Don't suggest adding return type for closures with default return type

Author: wafarm

Diff for: compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -832,7 +832,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 err.subdiagnostic(errors::ExpectedReturnTypeLabel::Unit { span });
                 return true;
             }
-            &hir::FnRetTy::DefaultReturn(span) if expected.is_unit() => {
+            // Don't suggest adding return types for closures with default return.
+            // They simply don't work.
+            &hir::FnRetTy::DefaultReturn(span)
+                if expected.is_unit() && !self.tcx.is_closure_like(fn_id.to_def_id()) =>
+            {
                 if let Some(found) = found.make_suggestable(self.tcx, false, None) {
                     err.subdiagnostic(errors::AddReturnTypeSuggestion::Add {
                         span,

Diff for: tests/ui/closures/add_semicolon_non_block_closure.rs

@@ -8,5 +8,4 @@ fn main() {
     foo(|| bar())
     //~^ ERROR mismatched types [E0308]
     //~| HELP consider using a semicolon here
-    //~| HELP try adding a return type
 }

Diff for: tests/ui/closures/add_semicolon_non_block_closure.stderr

@@ -8,10 +8,6 @@ help: consider using a semicolon here
    |
 LL |     foo(|| { bar(); })
    |            +      +++
-help: try adding a return type
-   |
-LL |     foo(|| -> i32 bar())
-   |            ++++++
 
 error: aborting due to 1 previous error
 

Diff for: tests/ui/suggestions/try-operator-dont-suggest-semicolon.rs

@@ -3,7 +3,6 @@
 
 fn main() -> Result<(), ()> {
     a(|| {
-        //~^ HELP: try adding a return type
         b()
         //~^ ERROR: mismatched types [E0308]
         //~| NOTE: expected `()`, found `i32`

Diff for: tests/ui/suggestions/try-operator-dont-suggest-semicolon.stderr

@@ -1,20 +1,13 @@
 error[E0308]: mismatched types
-  --> $DIR/try-operator-dont-suggest-semicolon.rs:7:9
+  --> $DIR/try-operator-dont-suggest-semicolon.rs:6:9
    |
 LL |         b()
-   |         ^^^ expected `()`, found `i32`
-   |
-help: consider using a semicolon here
-   |
-LL |         b();
-   |            +
-help: try adding a return type
-   |
-LL |     a(|| -> i32 {
-   |          ++++++
+   |         ^^^- help: consider using a semicolon here: `;`
+   |         |
+   |         expected `()`, found `i32`
 
 error[E0308]: mismatched types
-  --> $DIR/try-operator-dont-suggest-semicolon.rs:17:9
+  --> $DIR/try-operator-dont-suggest-semicolon.rs:16:9
    |
 LL | /     if true {
 LL | |

Diff for: tests/ui/typeck/issue-81943.stderr

@@ -2,32 +2,36 @@ error[E0308]: mismatched types
   --> $DIR/issue-81943.rs:7:9
    |
 LL |   f(|x| lib::d!(x));
-   |        -^^^^^^^^^^ expected `()`, found `i32`
-   |        |
-   |        help: try adding a return type: `-> i32`
+   |         ^^^^^^^^^^ expected `()`, found `i32`
    |
    = note: this error originates in the macro `lib::d` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0308]: mismatched types
   --> $DIR/issue-81943.rs:8:28
    |
 LL |   f(|x| match x { tmp => { g(tmp) } });
-   |                            ^^^^^^ expected `()`, found `i32`
+   |         -------------------^^^^^^----
+   |         |                  |
+   |         |                  expected `()`, found `i32`
+   |         expected this to be `()`
    |
 help: consider using a semicolon here
    |
 LL |   f(|x| match x { tmp => { g(tmp); } });
    |                                  +
-help: try adding a return type
+help: consider using a semicolon here
    |
-LL |   f(|x| -> i32 match x { tmp => { g(tmp) } });
-   |         ++++++
+LL |   f(|x| match x { tmp => { g(tmp) } };);
+   |                                      +
 
 error[E0308]: mismatched types
   --> $DIR/issue-81943.rs:10:38
    |
 LL |     ($e:expr) => { match $e { x => { g(x) } } }
-   |                                      ^^^^ expected `()`, found `i32`
+   |                    ------------------^^^^----
+   |                    |                 |
+   |                    |                 expected `()`, found `i32`
+   |                    expected this to be `()`
 LL |   }
 LL |   f(|x| d!(x));
    |         ----- in this macro invocation
@@ -37,10 +41,10 @@ help: consider using a semicolon here
    |
 LL |     ($e:expr) => { match $e { x => { g(x); } } }
    |                                          +
-help: try adding a return type
+help: consider using a semicolon here
    |
-LL |   f(|x| -> i32 d!(x));
-   |         ++++++
+LL |     ($e:expr) => { match $e { x => { g(x) } }; }
+   |                                              +
 
 error: aborting due to 3 previous errors