Change wording of missing return type suggestion

Author: estebank

compiler/rustc_typeck/src/check/fn_ctxt/suggestions.rs

@@ -530,13 +530,20 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 err.span_suggestion(
                     span,
                     "try adding a return type",
-                    format!("-> {} ", self.resolve_vars_with_obligations(found)),
+                    format!("-> {} ", found),
                     Applicability::MachineApplicable,
                 );
                 true
             }
             (&hir::FnRetTy::DefaultReturn(span), false, true, true) => {
-                err.span_label(span, "possibly return type missing here?");
+                // FIXME: if `found` could be `impl Iterator` or `impl Fn*`, we should suggest
+                // that.
+                err.span_suggestion(
+                    span,
+                    "a return type might be missing here",
+                    "-> _ ".to_string(),
+                    Applicability::HasPlaceholders,
+                );
                 true
             }
             (&hir::FnRetTy::DefaultReturn(span), _, false, true) => {

src/test/ui/block-result/issue-20862.stderr

@@ -2,7 +2,7 @@ error[E0308]: mismatched types
   --> $DIR/issue-20862.rs:2:5
    |
 LL | fn foo(x: i32) {
-   |                - possibly return type missing here?
+   |                - help: a return type might be missing here: `-> _`
 LL |     |y| x + y
    |     ^^^^^^^^^ expected `()`, found closure
    |

src/test/ui/never_type/diverging-tuple-parts-39485.stderr

@@ -1,13 +1,15 @@
 error[E0308]: mismatched types
   --> $DIR/diverging-tuple-parts-39485.rs:8:5
    |
-LL | fn g() {
-   |        - possibly return type missing here?
 LL |     &panic!()
    |     ^^^^^^^^^ expected `()`, found reference
    |
    = note: expected unit type `()`
               found reference `&_`
+help: a return type might be missing here
+   |
+LL | fn g() -> _ {
+   |        ++++
 help: consider removing the borrow
    |
 LL -     &panic!()

src/test/ui/typeck/issue-91334.stderr

@@ -40,7 +40,7 @@ error[E0308]: mismatched types
 LL | fn f(){||yield(((){),
    |       -^^^^^^^^^^^^^^^ expected `()`, found generator
    |       |
-   |       possibly return type missing here?
+   |       help: a return type might be missing here: `-> _`
    |
    = note: expected unit type `()`
               found generator `[generator@$DIR/issue-91334.rs:10:8: 10:23]`

src/test/ui/typeck/return_type_containing_closure.rs

@@ -1,10 +1,10 @@
 #[allow(unused)]
-fn foo() {
-    //~^ NOTE possibly return type missing here?
+fn foo() { //~ HELP a return type might be missing here
     vec!['a'].iter().map(|c| c)
     //~^ ERROR mismatched types [E0308]
     //~| NOTE expected `()`, found struct `Map`
     //~| NOTE expected unit type `()`
+    //~| HELP consider using a semicolon here
 }
 
 fn main() {}

src/test/ui/typeck/return_type_containing_closure.stderr

@@ -1,16 +1,19 @@
 error[E0308]: mismatched types
-  --> $DIR/return_type_containing_closure.rs:4:5
+  --> $DIR/return_type_containing_closure.rs:3:5
    |
-LL | fn foo() {
-   |          - possibly return type missing here?
-LL |
 LL |     vec!['a'].iter().map(|c| c)
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^- help: consider using a semicolon here: `;`
-   |     |
-   |     expected `()`, found struct `Map`
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found struct `Map`
    |
    = note: expected unit type `()`
-                 found struct `Map<std::slice::Iter<'_, char>, [closure@$DIR/return_type_containing_closure.rs:4:26: 4:31]>`
+                 found struct `Map<std::slice::Iter<'_, char>, [closure@$DIR/return_type_containing_closure.rs:3:26: 3:31]>`
+help: consider using a semicolon here
+   |
+LL |     vec!['a'].iter().map(|c| c);
+   |                                +
+help: a return type might be missing here
+   |
+LL | fn foo() -> _ {
+   |          ++++
 
 error: aborting due to previous error