Trust signature over return expr for generators.

This commit extends the logic used to determine what the expected
signature of a closure is so that it can also determine the expected
signature of a generator. This improves a diagnostic where the fn
signature was blamed instead of the generator body. It doesn't fix
fix the diagnostic for `async fn`.

src/test/ui/generator-yielding-or-returning-itself.rs

@@ -13,7 +13,7 @@ pub fn want_cyclic_generator_return<T>(_: T)
 
 fn supply_cyclic_generator_return() {
     want_cyclic_generator_return(|| {
-        //~^ ERROR type mismatch
+        //~^ ERROR closure/generator type that references itself
         if false { yield None.unwrap(); }
         None.unwrap()
     })

src/test/ui/generator-yielding-or-returning-itself.stderr

@@ -1,20 +1,17 @@
-error[E0271]: type mismatch resolving `<[generator@$DIR/generator-yielding-or-returning-itself.rs:15:34: 19:6 _] as std::ops::Generator>::Return == [generator@$DIR/generator-yielding-or-returning-itself.rs:15:34: 19:6 _]`
-  --> $DIR/generator-yielding-or-returning-itself.rs:15:5
+error[E0644]: closure/generator type that references itself
+  --> $DIR/generator-yielding-or-returning-itself.rs:15:34
    |
-LL |     want_cyclic_generator_return(|| {
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cyclic type of infinite size
+LL |       want_cyclic_generator_return(|| {
+   |  __________________________________^
+LL | |
+LL | |         if false { yield None.unwrap(); }
+LL | |         None.unwrap()
+LL | |     })
+   | |_____^ cyclic type of infinite size
    |
    = note: closures cannot capture themselves or take themselves as argument;
            this error may be the result of a recent compiler bug-fix,
            see https://github.com/rust-lang/rust/issues/46062 for more details
-note: required by `want_cyclic_generator_return`
-  --> $DIR/generator-yielding-or-returning-itself.rs:9:1
-   |
-LL | / pub fn want_cyclic_generator_return<T>(_: T)
-LL | |     where T: Generator<Yield = (), Return = T>
-LL | | {
-LL | | }
-   | |_^
 
 error[E0271]: type mismatch resolving `<[generator@$DIR/generator-yielding-or-returning-itself.rs:28:33: 32:6 _] as std::ops::Generator>::Yield == [generator@$DIR/generator-yielding-or-returning-itself.rs:28:33: 32:6 _]`
   --> $DIR/generator-yielding-or-returning-itself.rs:28:5
@@ -36,4 +33,5 @@ LL | | }
 
 error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0271`.
+Some errors have detailed explanations: E0271, E0644.
+For more information about an error, try `rustc --explain E0271`.

src/test/ui/generator/type-mismatch-signature-deduction.rs

@@ -2,15 +2,15 @@
 
 use std::ops::Generator;
 
-fn foo() -> impl Generator<Return = i32> { //~ ERROR type mismatch
+fn foo() -> impl Generator<Return = i32> {
     || {
         if false {
-            return Ok(6);
+            return Ok(6); //~ ERROR mismatched types [E0308]
         }
 
         yield ();
 
-        5 //~ ERROR mismatched types [E0308]
+        5
     }
 }
 

src/test/ui/generator/type-mismatch-signature-deduction.stderr

@@ -1,23 +1,12 @@
 error[E0308]: mismatched types
-  --> $DIR/type-mismatch-signature-deduction.rs:13:9
+  --> $DIR/type-mismatch-signature-deduction.rs:8:20
    |
-LL |         5
-   |         ^ expected enum `std::result::Result`, found integer
+LL |             return Ok(6);
+   |                    ^^^^^ expected i32, found enum `std::result::Result`
    |
-   = note: expected type `std::result::Result<{integer}, _>`
-              found type `{integer}`
+   = note: expected type `i32`
+              found type `std::result::Result<{integer}, _>`
 
-error[E0271]: type mismatch resolving `<[generator@$DIR/type-mismatch-signature-deduction.rs:6:5: 14:6 _] as std::ops::Generator>::Return == i32`
-  --> $DIR/type-mismatch-signature-deduction.rs:5:13
-   |
-LL | fn foo() -> impl Generator<Return = i32> {
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected enum `std::result::Result`, found i32
-   |
-   = note: expected type `std::result::Result<{integer}, _>`
-              found type `i32`
-   = note: the return type of a function must have a statically known size
+error: aborting due to previous error
 
-error: aborting due to 2 previous errors
-
-Some errors have detailed explanations: E0271, E0308.
-For more information about an error, try `rustc --explain E0271`.
+For more information about this error, try `rustc --explain E0308`.