show unit output when there is only output diff in diagnostics

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -1168,14 +1168,21 @@ fn cmp_fn_sig(
         let output1 = sig1.output();
         let output2 = sig2.output();
         let (x1, x2) = self.cmp(output1, output2);
-        if !output1.is_unit() {
+        let only_output_diff = !lifetime_diff
+            && sig1.c_variadic == sig2.c_variadic
+            && sig1.safety == sig2.safety
+            && sig1.abi == sig2.abi
+            && sig1.inputs() == sig2.inputs()
+            && x1 != x2;
+        if !output1.is_unit() || only_output_diff {
             values.0.push_normal(" -> ");
             (values.0).0.extend(x1.0);
         }
-        if !output2.is_unit() {
+        if !output2.is_unit() || only_output_diff {
             values.1.push_normal(" -> ");
             (values.1).0.extend(x2.0);
         }
+
         values
     }
 

tests/ui/compare-method/bad-self-type.stderr

@@ -41,7 +41,7 @@ note: type in trait
 LL |     fn bar(self) -> Option<()>;
    |                     ^^^^^^^^^^
    = note: expected signature `fn(MyFuture) -> Option<()>`
-              found signature `fn(MyFuture)`
+              found signature `fn(MyFuture) -> ()`
 help: change the output type to match the trait
    |
 LL |     fn bar(self) -> Option<()> {}

tests/ui/error-codes/E0308.stderr

@@ -5,7 +5,7 @@ LL |     fn size_of<T>();
    |                    ^ expected `usize`, found `()`
    |
    = note: expected signature `extern "rust-intrinsic" fn() -> usize`
-              found signature `extern "rust-intrinsic" fn()`
+              found signature `extern "rust-intrinsic" fn() -> ()`
 
 error: aborting due to 1 previous error
 

tests/ui/impl-trait/in-assoc-type-unconstrained.stderr

@@ -35,7 +35,7 @@ note: type in trait
 LL |         fn method() -> Self::Ty;
    |                        ^^^^^^^^
    = note: expected signature `fn() -> <() as compare_method::Trait>::Ty`
-              found signature `fn()`
+              found signature `fn() -> ()`
 note: this item must have the opaque type in its signature in order to be able to register hidden types
   --> $DIR/in-assoc-type-unconstrained.rs:22:12
    |

tests/ui/lang-items/start_lang_item_args.missing_ret.stderr

@@ -5,7 +5,7 @@ LL | fn start<T>(_main: fn() -> T, _argc: isize, _argv: *const *const u8, _sigpi
    |                                                                                   ^ expected `isize`, found `()`
    |
    = note: expected signature `fn(fn() -> _, _, _, _) -> isize`
-              found signature `fn(fn() -> _, _, _, _)`
+              found signature `fn(fn() -> _, _, _, _) -> ()`
 
 error: aborting due to 1 previous error
 

tests/ui/method-output-diff-issue-127263.rs

@@ -0,0 +1,8 @@
+fn bar() {}
+fn foo(x: i32) -> u32 {
+    0
+}
+fn main() {
+    let b: fn() -> u32 = bar; //~ ERROR mismatched types [E0308]
+    let f: fn(i32) = foo; //~ ERROR mismatched types [E0308]
+}

tests/ui/method-output-diff-issue-127263.stderr

@@ -0,0 +1,25 @@
+error[E0308]: mismatched types
+  --> $DIR/method-output-diff-issue-127263.rs:6:26
+   |
+LL |     let b: fn() -> u32 = bar;
+   |            -----------   ^^^ expected fn pointer, found fn item
+   |            |
+   |            expected due to this
+   |
+   = note: expected fn pointer `fn() -> u32`
+                 found fn item `fn() -> () {bar}`
+
+error[E0308]: mismatched types
+  --> $DIR/method-output-diff-issue-127263.rs:7:22
+   |
+LL |     let f: fn(i32) = foo;
+   |            -------   ^^^ expected fn pointer, found fn item
+   |            |
+   |            expected due to this
+   |
+   = note: expected fn pointer `fn(_) -> ()`
+                 found fn item `fn(_) -> u32 {foo}`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.