Add [unused_peekable] lint

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

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),

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,

clippy_lints/src/lib.register_suspicious.rs

@@ -32,5 +32,6 @@ store.register_group(true, "clippy::suspicious", Some("clippy_suspicious"), vec!
     LintId::of(suspicious_trait_impl::SUSPICIOUS_ARITHMETIC_IMPL),
     LintId::of(suspicious_trait_impl::SUSPICIOUS_OP_ASSIGN_IMPL),
     LintId::of(swap_ptr_to_ref::SWAP_PTR_TO_REF),
+    LintId::of(unused_peekable::UNUSED_PEEKABLE),
     LintId::of(write::POSITIONAL_NAMED_FORMAT_PARAMETERS),
 ])

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));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 

clippy_lints/src/unused_peekable.rs

@@ -0,0 +1,225 @@
+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, is_trait_method, 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_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::sym;
+
+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"
+}
+
+declare_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
+                && let PatKind::Binding(_, 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, binding);
+
+                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 self.found_peek_call {
+            return;
+        }
+
+        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;
+                                }
+
+                                if args.iter().any(|arg| {
+                                    matches!(arg.kind, ExprKind::Path(_)) && arg_is_mut_peekable(self.cx, arg)
+                                }) {
+                                    self.found_peek_call = true;
+                                    return;
+                                }
+                            },
+                            // Catch anything taking a Peekable mutably
+                            ExprKind::MethodCall(
+                                PathSegment {
+                                    ident: method_name_ident,
+                                    ..
+                                },
+                                [self_arg, remaining_args @ ..],
+                                _,
+                            ) => {
+                                let method_name = method_name_ident.name.as_str();
+
+                                // `Peekable` methods
+                                if matches!(method_name, "peek" | "peek_mut" | "next_if" | "next_if_eq")
+                                    && arg_is_mut_peekable(self.cx, self_arg)
+                                {
+                                    self.found_peek_call = true;
+                                    return;
+                                }
+
+                                // foo.some_method() excluding Iterator methods
+                                if remaining_args.iter().any(|arg| arg_is_mut_peekable(self.cx, arg))
+                                    && !is_trait_method(self.cx, expr, sym::Iterator)
+                                {
+                                    self.found_peek_call = true;
+                                    return;
+                                }
+
+                                // foo.by_ref(), keep checking for `peek`
+                                if method_name == "by_ref" {
+                                    continue;
+                                }
+
+                                return;
+                            },
+                            ExprKind::AddrOf(_, Mutability::Mut, _) | ExprKind::Unary(..) | ExprKind::DropTemps(_) => {
+                            },
+                            ExprKind::AddrOf(_, Mutability::Not, _) => return,
+                            _ => {
+                                self.found_peek_call = true;
+                                return;
+                            },
+                        }
+                    },
+                    Node::Local(Local { init: Some(init), .. }) => {
+                        if arg_is_mut_peekable(self.cx, init) {
+                            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);
+    }
+}
+
+fn arg_is_mut_peekable(cx: &LateContext<'_>, arg: &Expr<'_>) -> bool {
+    if let Some(ty) = cx.typeck_results().expr_ty_opt(arg)
+        && let (ty, _, Mutability::Mut) = peel_mid_ty_refs_is_mutable(ty)
+        && match_type(cx, ty, &paths::PEEKABLE)
+    {
+        true
+    } else {
+        false
+    }
+}

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"];

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) {