seen = true;
+        }
+
+        !seen
+    })
+    .visit_stmt(stmt);
+
+    seen
+}
+
+#[derive(Debug)]
+struct LocalAssign {
+    lhs_id: HirId,
+    lhs_span: Span,
+    rhs_span: Span,
+    span: Span,
+}
+
+impl LocalAssign {
+    fn from_expr(expr: &Expr<'_>, span: Span) -> Option<Self> {
+        if let ExprKind::Assign(lhs, rhs, _) = expr.kind {
+            if lhs.span.from_expansion() {
+                return None;
+            }
+
+            Some(Self {
+                lhs_id: path_to_local(lhs)?,
+                lhs_span: lhs.span,
+                rhs_span: rhs.span.source_callsite(),
+                span,
+            })
+        } else {
+            None
+        }
+    }
+
+    fn new<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>, binding_id: HirId) -> Option<LocalAssign> {
+        let assign = match expr.kind {
+            ExprKind::Block(Block { expr: Some(expr), .. }, _) => Self::from_expr(expr, expr.span),
+            ExprKind::Block(block, _) => {
+                if_chain! {
+                    if let Some((last, other_stmts)) = block.stmts.split_last();
+                    if let StmtKind::Expr(expr) | StmtKind::Semi(expr) = last.kind;
+
+                    let assign = Self::from_expr(expr, last.span)?;
+
+                    // avoid visiting if not needed
+                    if assign.lhs_id == binding_id;
+                    if other_stmts.iter().all(|stmt| !contains_assign_expr(cx, stmt));
+
+                    then {
+                        Some(assign)
+                    } else {
+                        None
+                    }
+                }
+            },
+            ExprKind::Assign(..) => Self::from_expr(expr, expr.span),
+            _ => None,
+        }?;
+
+        if assign.lhs_id == binding_id {
+            Some(assign)
+        } else {
+            None
+        }
+    }
+}
+
+fn assignment_suggestions<'tcx>(
+    cx: &LateContext<'tcx>,
+    binding_id: HirId,
+    exprs: impl IntoIterator<Item = &'tcx Expr<'tcx>>,
+) -> Option<(Applicability, Vec<(Span, String)>)> {
+    let mut assignments = Vec::new();
+
+    for expr in exprs {
+        let ty = cx.typeck_results().expr_ty(expr);
+
+        if ty.is_never() {
+            continue;
+        }
+        if !ty.is_unit() {
+            return None;
+        }
+
+        let assign = LocalAssign::new(cx, expr, binding_id)?;
+
+        assignments.push(assign);
+    }
+
+    let suggestions = assignments
+        .into_iter()
+        .map(|assignment| Some((assignment.span, snippet_opt(cx, assignment.rhs_span)?)))
+        .collect::<Option<Vec<(Span, String)>>>()?;
+
+    let applicability = if suggestions.len() > 1 {
+        // multiple suggestions don't work with rustfix in multipart_suggest
+        // https://github.com/rust-lang/rustfix/issues/141
+        Applicability::Unspecified
+    } else {
+        Applicability::MachineApplicable
+    };
+    Some((applicability, suggestions))
+}
+
+struct Usage<'tcx> {
+    stmt: &'tcx Stmt<'tcx>,
+    expr: &'tcx Expr<'tcx>,
+    needs_semi: bool,
+}
+
+fn first_usage<'tcx>(
+    cx: &LateContext<'tcx>,
+    binding_id: HirId,
+    local_stmt_id: HirId,
+    block: &'tcx Block<'tcx>,
+) -> Option<Usage<'tcx>> {
+    block
+        .stmts
+        .iter()
+        .skip_while(|stmt| stmt.hir_id != local_stmt_id)
+        .skip(1)
+        .find(|&stmt| is_local_used(cx, stmt, binding_id))
+        .and_then(|stmt| match stmt.kind {
+            StmtKind::Expr(expr) => Some(Usage {
+                stmt,
+                expr,
+                needs_semi: true,
+            }),
+            StmtKind::Semi(expr) => Some(Usage {
+                stmt,
+                expr,
+                needs_semi: false,
+            }),
+            _ => None,
+        })
+}
+
+fn local_snippet_without_semicolon(cx: &LateContext<'_>, local: &Local<'_>) -> Option<String> {
+    let span = local.span.with_hi(match local.ty {
+        // let <pat>: <ty>;
+        // ~~~~~~~~~~~~~~~
+        Some(ty) => ty.span.hi(),
+        // let <pat>;
+        // ~~~~~~~~~
+        None => local.pat.span.hi(),
+    });
+
+    snippet_opt(cx, span)
+}
+
+fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    local: &'tcx Local<'tcx>,
+    local_stmt: &'tcx Stmt<'tcx>,
+    block: &'tcx Block<'tcx>,
+    binding_id: HirId,
+) -> Option<()> {
+    let usage = first_usage(cx, binding_id, local_stmt.hir_id, block)?;
+    let binding_name = cx.tcx.hir().opt_name(binding_id)?;
+    let let_snippet = local_snippet_without_semicolon(cx, local)?;
+
+    match usage.expr.kind {
+        ExprKind::Assign(..) => {
+            let assign = LocalAssign::new(cx, usage.expr, binding_id)?;
+
+            span_lint_and_then(
+                cx,
+                NEEDLESS_LATE_INIT,
+                local_stmt.span,
+                "unneeded late initalization",
+                |diag| {
+                    diag.tool_only_span_suggestion(
+                        local_stmt.span,
+                        "remove the local",
+                        String::new(),
+                        Applicability::MachineApplicable,
+                    );
+
+                    diag.span_suggestion(
+                        assign.lhs_span,
+                        &format!("declare `{}` here", binding_name),
+                        let_snippet,
+                        Applicability::MachineApplicable,
+                    );
+                },
+            );
+        },
+        ExprKind::If(_, then_expr, Some(else_expr)) => {
+            let (applicability, suggestions) = assignment_suggestions(cx, binding_id, [then_expr, else_expr])?;
+
+            span_lint_and_then(
+                cx,
+                NEEDLESS_LATE_INIT,
+                local_stmt.span,
+                "unneeded late initalization",
+                |diag| {
+                    diag.tool_only_span_suggestion(local_stmt.span, "remove the local", String::new(), applicability);
+
+                    diag.span_suggestion_verbose(
+                        usage.stmt.span.shrink_to_lo(),
+                        &format!("declare `{}` here", binding_name),
+                        format!("{} = ", let_snippet),
+                        applicability,
+                    );
+
+                    diag.multipart_suggestion("remove the assignments from the branches", suggestions, applicability);
+
+                    if usage.needs_semi {
+                        diag.span_suggestion(
+                            usage.stmt.span.shrink_to_hi(),
+                            "add a semicolon after the `if` expression",
+                            ";".to_string(),
+                            applicability,
+                        );
+                    }
+                },
+            );
+        },
+        ExprKind::Match(_, arms, MatchSource::Normal) => {
+            let (applicability, suggestions) = assignment_suggestions(cx, binding_id, arms.iter().map(|arm| arm.body))?;
+
+            span_lint_and_then(
+                cx,
+                NEEDLESS_LATE_INIT,
+                local_stmt.span,
+                "unneeded late initalization",
+                |diag| {
+                    diag.tool_only_span_suggestion(local_stmt.span, "remove the local", String::new(), applicability);
+
+                    diag.span_suggestion_verbose(
+                        usage.stmt.span.shrink_to_lo(),
+                        &format!("declare `{}` here", binding_name),
+                        format!("{} = ", let_snippet),
+                        applicability,
+                    );
+
+                    diag.multipart_suggestion(
+                        "remove the assignments from the `match` arms",
+                        suggestions,
+                        applicability,
+                    );
+
+                    if usage.needs_semi {
+                        diag.span_suggestion(
+                            usage.stmt.span.shrink_to_hi(),
+                            "add a semicolon after the `match` expression",
+                            ";".to_string(),
+                            applicability,
+                        );
+                    }
+                },
+            );
+        },
+        _ => {},
+    };
+
+    Some(())
+}
+
+impl LateLintPass<'tcx> for NeedlessLateInit {
+    fn check_local(&mut self, cx: &LateContext<'tcx>, local: &'tcx Local<'tcx>) {
+        let mut parents = cx.tcx.hir().parent_iter(local.hir_id);
+
+        if_chain! {
+            if let Local {
+                init: None,
+                pat: &Pat {
+                    kind: PatKind::Binding(_, binding_id, _, None),
+                    ..
+                },
+                source: LocalSource::Normal,
+                ..
+            } = local;
+            if let Some((_, Node::Stmt(local_stmt))) = parents.next();
+            if let Some((_, Node::Block(block))) = parents.next();
+
+            then {
+                check(cx, local, local_stmt, block, binding_id);
+            }
+        }
+    }
+}
diff --git a/clippy_lints/src/no_effect.rs b/clippy_lints/src/no_effect.rs
index cd54de3ee4b..6fcc9ca29b9 100644
--- a/clippy_lints/src/no_effect.rs
+++ b/clippy_lints/src/no_effect.rs
@@ -164,12 +164,13 @@ fn check_unnecessary_operation(cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'_>) {
         if !&reduced.iter().any(|e| e.span.from_expansion());
         then {
             if let ExprKind::Index(..) = &expr.kind {
-                let snippet;
-                if let (Some(arr), Some(func)) = (snippet_opt(cx, reduced[0].span), snippet_opt(cx, reduced[1].span)) {
-                    snippet = format!("assert!({}.len() > {});", &arr, &func);
+                let snippet = if let (Some(arr), Some(func)) =
+                    (snippet_opt(cx, reduced[0].span), snippet_opt(cx, reduced[1].span))
+                {
+                    format!("assert!({}.len() > {});", &arr, &func)
                 } else {
                     return;
-                }
+                };
                 span_lint_hir_and_then(
                     cx,
                     UNNECESSARY_OPERATION,
diff --git a/tests/ui/let_if_seq.rs b/tests/ui/let_if_seq.rs
index 2d8f3c2f0e7..c5cb2eb1fe1 100644
--- a/tests/ui/let_if_seq.rs
+++ b/tests/ui/let_if_seq.rs
@@ -3,7 +3,8 @@
     unused_assignments,
     clippy::similar_names,
     clippy::blacklisted_name,
-    clippy::branches_sharing_code
+    clippy::branches_sharing_code,
+    clippy::needless_late_init
 )]
 #![warn(clippy::useless_let_if_seq)]
 
diff --git a/tests/ui/let_if_seq.stderr b/tests/ui/let_if_seq.stderr
index 9cf2e10a5ee..271ccce681c 100644
--- a/tests/ui/let_if_seq.stderr
+++ b/tests/ui/let_if_seq.stderr
@@ -1,5 +1,5 @@
 error: `if _ { .. } else { .. }` is an expression
-  --> $DIR/let_if_seq.rs:65:5
+  --> $DIR/let_if_seq.rs:66:5
    |
 LL | /     let mut foo = 0;
 LL | |     if f() {
@@ -11,7 +11,7 @@ LL | |     }
    = note: you might not need `mut` at all
 
 error: `if _ { .. } else { .. }` is an expression
-  --> $DIR/let_if_seq.rs:70:5
+  --> $DIR/let_if_seq.rs:71:5
    |
 LL | /     let mut bar = 0;
 LL | |     if f() {
@@ -25,7 +25,7 @@ LL | |     }
    = note: you might not need `mut` at all
 
 error: `if _ { .. } else { .. }` is an expression
-  --> $DIR/let_if_seq.rs:78:5
+  --> $DIR/let_if_seq.rs:79:5
    |
 LL | /     let quz;
 LL | |     if f() {
@@ -36,7 +36,7 @@ LL | |     }
    | |_____^ help: it is more idiomatic to write: `let quz = if f() { 42 } else { 0 };`
 
 error: `if _ { .. } else { .. }` is an expression
-  --> $DIR/let_if_seq.rs:107:5
+  --> $DIR/let_if_seq.rs:108:5
    |
 LL | /     let mut baz = 0;
 LL | |     if f() {
diff --git a/tests/ui/min_max.rs b/tests/ui/min_max.rs
index f7ed72a11cf..b2bc97f4744 100644
--- a/tests/ui/min_max.rs
+++ b/tests/ui/min_max.rs
@@ -19,8 +19,7 @@ impl NotOrd {
 }
 
 fn main() {
-    let x;
-    x = 2usize;
+    let x = 2usize;
     min(1, max(3, x));
     min(max(3, x), 1);
     max(min(x, 1), 3);
@@ -35,9 +34,7 @@ fn main() {
     let y = 2isize;
     min(max(y, -1), 3);
 
-    let s;
-    s = "Hello";
-
+    let s = "Hello";
     min("Apple", max("Zoo", s));
     max(min(s, "Apple"), "Zoo");
 
diff --git a/tests/ui/min_max.stderr b/tests/ui/min_max.stderr
index 9f8e26fa406..c70b77eabbd 100644
--- a/tests/ui/min_max.stderr
+++ b/tests/ui/min_max.stderr
@@ -1,5 +1,5 @@
 error: this `min`/`max` combination leads to constant result
-  --> $DIR/min_max.rs:24:5
+  --> $DIR/min_max.rs:23:5
    |
 LL |     min(1, max(3, x));
    |     ^^^^^^^^^^^^^^^^^
@@ -7,73 +7,73 @@ LL |     min(1, max(3, x));
    = note: `-D clippy::min-max` implied by `-D warnings`
 
 error: this `min`/`max` combination leads to constant result
-  --> $DIR/min_max.rs:25:5
+  --> $DIR/min_max.rs:24:5
    |
 LL |     min(max(3, x), 1);
    |     ^^^^^^^^^^^^^^^^^
 
 error: this `min`/`max` combination leads to constant result
-  --> $DIR/min_max.rs:26:5
+  --> $DIR/min_max.rs:25:5
    |
 LL |     max(min(x, 1), 3);
    |     ^^^^^^^^^^^^^^^^^
 
 error: this `min`/`max` combination leads to constant result
-  --> $DIR/min_max.rs:27:5
+  --> $DIR/min_max.rs:26:5
    |
 LL |     max(3, min(x, 1));
    |     ^^^^^^^^^^^^^^^^^
 
 error: this `min`/`max` combination leads to constant result
-  --> $DIR/min_max.rs:29:5
+  --> $DIR/min_max.rs:28:5
    |
 LL |     my_max(3, my_min(x, 1));
    |     ^^^^^^^^^^^^^^^^^^^^^^^
 
 error: this `min`/`max` combination leads to constant result
-  --> $DIR/min_max.rs:41:5
+  --> $DIR/min_max.rs:38:5
    |
 LL |     min("Apple", max("Zoo", s));
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: this `min`/`max` combination leads to constant result
-  --> $DIR/min_max.rs:42:5
+  --> $DIR/min_max.rs:39:5
    |
 LL |     max(min(s, "Apple"), "Zoo");
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: this `min`/`max` combination leads to constant result
-  --> $DIR/min_max.rs:47:5
+  --> $DIR/min_max.rs:44:5
    |
 LL |     x.min(1).max(3);
    |     ^^^^^^^^^^^^^^^
 
 error: this `min`/`max` combination leads to constant result
-  --> $DIR/min_max.rs:48:5
+  --> $DIR/min_max.rs:45:5
    |
 LL |     x.max(3).min(1);
    |     ^^^^^^^^^^^^^^^
 
 error: this `min`/`max` combination leads to constant result
-  --> $DIR/min_max.rs:49:5
+  --> $DIR/min_max.rs:46:5
    |
 LL |     f.max(3f32).min(1f32);
    |     ^^^^^^^^^^^^^^^^^^^^^
 
 error: this `min`/`max` combination leads to constant result
-  --> $DIR/min_max.rs:55:5
+  --> $DIR/min_max.rs:52:5
    |
 LL |     max(x.min(1), 3);
    |     ^^^^^^^^^^^^^^^^
 
 error: this `min`/`max` combination leads to constant result
-  --> $DIR/min_max.rs:58:5
+  --> $DIR/min_max.rs:55:5
    |
 LL |     s.max("Zoo").min("Apple");
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: this `min`/`max` combination leads to constant result
-  --> $DIR/min_max.rs:59:5
+  --> $DIR/min_max.rs:56:5
    |
 LL |     s.min("Apple").max("Zoo");
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/tests/ui/needless_late_init.rs b/tests/ui/needless_late_init.rs
new file mode 100644
index 00000000000..6fdb4cc566f
--- /dev/null
+++ b/tests/ui/needless_late_init.rs
@@ -0,0 +1,167 @@
+#![allow(unused)]
+
+fn main() {
+    let a;
+    let n = 1;
+    match n {
+        1 => a = "one",
+        _ => {
+            a = "two";
+        },
+    }
+
+    let b;
+    if n == 3 {
+        b = "four";
+    } else {
+        b = "five"
+    }
+
+    let c;
+    if let Some(n) = Some(5) {
+        c = n;
+    } else {
+        c = -50;
+    }
+
+    let d;
+    if true {
+        let temp = 5;
+        d = temp;
+    } else {
+        d = 15;
+    }
+
+    let e;
+    if true {
+        e = format!("{} {}", a, b);
+    } else {
+        e = format!("{}", c);
+    }
+
+    println!("{}", a);
+}
+
+async fn in_async() -> &'static str {
+    async fn f() -> &'static str {
+        "one"
+    }
+
+    let a;
+    let n = 1;
+    match n {
+        1 => a = f().await,
+        _ => {
+            a = "two";
+        },
+    }
+
+    a
+}
+
+const fn in_const() -> &'static str {
+    const fn f() -> &'static str {
+        "one"
+    }
+
+    let a;
+    let n = 1;
+    match n {
+        1 => a = f(),
+        _ => {
+            a = "two";
+        },
+    }
+
+    a
+}
+
+fn does_not_lint() {
+    let z;
+    if false {
+        z = 1;
+    }
+
+    let x;
+    let y;
+    if true {
+        x = 1;
+    } else {
+        y = 1;
+    }
+
+    let mut x;
+    if true {
+        x = 5;
+        x = 10 / x;
+    } else {
+        x = 2;
+    }
+
+    let x;
+    let _ = match 1 {
+        1 => x = 10,
+        _ => x = 20,
+    };
+
+    // using tuples would be possible, but not always preferable
+    let x;
+    let y;
+    if true {
+        x = 1;
+        y = 2;
+    } else {
+        x = 3;
+        y = 4;
+    }
+
+    // could match with a smarter heuristic to avoid multiple assignments
+    let x;
+    if true {
+        let mut y = 5;
+        y = 6;
+        x = y;
+    } else {
+        x = 2;
+    }
+
+    let (x, y);
+    if true {
+        x = 1;
+    } else {
+        x = 2;
+    }
+    y = 3;
+
+    macro_rules! assign {
+        ($i:ident) => {
+            $i = 1;
+        };
+    }
+    let x;
+    assign!(x);
+
+    let x;
+    if true {
+        assign!(x);
+    } else {
+        x = 2;
+    }
+
+    macro_rules! in_macro {
+        () => {
+            let x;
+            x = 1;
+
+            let x;
+            if true {
+                x = 1;
+            } else {
+                x = 2;
+            }
+        };
+    }
+    in_macro!();
+
+    println!("{}", x);
+}
diff --git a/tests/ui/needless_late_init.stderr b/tests/ui/needless_late_init.stderr
new file mode 100644
index 00000000000..cbb7538c63b
--- /dev/null
+++ b/tests/ui/needless_late_init.stderr
@@ -0,0 +1,150 @@
+error: unneeded late initalization
+  --> $DIR/needless_late_init.rs:4:5
+   |
+LL |     let a;
+   |     ^^^^^^
+   |
+   = note: `-D clippy::needless-late-init` implied by `-D warnings`
+help: declare `a` here
+   |
+LL |     let a = match n {
+   |     +++++++
+help: remove the assignments from the `match` arms
+   |
+LL ~         1 => "one",
+LL |         _ => {
+LL ~             "two"
+   |
+help: add a semicolon after the `match` expression
+   |
+LL |     };
+   |      +
+
+error: unneeded late initalization
+  --> $DIR/needless_late_init.rs:13:5
+   |
+LL |     let b;
+   |     ^^^^^^
+   |
+help: declare `b` here
+   |
+LL |     let b = if n == 3 {
+   |     +++++++
+help: remove the assignments from the branches
+   |
+LL ~         "four"
+LL |     } else {
+LL ~         "five"
+   |
+help: add a semicolon after the `if` expression
+   |
+LL |     };
+   |      +
+
+error: unneeded late initalization
+  --> $DIR/needless_late_init.rs:20:5
+   |
+LL |     let c;
+   |     ^^^^^^
+   |
+help: declare `c` here
+   |
+LL |     let c = if let Some(n) = Some(5) {
+   |     +++++++
+help: remove the assignments from the branches
+   |
+LL ~         n
+LL |     } else {
+LL ~         -50
+   |
+help: add a semicolon after the `if` expression
+   |
+LL |     };
+   |      +
+
+error: unneeded late initalization
+  --> $DIR/needless_late_init.rs:27:5
+   |
+LL |     let d;
+   |     ^^^^^^
+   |
+help: declare `d` here
+   |
+LL |     let d = if true {
+   |     +++++++
+help: remove the assignments from the branches
+   |
+LL ~         temp
+LL |     } else {
+LL ~         15
+   |
+help: add a semicolon after the `if` expression
+   |
+LL |     };
+   |      +
+
+error: unneeded late initalization
+  --> $DIR/needless_late_init.rs:35:5
+   |
+LL |     let e;
+   |     ^^^^^^
+   |
+help: declare `e` here
+   |
+LL |     let e = if true {
+   |     +++++++
+help: remove the assignments from the branches
+   |
+LL ~         format!("{} {}", a, b)
+LL |     } else {
+LL ~         format!("{}", c)
+   |
+help: add a semicolon after the `if` expression
+   |
+LL |     };
+   |      +
+
+error: unneeded late initalization
+  --> $DIR/needless_late_init.rs:50:5
+   |
+LL |     let a;
+   |     ^^^^^^
+   |
+help: declare `a` here
+   |
+LL |     let a = match n {
+   |     +++++++
+help: remove the assignments from the `match` arms
+   |
+LL ~         1 => f().await,
+LL |         _ => {
+LL ~             "two"
+   |
+help: add a semicolon after the `match` expression
+   |
+LL |     };
+   |      +
+
+error: unneeded late initalization
+  --> $DIR/needless_late_init.rs:67:5
+   |
+LL |     let a;
+   |     ^^^^^^
+   |
+help: declare `a` here
+   |
+LL |     let a = match n {
+   |     +++++++
+help: remove the assignments from the `match` arms
+   |
+LL ~         1 => f(),
+LL |         _ => {
+LL ~             "two"
+   |
+help: add a semicolon after the `match` expression
+   |
+LL |     };
+   |      +
+
+error: aborting due to 7 previous errors
+
diff --git a/tests/ui/needless_late_init_fixable.fixed b/tests/ui/needless_late_init_fixable.fixed
new file mode 100644
index 00000000000..32d5d04fde4
--- /dev/null
+++ b/tests/ui/needless_late_init_fixable.fixed
@@ -0,0 +1,38 @@
+// run-rustfix
+
+#![allow(unused, clippy::assign_op_pattern)]
+
+fn main() {
+    
+    let a = "zero";
+
+    
+    
+    let b = 1;
+    let c = 2;
+
+    
+    let d: usize = 1;
+
+    
+    let mut e = 1;
+    e = 2;
+
+    
+    let f = match 1 {
+        1 => "three",
+        _ => return,
+    }; // has semi
+
+    
+    let g: usize = if true {
+        5
+    } else {
+        panic!();
+    };
+
+    
+    let h = format!("{}", e);
+
+    println!("{}", a);
+}
diff --git a/tests/ui/needless_late_init_fixable.rs b/tests/ui/needless_late_init_fixable.rs
new file mode 100644
index 00000000000..6bc85f68632
--- /dev/null
+++ b/tests/ui/needless_late_init_fixable.rs
@@ -0,0 +1,38 @@
+// run-rustfix
+
+#![allow(unused, clippy::assign_op_pattern)]
+
+fn main() {
+    let a;
+    a = "zero";
+
+    let b;
+    let c;
+    b = 1;
+    c = 2;
+
+    let d: usize;
+    d = 1;
+
+    let mut e;
+    e = 1;
+    e = 2;
+
+    let f;
+    match 1 {
+        1 => f = "three",
+        _ => return,
+    }; // has semi
+
+    let g: usize;
+    if true {
+        g = 5;
+    } else {
+        panic!();
+    }
+
+    let h;
+    h = format!("{}", e);
+
+    println!("{}", a);
+}
diff --git a/tests/ui/needless_late_init_fixable.stderr b/tests/ui/needless_late_init_fixable.stderr
new file mode 100644
index 00000000000..a0ce4f812f4
--- /dev/null
+++ b/tests/ui/needless_late_init_fixable.stderr
@@ -0,0 +1,103 @@
+error: unneeded late initalization
+  --> $DIR/needless_late_init_fixable.rs:6:5
+   |
+LL |     let a;
+   |     ^^^^^^
+   |
+   = note: `-D clippy::needless-late-init` implied by `-D warnings`
+help: declare `a` here
+   |
+LL |     let a = "zero";
+   |     ~~~~~
+
+error: unneeded late initalization
+  --> $DIR/needless_late_init_fixable.rs:9:5
+   |
+LL |     let b;
+   |     ^^^^^^
+   |
+help: declare `b` here
+   |
+LL |     let b = 1;
+   |     ~~~~~
+
+error: unneeded late initalization
+  --> $DIR/needless_late_init_fixable.rs:10:5
+   |
+LL |     let c;
+   |     ^^^^^^
+   |
+help: declare `c` here
+   |
+LL |     let c = 2;
+   |     ~~~~~
+
+error: unneeded late initalization
+  --> $DIR/needless_late_init_fixable.rs:14:5
+   |
+LL |     let d: usize;
+   |     ^^^^^^^^^^^^^
+   |
+help: declare `d` here
+   |
+LL |     let d: usize = 1;
+   |     ~~~~~~~~~~~~
+
+error: unneeded late initalization
+  --> $DIR/needless_late_init_fixable.rs:17:5
+   |
+LL |     let mut e;
+   |     ^^^^^^^^^^
+   |
+help: declare `e` here
+   |
+LL |     let mut e = 1;
+   |     ~~~~~~~~~
+
+error: unneeded late initalization
+  --> $DIR/needless_late_init_fixable.rs:21:5
+   |
+LL |     let f;
+   |     ^^^^^^
+   |
+help: declare `f` here
+   |
+LL |     let f = match 1 {
+   |     +++++++
+help: remove the assignments from the `match` arms
+   |
+LL |         1 => "three",
+   |              ~~~~~~~
+
+error: unneeded late initalization
+  --> $DIR/needless_late_init_fixable.rs:27:5
+   |
+LL |     let g: usize;
+   |     ^^^^^^^^^^^^^
+   |
+help: declare `g` here
+   |
+LL |     let g: usize = if true {
+   |     ++++++++++++++
+help: remove the assignments from the branches
+   |
+LL |         5
+   |
+help: add a semicolon after the `if` expression
+   |
+LL |     };
+   |      +
+
+error: unneeded late initalization
+  --> $DIR/needless_late_init_fixable.rs:34:5
+   |
+LL |     let h;
+   |     ^^^^^^
+   |
+help: declare `h` here
+   |
+LL |     let h = format!("{}", e);
+   |     ~~~~~
+
+error: aborting due to 8 previous errors
+
diff --git a/tests/ui/redundant_closure_call_late.rs b/tests/ui/redundant_closure_call_late.rs
index 1f4864b7289..5612827bd39 100644
--- a/tests/ui/redundant_closure_call_late.rs
+++ b/tests/ui/redundant_closure_call_late.rs
@@ -1,6 +1,7 @@
 // non rustfixable, see redundant_closure_call_fixable.rs
 
 #![warn(clippy::redundant_closure_call)]
+#![allow(clippy::needless_late_init)]
 
 fn main() {
     let mut i = 1;
diff --git a/tests/ui/redundant_closure_call_late.stderr b/tests/ui/redundant_closure_call_late.stderr
index 7c8865f1bd3..4eca43a2b59 100644
--- a/tests/ui/redundant_closure_call_late.stderr
+++ b/tests/ui/redundant_closure_call_late.stderr
@@ -1,5 +1,5 @@
 error: closure called just once immediately after it was declared
-  --> $DIR/redundant_closure_call_late.rs:15:5
+  --> $DIR/redundant_closure_call_late.rs:16:5
    |
 LL |     i = redun_closure();
    |     ^^^^^^^^^^^^^^^^^^^
@@ -7,13 +7,13 @@ LL |     i = redun_closure();
    = note: `-D clippy::redundant-closure-call` implied by `-D warnings`
 
 error: closure called just once immediately after it was declared
-  --> $DIR/redundant_closure_call_late.rs:19:5
+  --> $DIR/redundant_closure_call_late.rs:20:5
    |
 LL |     i = shadowed_closure();
    |     ^^^^^^^^^^^^^^^^^^^^^^
 
 error: closure called just once immediately after it was declared
-  --> $DIR/redundant_closure_call_late.rs:21:5
+  --> $DIR/redundant_closure_call_late.rs:22:5
    |
 LL |     i = shadowed_closure();
    |     ^^^^^^^^^^^^^^^^^^^^^^
diff --git a/tests/ui/redundant_else.rs b/tests/ui/redundant_else.rs
index e8a6e940c01..64f566735cd 100644
--- a/tests/ui/redundant_else.rs
+++ b/tests/ui/redundant_else.rs
@@ -1,5 +1,5 @@
 #![warn(clippy::redundant_else)]
-#![allow(clippy::needless_return, clippy::if_same_then_else)]
+#![allow(clippy::needless_return, clippy::if_same_then_else, clippy::needless_late_init)]
 
 fn main() {
     loop {