Pretty print Fn traits in rustc_on_unimplemented

compiler/rustc_span/src/symbol.rs

@@ -295,6 +295,7 @@ symbols! {
         ToOwned,
         ToString,
         TokenStream,
+        Trait,
         Try,
         TryCaptureGeneric,
         TryCapturePrintable,

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

@@ -50,6 +50,7 @@ static ALLOWED_FORMAT_SYMBOLS: &[Symbol] = &[
     sym::float,
     sym::_Self,
     sym::crate_local,
+    sym::Trait,
 ];
 
 impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
@@ -180,6 +181,19 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             flags.push((sym::cause, Some("MainFunctionType".to_string())));
         }
 
+        if let Some(kind) = self.tcx.fn_trait_kind_from_def_id(trait_ref.def_id)
+            && let ty::Tuple(args) = trait_ref.args.type_at(1).kind()
+        {
+            let args = args
+                .iter()
+                .map(|ty| ty.to_string())
+                .collect::<Vec<_>>()
+                .join(", ");
+            flags.push((sym::Trait, Some(format!("{}({args})", kind.as_str()))));
+        } else {
+            flags.push((sym::Trait, Some(trait_ref.print_only_trait_path().to_string())));
+        }
+
         // Add all types without trimmed paths.
         ty::print::with_no_trimmed_paths!({
             let generics = self.tcx.generics_of(def_id);

library/core/src/ops/function.rs

@@ -56,7 +56,7 @@ use crate::marker::Tuple;
 #[lang = "fn"]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_paren_sugar]
-#[rustc_on_unimplemented(
+#[cfg_attr(not(bootstrap), rustc_on_unimplemented(
     on(
         Args = "()",
         note = "wrap the `{Self}` in a closure with no arguments: `|| {{ /* code */ }}`"
@@ -67,9 +67,9 @@ use crate::marker::Tuple;
         // 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 a `{Fn}<{Args}>` closure, found `{Self}`",
-    label = "expected an `Fn<{Args}>` closure, found `{Self}`"
-)]
+    message = "expected a `{Trait}` closure, found `{Self}`",
+    label = "expected an `{Trait}` closure, found `{Self}`"
+))]
 #[fundamental] // so that regex can rely that `&str: !FnMut`
 #[must_use = "closures are lazy and do nothing unless called"]
 // FIXME(effects) #[const_trait]
@@ -143,7 +143,7 @@ pub trait Fn<Args: Tuple>: FnMut<Args> {
 #[lang = "fn_mut"]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_paren_sugar]
-#[rustc_on_unimplemented(
+#[cfg_attr(not(bootstrap), rustc_on_unimplemented(
     on(
         Args = "()",
         note = "wrap the `{Self}` in a closure with no arguments: `|| {{ /* code */ }}`"
@@ -154,9 +154,9 @@ pub trait Fn<Args: Tuple>: 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 a `{FnMut}<{Args}>` closure, found `{Self}`",
-    label = "expected an `FnMut<{Args}>` closure, found `{Self}`"
-)]
+    message = "expected a `{Trait}` closure, found `{Self}`",
+    label = "expected an `{Trait}` closure, found `{Self}`"
+))]
 #[fundamental] // so that regex can rely that `&str: !FnMut`
 #[must_use = "closures are lazy and do nothing unless called"]
 // FIXME(effects) #[const_trait]
@@ -222,7 +222,7 @@ pub trait FnMut<Args: Tuple>: FnOnce<Args> {
 #[lang = "fn_once"]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_paren_sugar]
-#[rustc_on_unimplemented(
+#[cfg_attr(not(bootstrap), rustc_on_unimplemented(
     on(
         Args = "()",
         note = "wrap the `{Self}` in a closure with no arguments: `|| {{ /* code */ }}`"
@@ -233,9 +233,9 @@ pub 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 a `{FnOnce}<{Args}>` closure, found `{Self}`",
-    label = "expected an `FnOnce<{Args}>` closure, found `{Self}`"
-)]
+    message = "expected a `{Trait}` closure, found `{Self}`",
+    label = "expected an `{Trait}` closure, found `{Self}`"
+))]
 #[fundamental] // so that regex can rely that `&str: !FnMut`
 #[must_use = "closures are lazy and do nothing unless called"]
 // FIXME(effects) #[const_trait]

tests/ui/closures/closure-expected.rs

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

tests/ui/closures/closure-expected.stderr

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

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

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

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 a `Fn<()>` closure, found `extern "C" fn() {f}`
+    //~^ ERROR expected a `Fn()` closure, found `extern "C" fn() {f}`
 }

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

@@ -1,8 +1,8 @@
-error[E0277]: expected a `Fn<()>` closure, found `extern "C" fn() {f}`
+error[E0277]: expected a `Fn()` closure, 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 an `Fn()` closure, found `extern "C" fn() {f}`
    |     |
    |     required by a bound introduced by this call
    |

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

@@ -17,5 +17,5 @@ fn main() {
     //~| found struct `Box<dyn FnMut() -> isize>`
 
     needs_fn(1);
-    //~^ ERROR expected a `Fn<(isize,)>` closure, found `{integer}`
+    //~^ ERROR expected a `Fn(isize)` closure, 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 a `Fn<(isize,)>` closure, found `{integer}`
+error[E0277]: expected a `Fn(isize)` closure, found `{integer}`
   --> $DIR/fn-trait-formatting.rs:19:14
    |
 LL |     needs_fn(1);
-   |     -------- ^ expected an `Fn<(isize,)>` closure, found `{integer}`
+   |     -------- ^ expected an `Fn(isize)` closure, found `{integer}`
    |     |
    |     required by a bound introduced by this call
    |

tests/ui/generic-associated-types/issue-68642-broken-llvm-ir.rs

@@ -10,7 +10,7 @@ trait Fun {
 
 impl<T> Fun for T {
     type F<'a> = Self;
-    //~^ ERROR expected a `Fn<()>` closure, found `T`
+    //~^ ERROR expected a `Fn()` closure, found `T`
 }
 
 fn main() {

tests/ui/generic-associated-types/issue-68642-broken-llvm-ir.stderr

@@ -1,8 +1,8 @@
-error[E0277]: expected a `Fn<()>` closure, found `T`
+error[E0277]: expected a `Fn()` closure, found `T`
   --> $DIR/issue-68642-broken-llvm-ir.rs:12:18
    |
 LL |     type F<'a> = Self;
-   |                  ^^^^ expected an `Fn<()>` closure, found `T`
+   |                  ^^^^ expected an `Fn()` closure, found `T`
    |
    = note: wrap the `T` in a closure with no arguments: `|| { /* code */ }`
 note: required by a bound in `Fun::F`

tests/ui/generic-associated-types/issue-68643-broken-mir.rs

@@ -10,7 +10,7 @@ trait Fun {
 
 impl<T> Fun for T {
     type F<'a> = Self;
-    //~^ ERROR expected a `Fn<()>` closure, found `T`
+    //~^ ERROR expected a `Fn()` closure, found `T`
 }
 
 pub fn main() {

tests/ui/generic-associated-types/issue-68643-broken-mir.stderr

@@ -1,8 +1,8 @@
-error[E0277]: expected a `Fn<()>` closure, found `T`
+error[E0277]: expected a `Fn()` closure, found `T`
   --> $DIR/issue-68643-broken-mir.rs:12:18
    |
 LL |     type F<'a> = Self;
-   |                  ^^^^ expected an `Fn<()>` closure, found `T`
+   |                  ^^^^ expected an `Fn()` closure, found `T`
    |
    = note: wrap the `T` in a closure with no arguments: `|| { /* code */ }`
 note: required by a bound in `Fun::F`

tests/ui/generic-associated-types/issue-68644-codegen-selection.rs

@@ -10,7 +10,7 @@ trait Fun {
 
 impl<T> Fun for T {
     type F<'a> = Self;
-    //~^ ERROR expected a `Fn<()>` closure, found `T`
+    //~^ ERROR expected a `Fn()` closure, found `T`
 }
 
 fn main() {

tests/ui/generic-associated-types/issue-68644-codegen-selection.stderr

@@ -1,8 +1,8 @@
-error[E0277]: expected a `Fn<()>` closure, found `T`
+error[E0277]: expected a `Fn()` closure, found `T`
   --> $DIR/issue-68644-codegen-selection.rs:12:18
    |
 LL |     type F<'a> = Self;
-   |                  ^^^^ expected an `Fn<()>` closure, found `T`
+   |                  ^^^^ expected an `Fn()` closure, found `T`
    |
    = note: wrap the `T` in a closure with no arguments: `|| { /* code */ }`
 note: required by a bound in `Fun::F`

tests/ui/generic-associated-types/issue-68645-codegen-fulfillment.rs

@@ -10,7 +10,7 @@ trait Fun {
 
 impl<T> Fun for T {
     type F<'a> = Self;
-    //~^ ERROR expected a `Fn<()>` closure, found `T`
+    //~^ ERROR expected a `Fn()` closure, found `T`
 }
 
 fn main() {

tests/ui/generic-associated-types/issue-68645-codegen-fulfillment.stderr

@@ -1,8 +1,8 @@
-error[E0277]: expected a `Fn<()>` closure, found `T`
+error[E0277]: expected a `Fn()` closure, found `T`
   --> $DIR/issue-68645-codegen-fulfillment.rs:12:18
    |
 LL |     type F<'a> = Self;
-   |                  ^^^^ expected an `Fn<()>` closure, found `T`
+   |                  ^^^^ expected an `Fn()` closure, found `T`
    |
    = note: wrap the `T` in a closure with no arguments: `|| { /* code */ }`
 note: required by a bound in `Fun::F`

tests/ui/higher-ranked/trait-bounds/fn-ptr.classic.stderr

@@ -1,8 +1,8 @@
-error[E0277]: expected a `Fn<(&'w (),)>` closure, found `fn(&'w ())`
+error[E0277]: expected a `Fn(&'w ())` closure, found `fn(&'w ())`
   --> $DIR/fn-ptr.rs:12:5
    |
 LL |     ice();
-   |     ^^^ expected an `Fn<(&'w (),)>` closure, found `fn(&'w ())`
+   |     ^^^ expected an `Fn(&'w ())` closure, found `fn(&'w ())`
    |
    = help: the trait `for<'w> Fn<(&'w (),)>` is not implemented for `fn(&'w ())`
 note: required by a bound in `ice`

tests/ui/higher-ranked/trait-bounds/fn-ptr.rs

@@ -10,5 +10,5 @@ where
 
 fn main() {
     ice();
-    //[classic]~^ ERROR expected a `Fn<(&'w (),)>` closure, found `fn(&'w ())`
+    //[classic]~^ ERROR expected a `Fn(&'w ())` closure, found `fn(&'w ())`
 }

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

@@ -1,8 +1,8 @@
-error[E0277]: expected a `Fn<(<_ as ATC<'a>>::Type,)>` closure, found `F`
+error[E0277]: expected a `Fn(<_ as ATC<'a>>::Type)` closure, found `F`
   --> $DIR/issue-62529-3.rs:25:14
    |
 LL |         call(f, ());
-   |         ---- ^ expected an `Fn<(<_ as ATC<'a>>::Type,)>` closure, found `F`
+   |         ---- ^ expected an `Fn(<_ as ATC<'a>>::Type)` closure, found `F`
    |         |
    |         required by a bound introduced by this call
    |

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

@@ -35,8 +35,8 @@ impl<'a, T: 'a> Handle<'a, T, UIView<'a, T>, Result<(), io::Error>> for TUIHandl
         F: FnOnce(&mut UIView<'a, T>) -> Result<(), io::Error> + Send + 'static,
     {
         real_dispatch(f)
-        //~^ ERROR expected a `FnOnce<(&mut UIView<'_, T>,)>` closure, found `F`
-        //~| NOTE expected an `FnOnce<(&mut UIView<'_, T>,)>` closure, found `F`
+        //~^ ERROR expected a `FnOnce(&mut UIView<'_, T>)` closure, found `F`
+        //~| NOTE expected an `FnOnce(&mut UIView<'_, T>)` closure, found `F`
         //~| NOTE expected a closure with arguments
         //~| NOTE required by a bound introduced by this call
     }

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

@@ -1,8 +1,8 @@
-error[E0277]: expected a `FnOnce<(&mut UIView<'_, T>,)>` closure, found `F`
+error[E0277]: expected a `FnOnce(&mut UIView<'_, T>)` closure, found `F`
   --> $DIR/issue-100690.rs:37:23
    |
 LL |         real_dispatch(f)
-   |         ------------- ^ expected an `FnOnce<(&mut UIView<'_, T>,)>` closure, found `F`
+   |         ------------- ^ expected an `FnOnce(&mut UIView<'_, T>)` closure, found `F`
    |         |
    |         required by a bound introduced by this call
    |

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

@@ -8,8 +8,8 @@ const fn not_fn_items() {
     //~^ ERROR this argument must be a function item
     //~| ERROR this argument must be a function item
     const_eval_select((), 42, 0xDEADBEEF);
-    //~^ ERROR expected a `FnOnce<()>` closure
-    //~| ERROR expected a `FnOnce<()>` closure
+    //~^ ERROR expected a `FnOnce()` closure
+    //~| ERROR expected a `FnOnce()` closure
     //~| ERROR this argument must be a function item
     //~| ERROR this argument must be a function item
 }

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

@@ -25,11 +25,11 @@ LL |     const_eval_select((), 42, 0xDEADBEEF);
    = note: expected a function item, found {integer}
    = help: consult the documentation on `const_eval_select` for more information
 
-error[E0277]: expected a `FnOnce<()>` closure, found `{integer}`
+error[E0277]: expected a `FnOnce()` closure, found `{integer}`
   --> $DIR/const-eval-select-bad.rs:10:27
    |
 LL |     const_eval_select((), 42, 0xDEADBEEF);
-   |     -----------------     ^^ expected an `FnOnce<()>` closure, found `{integer}`
+   |     -----------------     ^^ expected an `FnOnce()` closure, found `{integer}`
    |     |
    |     required by a bound introduced by this call
    |
@@ -47,11 +47,11 @@ LL |     const_eval_select((), 42, 0xDEADBEEF);
    = note: expected a function item, found {integer}
    = help: consult the documentation on `const_eval_select` for more information
 
-error[E0277]: expected a `FnOnce<()>` closure, found `{integer}`
+error[E0277]: expected a `FnOnce()` closure, found `{integer}`
   --> $DIR/const-eval-select-bad.rs:10:31
    |
 LL |     const_eval_select((), 42, 0xDEADBEEF);
-   |     -----------------         ^^^^^^^^^^ expected an `FnOnce<()>` closure, found `{integer}`
+   |     -----------------         ^^^^^^^^^^ expected an `FnOnce()` closure, found `{integer}`
    |     |
    |     required by a bound introduced by this call
    |

tests/ui/issues/issue-22034.rs

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

tests/ui/issues/issue-22034.stderr

@@ -1,8 +1,8 @@
-error[E0277]: expected a `Fn<()>` closure, found `()`
+error[E0277]: expected a `Fn()` closure, found `()`
   --> $DIR/issue-22034.rs:8:16
    |
 LL |         &mut *(ptr as *mut dyn Fn())
-   |                ^^^ expected an `Fn<()>` closure, found `()`
+   |                ^^^ expected an `Fn()` closure, found `()`
    |
    = help: the trait `Fn<()>` is not implemented for `()`
    = note: wrap the `()` in a closure with no arguments: `|| { /* code */ }`

tests/ui/issues/issue-23966.stderr

@@ -1,8 +1,8 @@
-error[E0277]: expected a `FnMut<(_, char)>` closure, found `()`
+error[E0277]: expected a `FnMut(_, char)` closure, found `()`
   --> $DIR/issue-23966.rs:2:32
    |
 LL |     "".chars().fold(|_, _| (), ());
-   |                ----            ^^ expected an `FnMut<(_, char)>` closure, found `()`
+   |                ----            ^^ expected an `FnMut(_, char)` closure, found `()`
    |                |
    |                required by a bound introduced by this call
    |

tests/ui/issues/issue-59494.rs

@@ -19,5 +19,5 @@ fn main() {
     let g = |(a, _)| a;
     let t7 = |env| |a| |b| t7p(f, g)(((env, a), b));
     let t8 = t8n(t7, t7p(f, g));
-    //~^ ERROR: expected a `Fn<(_,)>` closure, found `impl Fn(((_, _), _))` [E0277]
+    //~^ ERROR: expected a `Fn(_)` closure, found `impl Fn(((_, _), _))` [E0277]
 }

tests/ui/issues/issue-59494.stderr

@@ -1,8 +1,8 @@
-error[E0277]: expected a `Fn<(_,)>` closure, found `impl Fn(((_, _), _))`
+error[E0277]: expected a `Fn(_)` closure, found `impl Fn(((_, _), _))`
   --> $DIR/issue-59494.rs:21:22
    |
 LL |     let t8 = t8n(t7, t7p(f, g));
-   |              ---     ^^^^^^^^^ expected an `Fn<(_,)>` closure, found `impl Fn(((_, _), _))`
+   |              ---     ^^^^^^^^^ expected an `Fn(_)` closure, found `impl Fn(((_, _), _))`
    |              |
    |              required by a bound introduced by this call
    |

tests/ui/mismatched_types/suggest-option-asderef-unfixable.rs

@@ -24,9 +24,9 @@ fn main() {
     let _ = produces_string().and_then(takes_str_but_too_many_refs);
     //~^ ERROR type mismatch in function arguments
     let _ = produces_string().and_then(takes_str_but_wrong_abi);
-    //~^ ERROR expected a `FnOnce<(String,)>` closure, found `for<'a> extern "C" fn(&'a str) -> Option<()> {takes_str_but_wrong_abi}`
+    //~^ ERROR expected a `FnOnce(String)` closure, found `for<'a> extern "C" fn(&'a str) -> Option<()> {takes_str_but_wrong_abi}`
     let _ = produces_string().and_then(takes_str_but_unsafe);
-    //~^ ERROR expected a `FnOnce<(String,)>` closure, found `for<'a> unsafe fn(&'a str) -> Option<()> {takes_str_but_unsafe}`
+    //~^ ERROR expected a `FnOnce(String)` closure, found `for<'a> unsafe fn(&'a str) -> Option<()> {takes_str_but_unsafe}`
     let _ = produces_string().and_then(no_args);
     //~^ ERROR function is expected to take 1 argument, but it takes 0 arguments
     let _ = Some(TypeWithoutDeref).and_then(takes_str_but_too_many_refs);