Suggest mutable references for FnMut closure arguments

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

@@ -1046,6 +1046,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 expected_kind,
                 trait_prefix,
             );
+            self.suggest_change_mut_ref_for_closure(&mut err, &obligation);
             self.note_obligation_cause(&mut err, &obligation);
             return Some(err.emit());
         }
@@ -3019,6 +3020,25 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         })
     }
 
+    fn suggest_change_mut_ref_for_closure(
+        &self,
+        err: &mut Diag<'_>,
+        obligation: &PredicateObligation<'tcx>,
+    ) {
+        if let ObligationCauseCode::FunctionArg { arg_hir_id, .. } = obligation.cause.code()
+            && let (_, Some(root_trait_pred)) =
+                obligation.cause.code().peel_derives_with_predicate()
+            && let Node::Expr(arg) = self.tcx.hir_node(*arg_hir_id)
+            && let hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Not, _) = arg.kind
+        {
+            let mut obligation = obligation.clone();
+            // Error reporting may narrow the cause span to the borrow's operand.
+            // Use the whole argument so `suggest_change_mut` can replace the shared borrow.
+            obligation.cause.span = arg.span;
+            self.suggest_change_mut(&obligation, err, root_trait_pred);
+        }
+    }
+
     pub fn note_obligation_cause(
         &self,
         err: &mut Diag<'_>,
@@ -3557,7 +3577,6 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 _ => {}
             }
         }
-
         self.dcx().create_err(err)
     }
 

tests/ui/closures/fnmut-shared-reference-requires-fn.rs

@@ -0,0 +1,11 @@
+//! Do not suggest a mutable reference when the root obligation genuinely requires `Fn`.
+
+fn requires_fn<F: Fn()>(_: F) {}
+
+fn main() {
+    let mut value = 0;
+    let mut func = || value += 1;
+    //~^ ERROR expected a closure that implements the `Fn` trait
+
+    requires_fn(&func);
+}

tests/ui/closures/fnmut-shared-reference-requires-fn.stderr

@@ -0,0 +1,23 @@
+error[E0525]: expected a closure that implements the `Fn` trait, but this closure only implements `FnMut`
+  --> $DIR/fnmut-shared-reference-requires-fn.rs:7:20
+   |
+LL |     let mut func = || value += 1;
+   |                    ^^ ----- closure is `FnMut` because it mutates the variable `value` here
+   |                    |
+   |                    this closure implements `FnMut`, not `Fn`
+...
+LL |     requires_fn(&func);
+   |     -----------  ---- the requirement to implement `Fn` derives from here
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = note: required for `&{closure@$DIR/fnmut-shared-reference-requires-fn.rs:7:20: 7:22}` to implement `Fn()`
+note: required by a bound in `requires_fn`
+  --> $DIR/fnmut-shared-reference-requires-fn.rs:3:19
+   |
+LL | fn requires_fn<F: Fn()>(_: F) {}
+   |                   ^^^^ required by this bound in `requires_fn`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0525`.

tests/ui/closures/fnmut-shared-reference-suggestion-issue-118843.fixed

@@ -0,0 +1,13 @@
+//@ run-rustfix
+
+//! Regression test for https://github.com/rust-lang/rust/issues/118843.
+//! A shared reference to an `FnMut` closure should suggest a mutable reference.
+
+fn main() {
+    let mut value = 0;
+    let mut func = |increment: usize| value += increment;
+    //~^ ERROR expected a closure that implements the `Fn` trait
+
+    (0..100).for_each(&mut func);
+    //~^ HELP consider changing this borrow's mutability
+}

tests/ui/closures/fnmut-shared-reference-suggestion-issue-118843.rs

@@ -0,0 +1,13 @@
+//@ run-rustfix
+
+//! Regression test for https://github.com/rust-lang/rust/issues/118843.
+//! A shared reference to an `FnMut` closure should suggest a mutable reference.
+
+fn main() {
+    let mut value = 0;
+    let mut func = |increment: usize| value += increment;
+    //~^ ERROR expected a closure that implements the `Fn` trait
+
+    (0..100).for_each(&func);
+    //~^ HELP consider changing this borrow's mutability
+}

tests/ui/closures/fnmut-shared-reference-suggestion-issue-118843.stderr

@@ -0,0 +1,24 @@
+error[E0525]: expected a closure that implements the `Fn` trait, but this closure only implements `FnMut`
+  --> $DIR/fnmut-shared-reference-suggestion-issue-118843.rs:8:20
+   |
+LL |     let mut func = |increment: usize| value += increment;
+   |                    ^^^^^^^^^^^^^^^^^^ ----- closure is `FnMut` because it mutates the variable `value` here
+   |                    |
+   |                    this closure implements `FnMut`, not `Fn`
+...
+LL |     (0..100).for_each(&func);
+   |              --------  ---- the requirement to implement `Fn` derives from here
+   |              |
+   |              required by a bound introduced by this call
+   |
+   = note: required for `&{closure@$DIR/fnmut-shared-reference-suggestion-issue-118843.rs:8:20: 8:38}` to implement `FnMut(usize)`
+note: required by a bound in `for_each`
+  --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
+help: consider changing this borrow's mutability
+   |
+LL |     (0..100).for_each(&mut func);
+   |                        +++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0525`.