Improve presentation of closure signature mismatch from Fn trait goal

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

@@ -2,6 +2,7 @@ use core::ops::ControlFlow;
 use std::borrow::Cow;
 use std::path::PathBuf;
 
+use rustc_abi::ExternAbi;
 use rustc_ast::TraitObjectSyntax;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::unord::UnordSet;
@@ -2799,32 +2800,57 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         }
 
         // Note any argument mismatches
-        let given_ty = params.skip_binder();
+        let ty::Tuple(given) = *params.skip_binder().kind() else {
+            return;
+        };
+
         let expected_ty = trait_pred.skip_binder().trait_ref.args.type_at(1);
-        if let ty::Tuple(given) = given_ty.kind()
+        let ty::Tuple(expected) = *expected_ty.kind() else {
-            && let ty::Tuple(expected) = expected_ty.kind()
+            return;
-        {
+        };
-            if expected.len() != given.len() {
+
-                // Note number of types that were expected and given
+        if expected.len() != given.len() {
-                err.note(
+            // Note number of types that were expected and given
-                    format!(
+            err.note(format!(
-                        "expected a closure taking {} argument{}, but one taking {} argument{} was given",
+                "expected a closure taking {} argument{}, but one taking {} argument{} was given",
-                        given.len(),
+                given.len(),
-                        pluralize!(given.len()),
+                pluralize!(given.len()),
-                        expected.len(),
+                expected.len(),
-                        pluralize!(expected.len()),
+                pluralize!(expected.len()),
-                    )
+            ));
-                );
+            return;
-            } else if !self.same_type_modulo_infer(given_ty, expected_ty) {
+        }
-                // Print type mismatch
+
-                let (expected_args, given_args) = self.cmp(given_ty, expected_ty);
+        let given_ty = Ty::new_fn_ptr(
-                err.note_expected_found(
+            self.tcx,
-                    &"a closure with arguments",
+            params.rebind(self.tcx.mk_fn_sig(
-                    expected_args,
+                given,
-                    &"a closure with arguments",
+                self.tcx.types.unit,
-                    given_args,
+                false,
-                );
+                hir::Safety::Safe,
-            }
+                ExternAbi::Rust,
+            )),
+        );
+        let expected_ty = Ty::new_fn_ptr(
+            self.tcx,
+            trait_pred.rebind(self.tcx.mk_fn_sig(
+                expected,
+                self.tcx.types.unit,
+                false,
+                hir::Safety::Safe,
+                ExternAbi::Rust,
+            )),
+        );
+
+        if !self.same_type_modulo_infer(given_ty, expected_ty) {
+            // Print type mismatch
+            let (expected_args, given_args) = self.cmp(expected_ty, given_ty);
+            err.note_expected_found(
+                &"a closure with signature",
+                expected_args,
+                &"a closure with signature",
+                given_args,
+            );
         }
     }
 

tests/ui/higher-ranked/trait-bounds/normalize-under-binder/issue-62529-3.stderr

@@ -6,8 +6,8 @@ LL |         call(f, ());
    |         |
    |         required by a bound introduced by this call
    |
-   = note: expected a closure with arguments `((),)`
+   = note: expected a closure with signature `for<'a> fn(<_ as ATC<'a>>::Type)`
-              found a closure with arguments `(<_ as ATC<'a>>::Type,)`
+              found a closure with signature `fn(())`
 note: this is a known limitation of the trait solver that will be lifted in the future
   --> $DIR/issue-62529-3.rs:25:14
    |

tests/ui/implied-bounds/issue-100690.stderr

@@ -6,8 +6,8 @@ LL |         real_dispatch(f)
    |         |
    |         required by a bound introduced by this call
    |
-   = note: expected a closure with arguments `(&mut UIView<'a, _>,)`
+   = note: expected a closure with signature `for<'a, 'b> fn(&'a mut UIView<'b, _>)`
-              found a closure with arguments `(&mut UIView<'_, _>,)`
+              found a closure with signature `fn(&mut UIView<'a, _>)`
 note: required by a bound in `real_dispatch`
   --> $DIR/issue-100690.rs:8:8
    |

tests/ui/mismatched_types/closure-arg-type-mismatch-issue-45727.next.stderr

@@ -7,8 +7,8 @@ LL |     let _ = (-10..=10).find(|x: i32| x.signum() == 0);
    |                        required by a bound introduced by this call
    |
    = help: the trait `for<'a> FnMut(&'a <std::ops::RangeInclusive<{integer}> as Iterator>::Item)` is not implemented for closure `{closure@$DIR/closure-arg-type-mismatch-issue-45727.rs:6:29: 6:37}`
-   = note: expected a closure with arguments `(i32,)`
+   = note: expected a closure with signature `for<'a> fn(&'a <std::ops::RangeInclusive<{integer}> as Iterator>::Item)`
-              found a closure with arguments `(&<std::ops::RangeInclusive<{integer}> as Iterator>::Item,)`
+              found a closure with signature `fn(i32)`
 note: required by a bound in `find`
   --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
 
@@ -27,8 +27,8 @@ LL |     let _ = (-10..=10).find(|x: &&&i32| x.signum() == 0);
    |                        required by a bound introduced by this call
    |
    = help: the trait `for<'a> FnMut(&'a <std::ops::RangeInclusive<{integer}> as Iterator>::Item)` is not implemented for closure `{closure@$DIR/closure-arg-type-mismatch-issue-45727.rs:9:29: 9:40}`
-   = note: expected a closure with arguments `(&&&i32,)`
+   = note: expected a closure with signature `for<'a> fn(&'a <std::ops::RangeInclusive<{integer}> as Iterator>::Item)`
-              found a closure with arguments `(&<std::ops::RangeInclusive<{integer}> as Iterator>::Item,)`
+              found a closure with signature `fn(&&&i32)`
 note: required by a bound in `find`
   --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
 

tests/ui/trait-bounds/mismatch-fn-trait.stderr

@@ -6,8 +6,8 @@ LL |     take(f)
    |     |
    |     required by a bound introduced by this call
    |
-   = note: expected a closure with arguments `(u32,)`
+   = note: expected a closure with signature `fn(i32)`
-              found a closure with arguments `(i32,)`
+              found a closure with signature `fn(u32)`
 note: required by a bound in `take`
   --> $DIR/mismatch-fn-trait.rs:1:18
    |
@@ -68,8 +68,8 @@ LL |     take(f)
    |     required by a bound introduced by this call
    |
    = note: `impl FnOnce(u32)` implements `FnOnce`, but it must implement `FnMut`, which is more general
-   = note: expected a closure with arguments `(u32,)`
+   = note: expected a closure with signature `fn(i32)`
-              found a closure with arguments `(i32,)`
+              found a closure with signature `fn(u32)`
 note: required by a bound in `take`
   --> $DIR/mismatch-fn-trait.rs:1:18
    |