peel _all_ extra &muts

see
https://rust-lang.zulipchat.com/#narrow/channel/257328-clippy/topic/problem.20when.20suggestion.20only.20removes.20one.20.22level.22.20of.20a.20lint/near/532889973

Author: ada4a

clippy_lints/src/mut_mut.rs

@@ -56,14 +56,30 @@ impl<'tcx> LateLintPass<'tcx> for MutMut {
             }
             self.seen_tys.insert(mty.ty.hir_id);
 
+            // if there is an even longer chain, like `&mut &mut &mut x`, suggest peeling off
+            // all extra ones at once
+            let (mut t, mut t2) = (mty.ty, mty2.ty);
+            let mut many_muts = false;
+            loop {
+                if let TyKind::Ref(_, next) = t2.kind
+                    && next.mutbl == Mutability::Mut
+                {
+                    (t, t2) = (t2, next.ty);
+                    many_muts = true;
+                } else {
+                    break;
+                }
+            }
+
             let mut applicability = Applicability::MaybeIncorrect;
-            let sugg = snippet_with_applicability(cx.sess(), mty.ty.span, "..", &mut applicability);
+            let sugg = snippet_with_applicability(cx.sess(), t.span, "..", &mut applicability);
+            let suffix = if many_muts { "s" } else { "" };
             span_lint_and_sugg(
                 cx,
                 MUT_MUT,
                 ty.span,
                 "a type of form `&mut &mut _`",
-                "remove the extra `&mut`",
+                format!("remove the extra `&mut`{suffix}"),
                 sugg.to_string(),
                 applicability,
             );
@@ -91,20 +107,43 @@ impl<'tcx> intravisit::Visitor<'tcx> for MutVisitor<'_, 'tcx> {
             intravisit::walk_expr(self, arg);
             intravisit::walk_expr(self, body);
         } else if let ExprKind::AddrOf(BorrowKind::Ref, Mutability::Mut, e) = expr.kind {
-            if let ExprKind::AddrOf(BorrowKind::Ref, Mutability::Mut, _) = e.kind {
+            if let ExprKind::AddrOf(BorrowKind::Ref, Mutability::Mut, e2) = e.kind {
                 if !expr.span.eq_ctxt(e.span) {
                     return;
                 }
+
+                // if there is an even longer chain, like `&mut &mut &mut x`, suggest peeling off
+                // all extra ones at once
+                let (mut e, mut e2) = (e, e2);
+                let mut many_muts = false;
+                loop {
+                    if !e.span.eq_ctxt(e2.span) {
+                        return;
+                    }
+                    if let ExprKind::AddrOf(BorrowKind::Ref, Mutability::Mut, next) = e2.kind {
+                        (e, e2) = (e2, next);
+                        many_muts = true;
+                    } else {
+                        break;
+                    }
+                }
+
                 let mut applicability = Applicability::MaybeIncorrect;
                 let sugg = Sugg::hir_with_applicability(self.cx, e, "..", &mut applicability);
+                let suffix = if many_muts { "s" } else { "" };
                 span_lint_hir_and_then(
                     self.cx,
                     MUT_MUT,
                     expr.hir_id,
                     expr.span,
                     "an expression of form `&mut &mut _`",
                     |diag| {
-                        diag.span_suggestion(expr.span, "remove the extra `&mut`", sugg, applicability);
+                        diag.span_suggestion(
+                            expr.span,
+                            format!("remove the extra `&mut`{suffix}"),
+                            sugg,
+                            applicability,
+                        );
                     },
                 );
             } else if let ty::Ref(_, ty, Mutability::Mut) = self.cx.typeck_results().expr_ty(e).kind()

tests/ui/mut_mut.fixed

@@ -43,7 +43,7 @@ fn main() {
     }
 
     {
-        let y: &mut &mut u32 = &mut &mut 2;
+        let y: &mut u32 = &mut 2;
         //~^ mut_mut
         //~| mut_mut
     }

tests/ui/mut_mut.stderr

@@ -35,13 +35,13 @@ error: an expression of form `&mut &mut _`
   --> tests/ui/mut_mut.rs:46:37
    |
 LL |         let y: &mut &mut &mut u32 = &mut &mut &mut 2;
-   |                                     ^^^^^^^^^^^^^^^^ help: remove the extra `&mut`: `&mut &mut 2`
+   |                                     ^^^^^^^^^^^^^^^^ help: remove the extra `&mut`s: `&mut 2`
 
 error: a type of form `&mut &mut _`
   --> tests/ui/mut_mut.rs:46:16
    |
 LL |         let y: &mut &mut &mut u32 = &mut &mut &mut 2;
-   |                ^^^^^^^^^^^^^^^^^^ help: remove the extra `&mut`: `&mut &mut u32`
+   |                ^^^^^^^^^^^^^^^^^^ help: remove the extra `&mut`s: `&mut u32`
 
 error: aborting due to 7 previous errors
 

tests/ui/mut_mut_unfixable.stderr

@@ -29,7 +29,7 @@ error: an expression of form `&mut &mut _`
   --> tests/ui/mut_mut_unfixable.rs:30:17
    |
 LL |         let y = &mut &mut &mut 2;
-   |                 ^^^^^^^^^^^^^^^^ help: remove the extra `&mut`: `&mut &mut 2`
+   |                 ^^^^^^^^^^^^^^^^ help: remove the extra `&mut`s: `&mut 2`
 
 error: aborting due to 5 previous errors