Add [`unused_peekable`] lint

10 files changed, 386 insertions, 1 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index edd7bc250a7..c42a03c04a0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4152,6 +4152,7 @@ Released 2018-09-13
 [`unused_collect`]: https://rust-lang.github.io/rust-clippy/master/index.html#unused_collect
 [`unused_io_amount`]: https://rust-lang.github.io/rust-clippy/master/index.html#unused_io_amount
 [`unused_label`]: https://rust-lang.github.io/rust-clippy/master/index.html#unused_label
+[`unused_peekable`]: https://rust-lang.github.io/rust-clippy/master/index.html#unused_peekable
 [`unused_rounding`]: https://rust-lang.github.io/rust-clippy/master/index.html#unused_rounding
 [`unused_self`]: https://rust-lang.github.io/rust-clippy/master/index.html#unused_self
 [`unused_unit`]: https://rust-lang.github.io/rust-clippy/master/index.html#unused_unit
diff --git a/clippy_lints/src/lib.register_all.rs b/clippy_lints/src/lib.register_all.rs
index a0a4b07a77e..de0514c1b66 100644
--- a/clippy_lints/src/lib.register_all.rs
+++ b/clippy_lints/src/lib.register_all.rs
@@ -337,6 +337,7 @@ store.register_group(true, "clippy::all", Some("clippy_all"), vec![
     LintId::of(unnecessary_sort_by::UNNECESSARY_SORT_BY),
     LintId::of(unsafe_removed_from_name::UNSAFE_REMOVED_FROM_NAME),
     LintId::of(unused_io_amount::UNUSED_IO_AMOUNT),
+    LintId::of(unused_peekable::UNUSED_PEEKABLE),
     LintId::of(unused_unit::UNUSED_UNIT),
     LintId::of(unwrap::PANICKING_UNWRAP),
     LintId::of(unwrap::UNNECESSARY_UNWRAP),
diff --git a/clippy_lints/src/lib.register_lints.rs b/clippy_lints/src/lib.register_lints.rs
index b7dbd30aa0c..eb23bb0c7db 100644
--- a/clippy_lints/src/lib.register_lints.rs
+++ b/clippy_lints/src/lib.register_lints.rs
@@ -573,6 +573,7 @@ store.register_lints(&[
     unsafe_removed_from_name::UNSAFE_REMOVED_FROM_NAME,
     unused_async::UNUSED_ASYNC,
     unused_io_amount::UNUSED_IO_AMOUNT,
+    unused_peekable::UNUSED_PEEKABLE,
     unused_rounding::UNUSED_ROUNDING,
     unused_self::UNUSED_SELF,
     unused_unit::UNUSED_UNIT,
diff --git a/clippy_lints/src/lib.register_suspicious.rs b/clippy_lints/src/lib.register_suspicious.rs
index 6e185f8d31e..5793d59bcad 100644
--- a/clippy_lints/src/lib.register_suspicious.rs
+++ b/clippy_lints/src/lib.register_suspicious.rs
@@ -33,4 +33,5 @@ store.register_group(true, "clippy::suspicious", Some("clippy_suspicious"), vec!
     LintId::of(suspicious_trait_impl::SUSPICIOUS_OP_ASSIGN_IMPL),
     LintId::of(swap_ptr_to_ref::SWAP_PTR_TO_REF),
     LintId::of(write::POSITIONAL_NAMED_FORMAT_PARAMETERS),
+    LintId::of(unused_peekable::UNUSED_PEEKABLE),
 ])
diff --git a/clippy_lints/src/lib.rs b/clippy_lints/src/lib.rs
index a041fbc7fd9..d4c6c4918cb 100644
--- a/clippy_lints/src/lib.rs
+++ b/clippy_lints/src/lib.rs
@@ -398,6 +398,7 @@ mod unnested_or_patterns;
 mod unsafe_removed_from_name;
 mod unused_async;
 mod unused_io_amount;
+mod unused_peekable;
 mod unused_rounding;
 mod unused_self;
 mod unused_unit;
@@ -935,6 +936,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|| Box::new(manual_instant_elapsed::ManualInstantElapsed));
     store.register_late_pass(|| Box::new(partialeq_to_none::PartialeqToNone));
     store.register_late_pass(|| Box::new(manual_empty_string_creations::ManualEmptyStringCreations));
+    store.register_late_pass(|| Box::new(unused_peekable::UnusedPeekable::default()));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 
diff --git a/clippy_lints/src/unused_peekable.rs b/clippy_lints/src/unused_peekable.rs
new file mode 100644
index 00000000000..4988ec0eca1
--- /dev/null
+++ b/clippy_lints/src/unused_peekable.rs
@@ -0,0 +1,208 @@
+use clippy_utils::diagnostics::span_lint_and_help;
+use clippy_utils::ty::{match_type, peel_mid_ty_refs_is_mutable};
+use clippy_utils::{fn_def_id, path_to_local_id, paths, peel_ref_operators};
+use rustc_ast::Mutability;
+use rustc_hir::intravisit::{walk_expr, Visitor};
+use rustc_hir::lang_items::LangItem;
+use rustc_hir::{Block, Expr, ExprKind, HirId, Local, Node, PatKind, PathSegment, StmtKind};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::ty::Ty;
+use rustc_session::{declare_tool_lint, impl_lint_pass};
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for the creation of a `peekable` iterator that is never `.peek()`ed
+    ///
+    /// ### Why is this bad?
+    /// Creating a peekable iterator without using any of its methods is likely a mistake,
+    /// or just a leftover after a refactor.
+    ///
+    /// ### Example
+    /// ```rust
+    /// let collection = vec![1, 2, 3];
+    /// let iter = collection.iter().peekable();
+    ///
+    /// for item in iter {
+    ///     // ...
+    /// }
+    /// ```
+    ///
+    /// Use instead:
+    /// ```rust
+    /// let collection = vec![1, 2, 3];
+    /// let iter = collection.iter();
+    ///
+    /// for item in iter {
+    ///     // ...
+    /// }
+    /// ```
+    #[clippy::version = "1.64.0"]
+    pub UNUSED_PEEKABLE,
+    suspicious,
+    "creating a peekable iterator without using any of its methods"
+}
+
+#[derive(Default)]
+pub struct UnusedPeekable {
+    visited: Vec<HirId>,
+}
+
+impl_lint_pass!(UnusedPeekable => [UNUSED_PEEKABLE]);
+
+impl<'tcx> LateLintPass<'tcx> for UnusedPeekable {
+    fn check_block(&mut self, cx: &LateContext<'tcx>, block: &Block<'tcx>) {
+        // Don't lint `Peekable`s returned from a block
+        if let Some(expr) = block.expr
+            && let Some(ty) = cx.typeck_results().expr_ty_opt(peel_ref_operators(cx, expr))
+            && match_type(cx, ty, &paths::PEEKABLE)
+        {
+            return;
+        }
+
+        for (idx, stmt) in block.stmts.iter().enumerate() {
+            if !stmt.span.from_expansion()
+                && let StmtKind::Local(local) = stmt.kind
+                && !self.visited.contains(&local.pat.hir_id)
+                && let PatKind::Binding(_, _, ident, _) = local.pat.kind
+                && let Some(init) = local.init
+                && !init.span.from_expansion()
+                && let Some(ty) = cx.typeck_results().expr_ty_opt(init)
+                && let (ty, _, Mutability::Mut) = peel_mid_ty_refs_is_mutable(ty)
+                && match_type(cx, ty, &paths::PEEKABLE)
+            {
+                let mut vis = PeekableVisitor::new(cx, local.pat.hir_id);
+
+                if idx + 1 == block.stmts.len() && block.expr.is_none() {
+                    return;
+                }
+
+                for stmt in &block.stmts[idx..] {
+                    vis.visit_stmt(stmt);
+                }
+
+                if let Some(expr) = block.expr {
+                    vis.visit_expr(expr);
+                }
+
+                if !vis.found_peek_call {
+                    span_lint_and_help(
+                        cx,
+                        UNUSED_PEEKABLE,
+                        ident.span,
+                        "`peek` never called on `Peekable` iterator",
+                        None,
+                        "consider removing the call to `peekable`"
+                   );
+                }
+            }
+        }
+    }
+}
+
+struct PeekableVisitor<'a, 'tcx> {
+    cx: &'a LateContext<'tcx>,
+    expected_hir_id: HirId,
+    found_peek_call: bool,
+}
+
+impl<'a, 'tcx> PeekableVisitor<'a, 'tcx> {
+    fn new(cx: &'a LateContext<'tcx>, expected_hir_id: HirId) -> Self {
+        Self {
+            cx,
+            expected_hir_id,
+            found_peek_call: false,
+        }
+    }
+}
+
+impl<'tcx> Visitor<'_> for PeekableVisitor<'_, 'tcx> {
+    fn visit_expr(&mut self, ex: &'_ Expr<'_>) {
+        if path_to_local_id(ex, self.expected_hir_id) {
+            for (_, node) in self.cx.tcx.hir().parent_iter(ex.hir_id) {
+                match node {
+                    Node::Expr(expr) => {
+                        match expr.kind {
+                            // some_function(peekable)
+                            //
+                            // If the Peekable is passed to a function, stop
+                            ExprKind::Call(_, args) => {
+                                if let Some(func_did) = fn_def_id(self.cx, expr)
+                                    && let Ok(into_iter_did) = self
+                                        .cx
+                                        .tcx
+                                        .lang_items()
+                                        .require(LangItem::IntoIterIntoIter)
+                                    && func_did == into_iter_did
+                                {
+                                    // Probably a for loop desugar, stop searching
+                                    return;
+                                }
+
+                                for arg in args.iter().map(|arg| peel_ref_operators(self.cx, arg)) {
+                                    if let ExprKind::Path(_) = arg.kind
+                                        && let Some(ty) = self
+                                            .cx
+                                            .typeck_results()
+                                            .expr_ty_opt(arg)
+                                            .map(Ty::peel_refs)
+                                        && match_type(self.cx, ty, &paths::PEEKABLE)
+                                    {
+                                        self.found_peek_call = true;
+                                        return;
+                                    }
+                                }
+                            },
+                            // Peekable::peek()
+                            ExprKind::MethodCall(PathSegment { ident: method_name, .. }, [arg, ..], _) => {
+                                let arg = peel_ref_operators(self.cx, arg);
+                                let method_name = method_name.name.as_str();
+
+                                if (method_name == "peek"
+                                    || method_name == "peek_mut"
+                                    || method_name == "next_if"
+                                    || method_name == "next_if_eq")
+                                    && let ExprKind::Path(_) = arg.kind
+                                    && let Some(ty) = self.cx.typeck_results().expr_ty_opt(arg).map(Ty::peel_refs)
+                                    && match_type(self.cx, ty, &paths::PEEKABLE)
+                                {
+                                    self.found_peek_call = true;
+                                    return;
+                                }
+                            },
+                            // Don't bother if moved into a struct
+                            ExprKind::Struct(..) => {
+                                self.found_peek_call = true;
+                                return;
+                            },
+                            _ => {},
+                        }
+                    },
+                    Node::Local(Local { init: Some(init), .. }) => {
+                        if let Some(ty) = self.cx.typeck_results().expr_ty_opt(init)
+                            && let (ty, _, Mutability::Mut) = peel_mid_ty_refs_is_mutable(ty)
+                            && match_type(self.cx, ty, &paths::PEEKABLE)
+                        {
+                            self.found_peek_call = true;
+                            return;
+                        }
+
+                        break;
+                    },
+                    Node::Stmt(stmt) => match stmt.kind {
+                        StmtKind::Expr(_) | StmtKind::Semi(_) => {},
+                        _ => {
+                            self.found_peek_call = true;
+                            return;
+                        },
+                    },
+                    Node::Block(_) => {},
+                    _ => {
+                        break;
+                    },
+                }
+            }
+        }
+
+        walk_expr(self, ex);
+    }
+}
diff --git a/clippy_utils/src/paths.rs b/clippy_utils/src/paths.rs
index 8d697a301c4..d3ae537a890 100644
--- a/clippy_utils/src/paths.rs
+++ b/clippy_utils/src/paths.rs
@@ -96,6 +96,7 @@ pub const PARKING_LOT_RWLOCK_READ_GUARD: [&str; 3] = ["lock_api", "rwlock", "RwL
 pub const PARKING_LOT_RWLOCK_WRITE_GUARD: [&str; 3] = ["lock_api", "rwlock", "RwLockWriteGuard"];
 pub const PATH_BUF_AS_PATH: [&str; 4] = ["std", "path", "PathBuf", "as_path"];
 pub const PATH_TO_PATH_BUF: [&str; 4] = ["std", "path", "Path", "to_path_buf"];
+pub const PEEKABLE: [&str; 5] = ["core", "iter", "adapters", "peekable", "Peekable"];
 pub const PERMISSIONS: [&str; 3] = ["std", "fs", "Permissions"];
 #[cfg_attr(not(unix), allow(clippy::invalid_paths))]
 pub const PERMISSIONS_FROM_MODE: [&str; 6] = ["std", "os", "unix", "fs", "PermissionsExt", "from_mode"];
diff --git a/clippy_utils/src/ty.rs b/clippy_utils/src/ty.rs
index e7d670766a0..edbe9c5a897 100644
--- a/clippy_utils/src/ty.rs
+++ b/clippy_utils/src/ty.rs
@@ -410,7 +410,7 @@ pub fn peel_mid_ty_refs(ty: Ty<'_>) -> (Ty<'_>, usize) {
     peel(ty, 0)
 }
 
-/// Peels off all references on the type.Returns the underlying type, the number of references
+/// Peels off all references on the type. Returns the underlying type, the number of references
 /// removed, and whether the pointer is ultimately mutable or not.
 pub fn peel_mid_ty_refs_is_mutable(ty: Ty<'_>) -> (Ty<'_>, usize, Mutability) {
     fn f(ty: Ty<'_>, count: usize, mutability: Mutability) -> (Ty<'_>, usize, Mutability) {
diff --git a/tests/ui/unused_peekable.rs b/tests/ui/unused_peekable.rs
new file mode 100644
index 00000000000..12cab995621
--- /dev/null
+++ b/tests/ui/unused_peekable.rs
@@ -0,0 +1,119 @@
+#![warn(clippy::unused_peekable)]
+#![allow(clippy::no_effect)]
+
+use std::iter::Empty;
+use std::iter::Peekable;
+
+fn main() {
+    invalid();
+    valid();
+}
+
+#[allow(clippy::unused_unit)]
+fn invalid() {
+    let peekable = std::iter::empty::<u32>().peekable();
+
+    // Only lint `new_local`
+    let old_local = std::iter::empty::<u32>().peekable();
+    let new_local = old_local;
+
+    // Behind mut ref
+    let mut by_mut_ref_test = std::iter::empty::<u32>().peekable();
+    let by_mut_ref = &mut by_mut_ref_test;
+
+    // Explicitly returns `Peekable`
+    fn returns_peekable() -> Peekable<Empty<u32>> {
+        std::iter::empty().peekable()
+    }
+
+    let peekable_from_fn = returns_peekable();
+
+    // Using a method not exclusive to `Peekable`
+    let mut peekable_using_iterator_method = std::iter::empty::<u32>().peekable();
+    peekable_using_iterator_method.next();
+
+    let mut peekable_in_for_loop = std::iter::empty::<u32>().peekable();
+    for x in peekable_in_for_loop {}
+}
+
+fn valid() {
+    fn takes_peekable(_peek: Peekable<Empty<u32>>) {}
+
+    // Passed to another function
+    let passed_along = std::iter::empty::<u32>().peekable();
+    takes_peekable(passed_along);
+
+    // `peek` called in another block
+    let mut peekable_in_block = std::iter::empty::<u32>().peekable();
+    {
+        peekable_in_block.peek();
+    }
+
+    // Check the other `Peekable` methods :)
+    {
+        let mut peekable_with_peek_mut = std::iter::empty::<u32>().peekable();
+        peekable_with_peek_mut.peek_mut();
+
+        let mut peekable_with_next_if = std::iter::empty::<u32>().peekable();
+        peekable_with_next_if.next_if(|_| true);
+
+        let mut peekable_with_next_if_eq = std::iter::empty::<u32>().peekable();
+        peekable_with_next_if_eq.next_if_eq(&3);
+    }
+
+    let mut peekable_in_closure = std::iter::empty::<u32>().peekable();
+    let call_peek = |p: &mut Peekable<Empty<u32>>| {
+        p.peek();
+    };
+    call_peek(&mut peekable_in_closure);
+
+    // From a macro
+    macro_rules! make_me_a_peekable_please {
+        () => {
+            std::iter::empty::<u32>().peekable()
+        };
+    }
+
+    let _unsuspecting_macro_user = make_me_a_peekable_please!();
+
+    // Generic Iterator returned
+    fn return_an_iter() -> impl Iterator<Item = u32> {
+        std::iter::empty::<u32>().peekable()
+    }
+
+    let _unsuspecting_user = return_an_iter();
+
+    // Call `peek` in a macro
+    macro_rules! peek_iter {
+        ($iter:ident) => {
+            $iter.peek();
+        };
+    }
+
+    let mut peek_in_macro = std::iter::empty::<u32>().peekable();
+    peek_iter!(peek_in_macro);
+
+    // Behind mut ref
+    let mut by_mut_ref_test = std::iter::empty::<u32>().peekable();
+    let by_mut_ref = &mut by_mut_ref_test;
+    by_mut_ref.peek();
+
+    // Behind ref
+    let mut by_ref_test = std::iter::empty::<u32>().peekable();
+    let by_ref = &by_ref_test;
+    by_ref_test.peek();
+
+    // In struct
+    struct PeekableWrapper {
+        f: Peekable<Empty<u32>>,
+    }
+
+    let struct_test = std::iter::empty::<u32>().peekable();
+    PeekableWrapper { f: struct_test };
+
+    // `peek` called in another block as the last expression
+    let mut peekable_last_expr = std::iter::empty::<u32>().peekable();
+    {
+        peekable_last_expr.peek();
+    }
+}
diff --git a/tests/ui/unused_peekable.stderr b/tests/ui/unused_peekable.stderr
new file mode 100644
index 00000000000..bd087f56e4c
--- /dev/null
+++ b/tests/ui/unused_peekable.stderr
@@ -0,0 +1,51 @@
+error: `peek` never called on `Peekable` iterator
+  --> $DIR/unused_peekable.rs:14:9
+   |
+LL |     let peekable = std::iter::empty::<u32>().peekable();
+   |         ^^^^^^^^
+   |
+   = note: `-D clippy::unused-peekable` implied by `-D warnings`
+   = help: consider removing the call to `peekable`
+
+error: `peek` never called on `Peekable` iterator
+  --> $DIR/unused_peekable.rs:18:9
+   |
+LL |     let new_local = old_local;
+   |         ^^^^^^^^^
+   |
+   = help: consider removing the call to `peekable`
+
+error: `peek` never called on `Peekable` iterator
+  --> $DIR/unused_peekable.rs:22:9
+   |
+LL |     let by_mut_ref = &mut by_mut_ref_test;
+   |         ^^^^^^^^^^
+   |
+   = help: consider removing the call to `peekable`
+
+error: `peek` never called on `Peekable` iterator
+  --> $DIR/unused_peekable.rs:29:9
+   |
+LL |     let peekable_from_fn = returns_peekable();
+   |         ^^^^^^^^^^^^^^^^
+   |
+   = help: consider removing the call to `peekable`
+
+error: `peek` never called on `Peekable` iterator
+  --> $DIR/unused_peekable.rs:32:13
+   |
+LL |     let mut peekable_using_iterator_method = std::iter::empty::<u32>().peekable();
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider removing the call to `peekable`
+
+error: `peek` never called on `Peekable` iterator
+  --> $DIR/unused_peekable.rs:35:13
+   |
+LL |     let mut peekable_in_for_loop = std::iter::empty::<u32>().peekable();
+   |             ^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider removing the call to `peekable`
+
+error: aborting due to 6 previous errors
+