Tweak E0271 wording

Author: estebank

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

@@ -1866,10 +1866,14 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
 
         with_forced_trimmed_paths! {
             if Some(pred.projection_ty.def_id) == self.tcx.lang_items().fn_once_output() {
+                let fn_kind = self_ty.prefix_string(self.tcx);
+                let item = match self_ty.kind() {
+                    ty::FnDef(def, _) => self.tcx.item_name(*def).to_string(),
+                    _ => self_ty.to_string(),
+                };
                 Some(format!(
-                    "expected `{self_ty}` to be a {fn_kind} that returns `{expected_ty}`, but it \
+                    "expected `{item}` to be a {fn_kind} that returns `{expected_ty}`, but it \
                      returns `{normalized_ty}`",
-                    fn_kind = self_ty.prefix_string(self.tcx)
                 ))
             } else if Some(trait_def_id) == self.tcx.lang_items().future_trait() {
                 Some(format!(

tests/ui/async-await/issue-98634.rs

@@ -43,8 +43,8 @@ impl Runtime {
 fn main() {
     Runtime.block_on(async {
         StructAsync { callback }.await;
-        //~^ ERROR expected `fn() -> impl Future<Output = ()> {callback}` to be a fn item that returns `Pin<Box<dyn Future<Output = ()>>>`, but it returns `impl Future<Output = ()>`
-        //~| ERROR expected `fn() -> impl Future<Output = ()> {callback}` to be a fn item that returns `Pin<Box<dyn Future<Output = ()>>>`, but it returns `impl Future<Output = ()>`
-        //~| ERROR expected `fn() -> impl Future<Output = ()> {callback}` to be a fn item that returns `Pin<Box<dyn Future<Output = ()>>>`, but it returns `impl Future<Output = ()>`
+        //~^ ERROR expected `callback` to be a fn item that returns `Pin<Box<dyn Future<Output = ()>>>`, but it returns `impl Future<Output = ()>`
+        //~| ERROR expected `callback` to be a fn item that returns `Pin<Box<dyn Future<Output = ()>>>`, but it returns `impl Future<Output = ()>`
+        //~| ERROR expected `callback` to be a fn item that returns `Pin<Box<dyn Future<Output = ()>>>`, but it returns `impl Future<Output = ()>`
     });
 }

tests/ui/async-await/issue-98634.stderr

@@ -1,4 +1,4 @@
-error[E0271]: expected `fn() -> impl Future<Output = ()> {callback}` to be a fn item that returns `Pin<Box<dyn Future<Output = ()>>>`, but it returns `impl Future<Output = ()>`
+error[E0271]: expected `callback` to be a fn item that returns `Pin<Box<dyn Future<Output = ()>>>`, but it returns `impl Future<Output = ()>`
   --> $DIR/issue-98634.rs:45:23
    |
 LL |         StructAsync { callback }.await;
@@ -17,7 +17,7 @@ note: required by a bound in `StructAsync`
 LL | pub struct StructAsync<F: Fn() -> Pin<Box<dyn Future<Output = ()>>>> {
    |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `StructAsync`
 
-error[E0271]: expected `fn() -> impl Future<Output = ()> {callback}` to be a fn item that returns `Pin<Box<dyn Future<Output = ()>>>`, but it returns `impl Future<Output = ()>`
+error[E0271]: expected `callback` to be a fn item that returns `Pin<Box<dyn Future<Output = ()>>>`, but it returns `impl Future<Output = ()>`
   --> $DIR/issue-98634.rs:45:9
    |
 LL |         StructAsync { callback }.await;
@@ -36,7 +36,7 @@ note: required by a bound in `StructAsync`
 LL | pub struct StructAsync<F: Fn() -> Pin<Box<dyn Future<Output = ()>>>> {
    |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `StructAsync`
 
-error[E0271]: expected `fn() -> impl Future<Output = ()> {callback}` to be a fn item that returns `Pin<Box<dyn Future<Output = ()>>>`, but it returns `impl Future<Output = ()>`
+error[E0271]: expected `callback` to be a fn item that returns `Pin<Box<dyn Future<Output = ()>>>`, but it returns `impl Future<Output = ()>`
   --> $DIR/issue-98634.rs:45:33
    |
 LL |         StructAsync { callback }.await;

tests/ui/intrinsics/const-eval-select-bad.rs

@@ -30,7 +30,7 @@ fn baz(n: bool) -> i32 {
 
 const fn return_ty_mismatch() {
     const_eval_select((1,), foo, bar);
-    //~^ ERROR expected `fn(i32) -> bool {bar}` to be a fn item that returns `i32`, but it returns `bool`
+    //~^ ERROR expected `bar` to be a fn item that returns `i32`, but it returns `bool`
 }
 
 const fn args_ty_mismatch() {

tests/ui/intrinsics/const-eval-select-bad.stderr

@@ -60,7 +60,7 @@ LL |     const_eval_select((), 42, 0xDEADBEEF);
 note: required by a bound in `const_eval_select`
   --> $SRC_DIR/core/src/intrinsics.rs:LL:COL
 
-error[E0271]: expected `fn(i32) -> bool {bar}` to be a fn item that returns `i32`, but it returns `bool`
+error[E0271]: expected `bar` to be a fn item that returns `i32`, but it returns `bool`
   --> $DIR/const-eval-select-bad.rs:32:34
    |
 LL |     const_eval_select((1,), foo, bar);

tests/ui/type-alias-impl-trait/issue-98604.rs

@@ -7,5 +7,5 @@ async fn test() {}
 #[allow(unused_must_use)]
 fn main() {
     Box::new(test) as AsyncFnPtr;
-    //~^ ERROR expected `fn() -> impl Future<Output = ()> {test}` to be a fn item that returns `Pin<Box<dyn Future<Output = ()>>>`, but it returns `impl Future<Output = ()>
+    //~^ ERROR expected `test` to be a fn item that returns `Pin<Box<dyn Future<Output = ()>>>`, but it returns `impl Future<Output = ()>
 }

tests/ui/type-alias-impl-trait/issue-98604.stderr

@@ -1,4 +1,4 @@
-error[E0271]: expected `fn() -> impl Future<Output = ()> {test}` to be a fn item that returns `Pin<Box<dyn Future<Output = ()>>>`, but it returns `impl Future<Output = ()>`
+error[E0271]: expected `test` to be a fn item that returns `Pin<Box<dyn Future<Output = ()>>>`, but it returns `impl Future<Output = ()>`
   --> $DIR/issue-98604.rs:9:5
    |
 LL |     Box::new(test) as AsyncFnPtr;

tests/ui/type-alias-impl-trait/issue-98608.rs

@@ -4,7 +4,7 @@ fn hi() -> impl Sized {
 
 fn main() {
     let b: Box<dyn Fn() -> Box<u8>> = Box::new(hi);
-    //~^ ERROR expected `fn() -> impl Sized {hi}` to be a fn item that returns `Box<u8>`, but it returns `impl Sized`
+    //~^ ERROR expected `hi` to be a fn item that returns `Box<u8>`, but it returns `impl Sized`
     let boxed = b();
     let null = *boxed;
     println!("{null:?}");

tests/ui/type-alias-impl-trait/issue-98608.stderr

@@ -1,4 +1,4 @@
-error[E0271]: expected `fn() -> impl Sized {hi}` to be a fn item that returns `Box<u8>`, but it returns `impl Sized`
+error[E0271]: expected `hi` to be a fn item that returns `Box<u8>`, but it returns `impl Sized`
   --> $DIR/issue-98608.rs:6:39
    |
 LL | fn hi() -> impl Sized {