Improve output

Author: estebank

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -34,7 +34,7 @@ use crate::traits::{
     ImplSourceUserDefinedData,
 };
 use crate::traits::{ObjectCastObligation, PredicateObligation, TraitObligation};
-use crate::traits::{Obligation, ObligationCause};
+use crate::traits::{Obligation, ObligationCause, ObligationCauseCode};
 use crate::traits::{SelectionError, Unimplemented};
 
 use super::BuiltinImplConditions;
@@ -533,6 +533,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let lang_items = self.tcx().lang_items();
         if [lang_items.fn_once_trait(), lang_items.fn_trait(), lang_items.fn_mut_trait()]
             .contains(&Some(obligation.predicate.def_id()))
+            // Skip `MiscObligation`s for better output.
+            && matches!(obligation.cause.code, ObligationCauseCode::BindingObligation(_, _))
         {
             // Do not allow `foo::<fn() -> A>();` for `A: !Sized` (#82633)
             let fn_sig = obligation.predicate.self_ty().skip_binder().fn_sig(self.tcx());

src/test/ui/closures/closure-return-type-must-be-sized.stderr

@@ -1,6 +1,9 @@
 error[E0277]: the size for values of type `(dyn A + 'static)` cannot be known at compilation time
   --> $DIR/closure-return-type-must-be-sized.rs:54:5
    |
+LL |     pub fn foo<F: FnOnce<()>>() where F::Output: A {
+   |                   ---------- required by this bound in `a::foo`
+...
 LL |     a::foo::<fn() -> dyn A>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
    |