Print note with closure signature on type mismatch

Author: wutchzone

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

@@ -1917,6 +1917,14 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                     );
                     if !is_simple_error || terr.must_include_note() {
                         diag.note_expected_found(&expected_label, expected, &found_label, found);
+
+                        if let Some(expected_type_found) = exp_found {
+                            if let ty::Closure(_, args) = expected_type_found.found.kind() {
+                                diag.note_closure_signature(
+                                    args.as_closure().print_signature().to_string(),
+                                );
+                            }
+                        }
                     }
                 }
             }

tests/ui/inference/hint-closure-signature-119266.rs

@@ -0,0 +1,11 @@
+fn main() {
+    let x = |a: u8, b: (usize, u32), c: fn() -> char| -> String { "I love beans.".to_string() };
+    //~^ NOTE: the found closure
+
+    let x: fn(i32) = x;
+    //~^ ERROR: 5:22: 5:23: mismatched types [E0308]
+    //~| NOTE: incorrect number of function parameters
+    //~| NOTE: expected due to this
+    //~| NOTE: expected fn pointer `fn(i32)`
+    //~| NOTE: closure has signature: `fn(u8, (usize, u32), fn() -> char) -> String`
+}

tests/ui/inference/hint-closure-signature-119266.stderr

@@ -0,0 +1,18 @@
+error[E0308]: mismatched types
+  --> $DIR/hint-closure-signature-119266.rs:5:22
+   |
+LL |     let x = |a: u8, b: (usize, u32), c: fn() -> char| -> String { "I love beans.".to_string() };
+   |             --------------------------------------------------- the found closure
+...
+LL |     let x: fn(i32) = x;
+   |            -------   ^ incorrect number of function parameters
+   |            |
+   |            expected due to this
+   |
+   = note: expected fn pointer `fn(i32)`
+                 found closure `{closure@$DIR/hint-closure-signature-119266.rs:2:13: 2:64}`
+   = note: closure has signature: `fn(u8, (usize, u32), fn() -> char) -> String`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.