1 files changed, 114 insertions, 0 deletions
diff --git a/src/tools/clippy/clippy_lints/src/mut_mut.rs b/src/tools/clippy/clippy_lints/src/mut_mut.rs
new file mode 100644
index 000000000..cb16f0004
--- /dev/null
+++ b/src/tools/clippy/clippy_lints/src/mut_mut.rs
@@ -0,0 +1,114 @@
+use clippy_utils::diagnostics::span_lint;
+use clippy_utils::higher;
+use rustc_hir as hir;
+use rustc_hir::intravisit;
+use rustc_lint::{LateContext, LateLintPass, LintContext};
+use rustc_middle::lint::in_external_macro;
+use rustc_middle::ty;
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for instances of `mut mut` references.
+    ///
+    /// ### Why is this bad?
+    /// Multiple `mut`s don't add anything meaningful to the
+    /// source. This is either a copy'n'paste error, or it shows a fundamental
+    /// misunderstanding of references.
+    ///
+    /// ### Example
+    /// ```rust
+    /// # let mut y = 1;
+    /// let x = &mut &mut y;
+    /// ```
+    #[clippy::version = "pre 1.29.0"]
+    pub MUT_MUT,
+    pedantic,
+    "usage of double-mut refs, e.g., `&mut &mut ...`"
+}
+
+declare_lint_pass!(MutMut => [MUT_MUT]);
+
+impl<'tcx> LateLintPass<'tcx> for MutMut {
+    fn check_block(&mut self, cx: &LateContext<'tcx>, block: &'tcx hir::Block<'_>) {
+        intravisit::walk_block(&mut MutVisitor { cx }, block);
+    }
+
+    fn check_ty(&mut self, cx: &LateContext<'tcx>, ty: &'tcx hir::Ty<'_>) {
+        use rustc_hir::intravisit::Visitor;
+
+        MutVisitor { cx }.visit_ty(ty);
+    }
+}
+
+pub struct MutVisitor<'a, 'tcx> {
+    cx: &'a LateContext<'tcx>,
+}
+
+impl<'a, 'tcx> intravisit::Visitor<'tcx> for MutVisitor<'a, 'tcx> {
+    fn visit_expr(&mut self, expr: &'tcx hir::Expr<'_>) {
+        if in_external_macro(self.cx.sess(), expr.span) {
+            return;
+        }
+
+        if let Some(higher::ForLoop { arg, body, .. }) = higher::ForLoop::hir(expr) {
+            // A `for` loop lowers to:
+            // ```rust
+            // match ::std::iter::Iterator::next(&mut iter) {
+            // //                                ^^^^
+            // ```
+            // Let's ignore the generated code.
+            intravisit::walk_expr(self, arg);
+            intravisit::walk_expr(self, body);
+        } else if let hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Mut, e) = expr.kind {
+            if let hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Mut, _) = e.kind {
+                span_lint(
+                    self.cx,
+                    MUT_MUT,
+                    expr.span,
+                    "generally you want to avoid `&mut &mut _` if possible",
+                );
+            } else if let ty::Ref(_, _, hir::Mutability::Mut) = self.cx.typeck_results().expr_ty(e).kind() {
+                span_lint(
+                    self.cx,
+                    MUT_MUT,
+                    expr.span,
+                    "this expression mutably borrows a mutable reference. Consider reborrowing",
+                );
+            }
+        }
+    }
+
+    fn visit_ty(&mut self, ty: &'tcx hir::Ty<'_>) {
+        if in_external_macro(self.cx.sess(), ty.span) {
+            return;
+        }
+
+        if let hir::TyKind::Rptr(
+            _,
+            hir::MutTy {
+                ty: pty,
+                mutbl: hir::Mutability::Mut,
+            },
+        ) = ty.kind
+        {
+            if let hir::TyKind::Rptr(
+                _,
+                hir::MutTy {
+                    mutbl: hir::Mutability::Mut,
+                    ..
+                },
+            ) = pty.kind
+            {
+                span_lint(
+                    self.cx,
+                    MUT_MUT,
+                    ty.span,
+                    "generally you want to avoid `&mut &mut _` if possible",
+                );
+            }
+        }
+
+        intravisit::walk_ty(self, ty);
+    }
+}