only lint once on 3+ &muts

for some reason this seems to already be the case with exprs, so only do
it for tys

Author: ada4a

clippy_lints/src/lib.rs

@@ -481,7 +481,7 @@ pub fn register_lint_passes(store: &mut rustc_lint::LintStore, conf: &'static Co
     store.register_late_pass(|_| Box::new(needless_for_each::NeedlessForEach));
     store.register_late_pass(|_| Box::new(misc::LintPass));
     store.register_late_pass(|_| Box::new(eta_reduction::EtaReduction));
-    store.register_late_pass(|_| Box::new(mut_mut::MutMut));
+    store.register_late_pass(|_| Box::new(mut_mut::MutMut::default()));
     store.register_late_pass(|_| Box::new(mut_reference::UnnecessaryMutPassed));
     store.register_late_pass(|_| Box::<significant_drop_tightening::SignificantDropTightening<'_>>::default());
     store.register_late_pass(|_| Box::new(len_zero::LenZero));

clippy_lints/src/mut_mut.rs

@@ -2,11 +2,12 @@ use clippy_utils::diagnostics::{span_lint_and_sugg, span_lint_hir_and_then};
 use clippy_utils::higher;
 use clippy_utils::source::snippet_with_applicability;
 use clippy_utils::sugg::Sugg;
+use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::Applicability;
-use rustc_hir::{self as hir, AmbigArg, intravisit};
+use rustc_hir::{self as hir, AmbigArg, HirId, intravisit};
 use rustc_lint::{LateContext, LateLintPass, LintContext};
 use rustc_middle::ty;
-use rustc_session::declare_lint_pass;
+use rustc_session::impl_lint_pass;
 
 declare_clippy_lint! {
     /// ### What it does
@@ -28,7 +29,12 @@ declare_clippy_lint! {
     "usage of double-mut refs, e.g., `&mut &mut ...`"
 }
 
-declare_lint_pass!(MutMut => [MUT_MUT]);
+impl_lint_pass!(MutMut => [MUT_MUT]);
+
+#[derive(Default)]
+pub(crate) struct MutMut {
+    seen_tys: FxHashSet<HirId>,
+}
 
 impl<'tcx> LateLintPass<'tcx> for MutMut {
     fn check_block(&mut self, cx: &LateContext<'tcx>, block: &'tcx hir::Block<'_>) {
@@ -42,6 +48,14 @@ impl<'tcx> LateLintPass<'tcx> for MutMut {
             && mty2.mutbl == hir::Mutability::Mut
             && !ty.span.in_external_macro(cx.sess().source_map())
         {
+            // we have 2+ `&mut`s, e.g., `&mut &mut &mut x`
+            // and we have already flagged on the outermost `&mut &mut (&mut x)`,
+            // so don't flag the inner `&mut &mut (x)`
+            if self.seen_tys.contains(&ty.hir_id) {
+                return;
+            }
+            self.seen_tys.insert(mty.ty.hir_id);
+
             let mut applicability = Applicability::MaybeIncorrect;
             let sugg = snippet_with_applicability(cx.sess(), mty.ty.span, "..", &mut applicability);
             span_lint_and_sugg(

tests/ui/mut_mut.fixed

@@ -12,7 +12,7 @@
 extern crate proc_macros;
 use proc_macros::{external, inline_macros};
 
-fn fun(x: &mut &mut u32) {
+fn fun(x: &mut u32) {
     //~^ mut_mut
 }
 
@@ -37,16 +37,15 @@ fn main() {
     }
 
     {
-        let y: &mut &mut u32 = &mut 2;
+        let y: &mut u32 = &mut 2;
         //~^ mut_mut
         //~| mut_mut
     }
 
     {
-        let y: &mut &mut &mut u32 = &mut &mut 2;
+        let y: &mut &mut u32 = &mut &mut 2;
         //~^ mut_mut
         //~| mut_mut
-        //~| mut_mut
     }
 
     let mut z = inline!(&mut $(&mut 3u32));

tests/ui/mut_mut.rs

@@ -46,7 +46,6 @@ fn main() {
         let y: &mut &mut &mut u32 = &mut &mut &mut 2;
         //~^ mut_mut
         //~| mut_mut
-        //~| mut_mut
     }
 
     let mut z = inline!(&mut $(&mut 3u32));

tests/ui/mut_mut.stderr

@@ -43,11 +43,5 @@ error: a type of form `&mut &mut _`
 LL |         let y: &mut &mut &mut u32 = &mut &mut &mut 2;
    |                ^^^^^^^^^^^^^^^^^^ help: remove the extra `&mut`: `&mut &mut u32`
 
-error: a type of form `&mut &mut _`
-  --> tests/ui/mut_mut.rs:46:21
-   |
-LL |         let y: &mut &mut &mut u32 = &mut &mut &mut 2;
-   |                     ^^^^^^^^^^^^^ help: remove the extra `&mut`: `&mut u32`
-
-error: aborting due to 8 previous errors
+error: aborting due to 7 previous errors