Add test case where FnMut used once needs &mut

tests/ui/eta.fixed

@@ -305,7 +305,7 @@ fn not_general_enough() {
 }
 
 // https://github.com/rust-lang/rust-clippy/issues/9369
-pub fn mutable_impl_fn_mut(mut f: impl FnMut()) {
+pub fn mutable_impl_fn_mut(mut f: impl FnMut(), mut f_used_once: impl FnMut()) -> impl FnMut() {
     fn takes_fn_mut(_: impl FnMut()) {}
     takes_fn_mut(&mut f);
 
@@ -313,4 +313,6 @@ pub fn mutable_impl_fn_mut(mut f: impl FnMut()) {
     takes_fn_once(&mut f);
 
     f();
+
+    move || takes_fn_mut(&mut f_used_once)
 }

tests/ui/eta.rs

@@ -305,7 +305,7 @@ fn f(_: impl FnMut(&Path) -> std::io::Result<()>) {}
 }
 
 // https://github.com/rust-lang/rust-clippy/issues/9369
-pub fn mutable_impl_fn_mut(mut f: impl FnMut()) {
+pub fn mutable_impl_fn_mut(mut f: impl FnMut(), mut f_used_once: impl FnMut()) -> impl FnMut() {
     fn takes_fn_mut(_: impl FnMut()) {}
     takes_fn_mut(|| f());
 
@@ -313,4 +313,6 @@ fn takes_fn_once(_: impl FnOnce()) {}
     takes_fn_once(|| f());
 
     f();
+
+    move || takes_fn_mut(|| f_used_once())
 }

tests/ui/eta.stderr

@@ -128,5 +128,11 @@ error: redundant closure
 LL |     takes_fn_once(|| f());
    |                   ^^^^^^ help: replace the closure with the function itself: `&mut f`
 
-error: aborting due to 21 previous errors
+error: redundant closure
+  --> $DIR/eta.rs:317:26
+   |
+LL |     move || takes_fn_mut(|| f_used_once())
+   |                          ^^^^^^^^^^^^^^^^ help: replace the closure with the function itself: `&mut f_used_once`
+
+error: aborting due to 22 previous errors