Print note with closure signature on type mismatch

compiler/rustc_errors/src/diagnostic.rs

@@ -730,7 +730,7 @@ impl<'a, G: EmissionGuarantee> Diag<'a, G> {
     } }
 
     #[rustc_lint_diagnostics]
-    fn highlighted_note(&mut self, msg: Vec<StringPart>) -> &mut Self {
+    pub fn highlighted_note(&mut self, msg: Vec<StringPart>) -> &mut Self {
         self.sub_with_highlights(Level::Note, msg, MultiSpan::new());
         self
     }

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

@@ -61,7 +61,7 @@ use crate::traits::{
 use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
 use rustc_errors::{
     codes::*, pluralize, struct_span_code_err, Applicability, Diag, DiagCtxt, DiagStyledString,
-    ErrorGuaranteed, IntoDiagArg,
+    ErrorGuaranteed, IntoDiagArg, StringPart,
 };
 use rustc_hir as hir;
 use rustc_hir::def::DefKind;
@@ -1917,6 +1917,23 @@ 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(ty::Closure(_, args)) =
+                            exp_found.map(|expected_type_found| expected_type_found.found.kind())
+                        {
+                            diag.highlighted_note(vec![
+                                StringPart::normal("closure has signature: `"),
+                                StringPart::highlighted(
+                                    self.tcx
+                                        .signature_unclosure(
+                                            args.as_closure().sig(),
+                                            rustc_hir::Unsafety::Normal,
+                                        )
+                                        .to_string(),
+                                ),
+                                StringPart::normal("`"),
+                            ]);
+                        }
                     }
                 }
             }

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`.