Do not recomment cloning explicit &mut expressions

Author: estebank

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -1033,6 +1033,11 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         while let hir::ExprKind::AddrOf(.., inner) | hir::ExprKind::Unary(hir::UnOp::Deref, inner) =
             &inner_expr.kind
         {
+            if let hir::ExprKind::AddrOf(_, hir::Mutability::Mut, _) = inner_expr.kind {
+                // We assume that `&mut` refs are desired for their side-effects, so cloning the
+                // value wouldn't do what the user wanted.
+                return;
+            }
             inner_expr = inner;
         }
         if inner_expr.span.lo() != expr.span.lo() {

tests/ui/borrowck/borrowck-overloaded-index-move-index.stderr

@@ -11,12 +11,6 @@ LL |     println!("{}", f[s]);
 ...
 LL |     use_mut(rs);
    |             -- borrow later used here
-   |
-help: consider cloning the value if the performance cost is acceptable
-   |
-LL -     let rs = &mut s;
-LL +     let rs = s.clone();
-   |
 
 error[E0505]: cannot move out of `s` because it is borrowed
   --> $DIR/borrowck-overloaded-index-move-index.rs:53:7
@@ -31,12 +25,6 @@ LL |     f[s] = 10;
 ...
 LL |     use_mut(rs);
    |             -- borrow later used here
-   |
-help: consider cloning the value if the performance cost is acceptable
-   |
-LL -     let rs = &mut s;
-LL +     let rs = s.clone();
-   |
 
 error[E0382]: use of moved value: `s`
   --> $DIR/borrowck-overloaded-index-move-index.rs:53:7

tests/ui/nll/polonius/polonius-smoke-test.stderr

@@ -27,12 +27,6 @@ LL |     let z = x;
    |             ^ move out of `x` occurs here
 LL |     y
    |     - returning this value requires that `*x` is borrowed for `'1`
-   |
-help: consider cloning the value if the performance cost is acceptable
-   |
-LL -     let y = &mut *x;
-LL +     let y = x.clone();
-   |
 
 error[E0505]: cannot move out of `s` because it is borrowed
   --> $DIR/polonius-smoke-test.rs:42:5
@@ -46,12 +40,6 @@ LL |     s;
    |     ^ move out of `s` occurs here
 LL |     tmp;
    |     --- borrow later used here
-   |
-help: consider cloning the value if the performance cost is acceptable
-   |
-LL -     let r = &mut *s;
-LL +     let r = s.clone();
-   |
 
 error: aborting due to 4 previous errors