Tweak wording of "expected/found signature" errors

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

@@ -3632,9 +3632,12 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             // Print type mismatch
             let (expected_args, given_args) = self.cmp(expected_ty, given_ty);
             err.note_expected_found(
-                "a closure with signature",
+                "type with signature",
                 expected_args,
-                "a closure with signature",
+                &format!(
+                    "{} with signature",
+                    trait_pred.self_ty().skip_binder().prefix_string(self.tcx)
+                ),
                 given_args,
             );
         }

library/core/src/ops/function.rs

@@ -67,8 +67,8 @@ use crate::ptr::NonNull;
         // SAFETY: tidy is not smart enough to tell that the below unsafe block is a string
         label = "call the function in a closure: `|| unsafe {{ /* code */ }}`"
     ),
-    message = "expected an `{This:resolved}` closure, found `{Self}`",
-    label = "expected an `{This:resolved}` closure, found `{Self}`"
+    message = "expected `{This:resolved}` signature, found `{Self}`",
+    label = "expected `{This:resolved}` signature, found `{Self}`"
 )]
 #[fundamental] // so that regex can rely that `&str: !FnMut`
 #[must_use = "closures are lazy and do nothing unless called"]
@@ -154,8 +154,8 @@ pub const trait Fn<Args: Tuple>: [const] FnMut<Args> {
         // SAFETY: tidy is not smart enough to tell that the below unsafe block is a string
         label = "call the function in a closure: `|| unsafe {{ /* code */ }}`"
     ),
-    message = "expected an `{This:resolved}` closure, found `{Self}`",
-    label = "expected an `{This:resolved}` closure, found `{Self}`"
+    message = "expected `{This:resolved}` signature, found `{Self}`",
+    label = "expected `{This:resolved}` signature, found `{Self}`"
 )]
 #[fundamental] // so that regex can rely that `&str: !FnMut`
 #[must_use = "closures are lazy and do nothing unless called"]
@@ -233,8 +233,8 @@ pub const trait FnMut<Args: Tuple>: FnOnce<Args> {
         // SAFETY: tidy is not smart enough to tell that the below unsafe block is a string
         label = "call the function in a closure: `|| unsafe {{ /* code */ }}`"
     ),
-    message = "expected an `{This:resolved}` closure, found `{Self}`",
-    label = "expected an `{This:resolved}` closure, found `{Self}`"
+    message = "expected `{This:resolved}` signature, found `{Self}`",
+    label = "expected `{This:resolved}` signature, found `{Self}`"
 )]
 #[fundamental] // so that regex can rely that `&str: !FnMut`
 #[must_use = "closures are lazy and do nothing unless called"]

tests/ui/abi/bad-custom.rs

@@ -142,7 +142,7 @@ async unsafe extern "custom" fn no_async_fn() {
 }
 
 fn no_promotion_to_fn_trait(f: unsafe extern "custom" fn()) -> impl Fn() {
-    //~^ ERROR expected an `Fn()` closure, found `unsafe extern "custom" fn()`
+    //~^ ERROR expected `Fn()` signature, found `unsafe extern "custom" fn()`
     f
 }
 

tests/ui/abi/bad-custom.stderr

@@ -418,7 +418,7 @@ LL | unsafe extern "custom" fn no_cold_attribute() {
    |        |
    |        `extern "custom"` because of this
 
-error[E0277]: expected an `Fn()` closure, found `unsafe extern "custom" fn()`
+error[E0277]: expected `Fn()` signature, found `unsafe extern "custom" fn()`
   --> $DIR/bad-custom.rs:144:64
    |
 LL | fn no_promotion_to_fn_trait(f: unsafe extern "custom" fn()) -> impl Fn() {

tests/ui/associated-types/normalization-ice-issue-149746.rs

@@ -4,14 +4,14 @@ trait Owner { type Ty<T: FnMut()>; }
 impl Owner for () { type Ty<T: FnMut()> = T; }
 pub struct Warns<T> {
     _significant_drop: <() as Owner>::Ty<T>,
-    //~^ ERROR expected an `FnMut()` closure, found `T`
+    //~^ ERROR expected `FnMut()` signature, found `T`
     field: String,
 }
 pub fn test<T>(w: Warns<T>) {
-    //~^ ERROR expected an `FnMut()` closure, found `T`
+    //~^ ERROR expected `FnMut()` signature, found `T`
     _ = || w.field
-    //~^ ERROR expected an `FnMut()` closure, found `T`
-    //~| ERROR expected an `FnMut()` closure, found `T`
+    //~^ ERROR expected `FnMut()` signature, found `T`
+    //~| ERROR expected `FnMut()` signature, found `T`
     //~| WARN: changes to closure capture in Rust 2021 will affect drop order
 }
 fn main() {}

tests/ui/associated-types/normalization-ice-issue-149746.stderr

@@ -1,8 +1,8 @@
-error[E0277]: expected an `FnMut()` closure, found `T`
+error[E0277]: expected `FnMut()` signature, found `T`
   --> $DIR/normalization-ice-issue-149746.rs:6:24
    |
 LL |     _significant_drop: <() as Owner>::Ty<T>,
-   |                        ^^^^^^^^^^^^^^^^^^^^ expected an `FnMut()` closure, found `T`
+   |                        ^^^^^^^^^^^^^^^^^^^^ expected `FnMut()` signature, found `T`
    |
    = note: wrap the `T` in a closure with no arguments: `|| { /* code */ }`
 note: required by a bound in `Owner::Ty`
@@ -35,11 +35,11 @@ help: add a dummy let to cause `w` to be fully captured
 LL |     _ = || { let _ = &w; w.field }
    |            +++++++++++++         +
 
-error[E0277]: expected an `FnMut()` closure, found `T`
+error[E0277]: expected `FnMut()` signature, found `T`
   --> $DIR/normalization-ice-issue-149746.rs:12:9
    |
 LL |     _ = || w.field
-   |         ^^^^^^^^^^ expected an `FnMut()` closure, found `T`
+   |         ^^^^^^^^^^ expected `FnMut()` signature, found `T`
    |
    = note: wrap the `T` in a closure with no arguments: `|| { /* code */ }`
 note: required by a bound in `Owner::Ty`
@@ -48,11 +48,11 @@ note: required by a bound in `Owner::Ty`
 LL | trait Owner { type Ty<T: FnMut()>; }
    |                          ^^^^^^^ required by this bound in `Owner::Ty`
 
-error[E0277]: expected an `FnMut()` closure, found `T`
+error[E0277]: expected `FnMut()` signature, found `T`
   --> $DIR/normalization-ice-issue-149746.rs:10:16
    |
 LL | pub fn test<T>(w: Warns<T>) {
-   |                ^ expected an `FnMut()` closure, found `T`
+   |                ^ expected `FnMut()` signature, found `T`
    |
    = note: wrap the `T` in a closure with no arguments: `|| { /* code */ }`
 note: required by a bound in `Owner::Ty`
@@ -61,11 +61,11 @@ note: required by a bound in `Owner::Ty`
 LL | trait Owner { type Ty<T: FnMut()>; }
    |                          ^^^^^^^ required by this bound in `Owner::Ty`
 
-error[E0277]: expected an `FnMut()` closure, found `T`
+error[E0277]: expected `FnMut()` signature, found `T`
   --> $DIR/normalization-ice-issue-149746.rs:12:9
    |
 LL |     _ = || w.field
-   |         ^^^^^^^^^^ expected an `FnMut()` closure, found `T`
+   |         ^^^^^^^^^^ expected `FnMut()` signature, found `T`
    |
    = note: wrap the `T` in a closure with no arguments: `|| { /* code */ }`
 note: required by a bound in `Owner::Ty`

tests/ui/async-await/async-fn/impl-header.rs

@@ -6,7 +6,7 @@ impl async Fn<()> for F {}
 //~^ ERROR `async` trait implementations are unsupported
 //~| ERROR the precise format of `Fn`-family traits' type parameters is subject to change
 //~| ERROR manual implementations of `Fn` are experimental
-//~| ERROR expected an `FnMut()` closure, found `F`
+//~| ERROR expected `FnMut()` signature, found `F`
 //~| ERROR not all trait items implemented, missing: `call`
 
 fn main() {}

tests/ui/async-await/async-fn/impl-header.stderr

@@ -30,11 +30,11 @@ LL | impl async Fn<()> for F {}
    |
    = help: implement the missing item: `fn call(&self, _: ()) -> <Self as FnOnce<()>>::Output { todo!() }` (unstable, requires feature `fn_traits`)
 
-error[E0277]: expected an `FnMut()` closure, found `F`
+error[E0277]: expected `FnMut()` signature, found `F`
   --> $DIR/impl-header.rs:5:23
    |
 LL | impl async Fn<()> for F {}
-   |                       ^ expected an `FnMut()` closure, found `F`
+   |                       ^ expected `FnMut()` signature, found `F`
    |
 help: the trait `FnMut()` is not implemented for `F`
   --> $DIR/impl-header.rs:3:1

tests/ui/cast/raw-ptr-to-dyn-fn-raw-ptr.rs

@@ -4,6 +4,6 @@ fn main() {
     let ptr: *mut () = core::ptr::null_mut();
     let _: &mut dyn Fn() = unsafe {
         &mut *(ptr as *mut dyn Fn())
-        //~^ ERROR expected an `Fn()` closure, found `()`
+        //~^ ERROR expected `Fn()` signature, found `()`
     };
 }

tests/ui/cast/raw-ptr-to-dyn-fn-raw-ptr.stderr

@@ -1,8 +1,8 @@
-error[E0277]: expected an `Fn()` closure, found `()`
+error[E0277]: expected `Fn()` signature, found `()`
   --> $DIR/raw-ptr-to-dyn-fn-raw-ptr.rs:6:16
    |
 LL |         &mut *(ptr as *mut dyn Fn())
-   |                ^^^ expected an `Fn()` closure, found `()`
+   |                ^^^ expected `Fn()` signature, found `()`
    |
    = help: the trait `Fn()` is not implemented for `()`
    = note: wrap the `()` in a closure with no arguments: `|| { /* code */ }`

tests/ui/closures/closure-expected.rs

@@ -1,5 +1,5 @@
 fn main() {
     let x = Some(1);
     let y = x.or_else(4);
-    //~^ ERROR expected an `FnOnce()` closure, found `{integer}`
+    //~^ ERROR expected `FnOnce()` signature, found `{integer}`
 }

tests/ui/closures/closure-expected.stderr

@@ -1,8 +1,8 @@
-error[E0277]: expected an `FnOnce()` closure, found `{integer}`
+error[E0277]: expected `FnOnce()` signature, found `{integer}`
   --> $DIR/closure-expected.rs:3:23
    |
 LL |     let y = x.or_else(4);
-   |               ------- ^ expected an `FnOnce()` closure, found `{integer}`
+   |               ------- ^ expected `FnOnce()` signature, found `{integer}`
    |               |
    |               required by a bound introduced by this call
    |

tests/ui/closures/closure-ref-fn-kind-mismatch-issue-161327.stderr

@@ -79,11 +79,11 @@ note: required by a bound in `req_fn`
 LL | fn req_fn(_: impl Fn(&'static str) -> String) {}
    |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `req_fn`
 
-error[E0277]: expected an `Fn(&'static str)` closure, found `&mut {closure@$DIR/closure-ref-fn-kind-mismatch-issue-161327.rs:45:28: 45:45}`
+error[E0277]: expected `Fn(&'static str)` signature, found `&mut {closure@$DIR/closure-ref-fn-kind-mismatch-issue-161327.rs:45:28: 45:45}`
   --> $DIR/closure-ref-fn-kind-mismatch-issue-161327.rs:46:12
    |
 LL |     req_fn(&mut pure_closure);
-   |     ------ ^^^^^^^^^^^^^^^^^ expected an `Fn(&'static str)` closure, found `&mut {closure@$DIR/closure-ref-fn-kind-mismatch-issue-161327.rs:45:28: 45:45}`
+   |     ------ ^^^^^^^^^^^^^^^^^ expected `Fn(&'static str)` signature, found `&mut {closure@$DIR/closure-ref-fn-kind-mismatch-issue-161327.rs:45:28: 45:45}`
    |     |
    |     required by a bound introduced by this call
    |
@@ -99,11 +99,11 @@ LL -     req_fn(&mut pure_closure);
 LL +     req_fn(pure_closure);
    |
 
-error[E0277]: expected an `Fn(&'static str)` closure, found `&mut {closure@$DIR/closure-ref-fn-kind-mismatch-issue-161327.rs:53:21: 53:25}`
+error[E0277]: expected `Fn(&'static str)` signature, found `&mut {closure@$DIR/closure-ref-fn-kind-mismatch-issue-161327.rs:53:21: 53:25}`
   --> $DIR/closure-ref-fn-kind-mismatch-issue-161327.rs:59:12
    |
 LL |     req_fn(&mut outer);
-   |     ------ ^^^^^^^^^^ expected an `Fn(&'static str)` closure, found `&mut {closure@$DIR/closure-ref-fn-kind-mismatch-issue-161327.rs:53:21: 53:25}`
+   |     ------ ^^^^^^^^^^ expected `Fn(&'static str)` signature, found `&mut {closure@$DIR/closure-ref-fn-kind-mismatch-issue-161327.rs:53:21: 53:25}`
    |     |
    |     required by a bound introduced by this call
    |
@@ -119,11 +119,11 @@ LL -     req_fn(&mut outer);
 LL +     req_fn(outer);
    |
 
-error[E0277]: expected an `Fn(&'static str)` closure, found `&mut {closure@$DIR/closure-ref-fn-kind-mismatch-issue-161327.rs:65:17: 65:21}`
+error[E0277]: expected `Fn(&'static str)` signature, found `&mut {closure@$DIR/closure-ref-fn-kind-mismatch-issue-161327.rs:65:17: 65:21}`
   --> $DIR/closure-ref-fn-kind-mismatch-issue-161327.rs:69:12
    |
 LL |     req_fn(&mut c);
-   |     ------ ^^^^^^ expected an `Fn(&'static str)` closure, found `&mut {closure@$DIR/closure-ref-fn-kind-mismatch-issue-161327.rs:65:17: 65:21}`
+   |     ------ ^^^^^^ expected `Fn(&'static str)` signature, found `&mut {closure@$DIR/closure-ref-fn-kind-mismatch-issue-161327.rs:65:17: 65:21}`
    |     |
    |     required by a bound introduced by this call
    |

tests/ui/closures/coerce-unsafe-to-closure.stderr

@@ -1,4 +1,4 @@
-error[E0277]: expected an `FnOnce(&str)` closure, found `unsafe fn(_) -> _ {std::intrinsics::transmute::<_, _>}`
+error[E0277]: expected `FnOnce(&str)` signature, found `unsafe fn(_) -> _ {std::intrinsics::transmute::<_, _>}`
   --> $DIR/coerce-unsafe-to-closure.rs:2:44
    |
 LL |     let x: Option<&[u8]> = Some("foo").map(std::mem::transmute);

tests/ui/expr/malformed_closure/block_instead_of_closure_in_arg.rs

@@ -1,6 +1,6 @@
 fn main() {
     let number = 2;
-    Some(true).filter({ //~ ERROR expected an `FnOnce(&bool)` closure, found `bool`
+    Some(true).filter({ //~ ERROR expected `FnOnce(&bool)` signature, found `bool`
         if number % 2 == 0 {
             number == 0
         } else {

tests/ui/expr/malformed_closure/block_instead_of_closure_in_arg.stderr

@@ -1,4 +1,4 @@
-error[E0277]: expected an `FnOnce(&bool)` closure, found `bool`
+error[E0277]: expected `FnOnce(&bool)` signature, found `bool`
   --> $DIR/block_instead_of_closure_in_arg.rs:3:23
    |
 LL |        Some(true).filter({
@@ -12,7 +12,7 @@ LL | ||             number != 0
 LL | ||         }
    | ||_________- this tail expression is of type `bool`
 LL | |      });
-   | |______^ expected an `FnOnce(&bool)` closure, found `bool`
+   | |______^ expected `FnOnce(&bool)` signature, found `bool`
    |
    = help: the trait `for<'a> FnOnce(&'a bool)` is not implemented for `bool`
 note: required by a bound in `Option::<T>::filter`

tests/ui/expr/malformed_closure/ruby_style_closure_successful_parse.rs

@@ -1,6 +1,6 @@
 const x: usize =42;
 fn main() {
-    let p = Some(45).and_then({|x| //~ ERROR expected an `FnOnce({integer})` closure, found `Option<usize>`
+    let p = Some(45).and_then({|x| //~ ERROR expected `FnOnce({integer})` signature, found `Option<usize>`
         1 + 1;
         Some(x * 2)
     });

tests/ui/expr/malformed_closure/ruby_style_closure_successful_parse.stderr

@@ -1,4 +1,4 @@
-error[E0277]: expected an `FnOnce({integer})` closure, found `Option<usize>`
+error[E0277]: expected `FnOnce({integer})` signature, found `Option<usize>`
   --> $DIR/ruby_style_closure_successful_parse.rs:3:31
    |
 LL |       let p = Some(45).and_then({|x|
@@ -9,7 +9,7 @@ LL | |         1 + 1;
 LL | |         Some(x * 2)
    | |         ----------- this tail expression is of type `Option<usize>`
 LL | |     });
-   | |_____^ expected an `FnOnce({integer})` closure, found `Option<usize>`
+   | |_____^ expected `FnOnce({integer})` signature, found `Option<usize>`
    |
    = help: the trait `FnOnce({integer})` is not implemented for `Option<usize>`
 note: required by a bound in `Option::<T>::and_then`

tests/ui/extern/extern-wrong-value-type.rs

@@ -7,5 +7,5 @@ fn main() {
     // extern functions are extern "C" fn
     let _x: extern "C" fn() = f; // OK
     is_fn(f);
-    //~^ ERROR expected an `Fn()` closure, found `extern "C" fn() {f}`
+    //~^ ERROR expected `Fn()` signature, found `extern "C" fn() {f}`
 }

tests/ui/extern/extern-wrong-value-type.stderr

@@ -1,8 +1,8 @@
-error[E0277]: expected an `Fn()` closure, found `extern "C" fn() {f}`
+error[E0277]: expected `Fn()` signature, found `extern "C" fn() {f}`
   --> $DIR/extern-wrong-value-type.rs:9:11
    |
 LL |     is_fn(f);
-   |     ----- ^ expected an `Fn()` closure, found `extern "C" fn() {f}`
+   |     ----- ^ expected `Fn()` signature, found `extern "C" fn() {f}`
    |     |
    |     required by a bound introduced by this call
    |

tests/ui/feature-gates/feature-gate-unboxed-closures-manual-impls.rs

@@ -9,7 +9,7 @@ struct Foo;
 impl Fn<()> for Foo {
     //~^ ERROR the precise format of `Fn`-family traits' type parameters is subject to change
     //~| ERROR manual implementations of `Fn` are experimental
-    //~| ERROR expected an `FnMut()` closure, found `Foo`
+    //~| ERROR expected `FnMut()` signature, found `Foo`
     extern "rust-call" fn call(self, args: ()) -> () {}
     //~^ ERROR "rust-call" ABI is experimental and subject to change
     //~| ERROR `call` has an incompatible type for trait
@@ -26,7 +26,7 @@ struct Bar;
 impl FnMut<()> for Bar {
     //~^ ERROR the precise format of `Fn`-family traits' type parameters is subject to change
     //~| ERROR manual implementations of `FnMut` are experimental
-    //~| ERROR expected an `FnOnce()` closure, found `Bar`
+    //~| ERROR expected `FnOnce()` signature, found `Bar`
     extern "rust-call" fn call_mut(&self, args: ()) -> () {}
     //~^ ERROR "rust-call" ABI is experimental and subject to change
     //~| ERROR incompatible type for trait

tests/ui/feature-gates/feature-gate-unboxed-closures-manual-impls.stderr

@@ -87,11 +87,11 @@ LL | impl FnMut<()> for Bar {
    |
    = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
 
-error[E0277]: expected an `FnMut()` closure, found `Foo`
+error[E0277]: expected `FnMut()` signature, found `Foo`
   --> $DIR/feature-gate-unboxed-closures-manual-impls.rs:9:17
    |
 LL | impl Fn<()> for Foo {
-   |                 ^^^ expected an `FnMut()` closure, found `Foo`
+   |                 ^^^ expected `FnMut()` signature, found `Foo`
    |
 help: the trait `FnMut()` is not implemented for `Foo`
   --> $DIR/feature-gate-unboxed-closures-manual-impls.rs:8:1
@@ -161,11 +161,11 @@ help: change the self-receiver type to match the trait
 LL |     extern "rust-call" fn call_mut(&mut self, args: ()) -> () {}
    |                                     +++
 
-error[E0277]: expected an `FnOnce()` closure, found `Bar`
+error[E0277]: expected `FnOnce()` signature, found `Bar`
   --> $DIR/feature-gate-unboxed-closures-manual-impls.rs:26:20
    |
 LL | impl FnMut<()> for Bar {
-   |                    ^^^ expected an `FnOnce()` closure, found `Bar`
+   |                    ^^^ expected `FnOnce()` signature, found `Bar`
    |
 help: the trait `FnOnce()` is not implemented for `Bar`
   --> $DIR/feature-gate-unboxed-closures-manual-impls.rs:25:1

tests/ui/fn/fn-trait-formatting.rs

@@ -17,5 +17,5 @@ fn main() {
     //~| NOTE found struct `Box<dyn FnMut() -> isize>`
 
     needs_fn(1);
-    //~^ ERROR expected an `Fn(isize)` closure, found `{integer}`
+    //~^ ERROR expected `Fn(isize)` signature, found `{integer}`
 }

tests/ui/fn/fn-trait-formatting.stderr

@@ -39,11 +39,11 @@ LL |     let _: () = Box::new(|| -> isize { unimplemented!() }) as Box<dyn FnMut
    = note: expected unit type `()`
                  found struct `Box<dyn FnMut() -> isize>`
 
-error[E0277]: expected an `Fn(isize)` closure, found `{integer}`
+error[E0277]: expected `Fn(isize)` signature, found `{integer}`
   --> $DIR/fn-trait-formatting.rs:19:14
    |
 LL |     needs_fn(1);
-   |     -------- ^ expected an `Fn(isize)` closure, found `{integer}`
+   |     -------- ^ expected `Fn(isize)` signature, found `{integer}`
    |     |
    |     required by a bound introduced by this call
    |

tests/ui/fn/issue-39259.rs

@@ -5,7 +5,7 @@ struct S;
 
 impl Fn(u32) -> u32 for S {
     //~^ ERROR associated item constraints are not allowed here [E0229]
-    //~| ERROR expected an `FnMut(u32)` closure, found `S`
+    //~| ERROR expected `FnMut(u32)` signature, found `S`
     fn call(&self) -> u32 {
         //~^ ERROR method `call` has 1 parameter but the declaration in trait `call` has 2
         5

tests/ui/fn/issue-39259.stderr

@@ -22,11 +22,11 @@ help: add the missing parameter from the trait
 LL |     fn call(&self, args: Args) -> u32 {
    |                  ++++++++++++
 
-error[E0277]: expected an `FnMut(u32)` closure, found `S`
+error[E0277]: expected `FnMut(u32)` signature, found `S`
   --> $DIR/issue-39259.rs:6:25
    |
 LL | impl Fn(u32) -> u32 for S {
-   |                         ^ expected an `FnMut(u32)` closure, found `S`
+   |                         ^ expected `FnMut(u32)` signature, found `S`
    |
 help: the trait `FnMut(u32)` is not implemented for `S`
   --> $DIR/issue-39259.rs:4:1