Rollup merge of #106581 - estebank:bad-suggestion, r=compiler-errors

Do not emit wrong E0308 suggestion for closure mismatch

Found in #76353.

Author: Yuki Okushi

compiler/rustc_hir_typeck/src/demand.rs

@@ -1379,7 +1379,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         }
                     }
                     // If we've reached our target type with just removing `&`, then just print now.
-                    if steps == 0 {
+                    if steps == 0 && !remove.trim().is_empty() {
                         return Some((
                             prefix_span,
                             format!("consider removing the `{}`", remove.trim()),
@@ -1438,6 +1438,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         } else {
                             (prefix_span, format!("{}{}", prefix, "*".repeat(steps)))
                         };
+                        if suggestion.trim().is_empty() {
+                            return None;
+                        }
 
                         return Some((
                             span,

src/test/ui/type/closure-with-wrong-borrows.rs

@@ -0,0 +1,10 @@
+struct S<'a>(&'a str);
+
+fn f(inner: fn(&str, &S)) {
+}
+
+#[allow(unreachable_code)]
+fn main() {
+    let inner: fn(_, _) = unimplemented!();
+    f(inner); //~ ERROR mismatched types
+}

src/test/ui/type/closure-with-wrong-borrows.stderr

@@ -0,0 +1,19 @@
+error[E0308]: mismatched types
+  --> $DIR/closure-with-wrong-borrows.rs:9:7
+   |
+LL |     f(inner);
+   |     - ^^^^^ one type is more general than the other
+   |     |
+   |     arguments to this function are incorrect
+   |
+   = note: expected fn pointer `for<'a, 'b, 'c> fn(&'a str, &'b S<'c>)`
+              found fn pointer `fn(_, _)`
+note: function defined here
+  --> $DIR/closure-with-wrong-borrows.rs:3:4
+   |
+LL | fn f(inner: fn(&str, &S)) {
+   |    ^ -------------------
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.