Pretty print Fn traits in rustc_on_unimplemented

Author: compiler-errors

compiler/rustc_span/src/symbol.rs

@@ -316,6 +316,7 @@ symbols! {
         ToOwned,
         ToString,
         TokenStream,
+        Trait,
         Try,
         TryCaptureGeneric,
         TryCapturePrintable,

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

@@ -53,6 +53,7 @@ static ALLOWED_FORMAT_SYMBOLS: &[Symbol] = &[
     sym::float,
     sym::_Self,
     sym::crate_local,
+    sym::Trait,
 ];
 
 impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
@@ -183,6 +184,19 @@ 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 or visible paths, ensuring they end up with
         // their "canonical" def path.
         ty::print::with_no_trimmed_paths!(ty::print::with_no_visible_paths!({

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/expr/malformed_closure/block_instead_of_closure_in_arg.rs

@@ -1,6 +1,6 @@
 fn main() {
     let number = 2;
-    Some(true).filter({ //~ ERROR expected a `FnOnce<(&bool,)>` closure, found `bool`
+    Some(true).filter({ //~ ERROR expected a `FnOnce(&bool)` closure, 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 a `FnOnce<(&bool,)>` closure, found `bool`
+error[E0277]: expected a `FnOnce(&bool)` closure, 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 an `FnOnce(&bool)` closure, 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 a `FnOnce<({integer},)>` closure, found `Option<usize>`
+    let p = Some(45).and_then({|x| //~ ERROR expected a `FnOnce({integer})` closure, 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 a `FnOnce<({integer},)>` closure, found `Option<usize>`
+error[E0277]: expected a `FnOnce({integer})` closure, 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 an `FnOnce({integer})` closure, found `Option<usize>`
    |
    = help: the trait `FnOnce<({integer},)>` is not implemented for `Option<usize>`
 note: required by a bound in `Option::<T>::and_then`