better error message for normalizes-to ambiguities

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

@@ -2705,6 +2705,22 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
  ),
  );
  }
+
+ ty::PredicateKind::NormalizesTo(ty::NormalizesTo { alias, term })
+ if term.is_infer() =>
+ {
+ if let Some(e) = self.tainted_by_errors() {
+ return e;
+ }
+ struct_span_code_err!(
+ self.dcx(),
+ span,
+ E0284,
+ "type annotations needed: cannot normalize `{alias}`",
+ )
+ .with_span_label(span, format!("cannot normalize `{alias}`"))
+ }
+
  _ => {
  if let Some(e) = self.tainted_by_errors() {
  return e;

tests/ui/coherence/indirect-impl-for-trait-obj-coherence.next.stderr

@@ -1,8 +1,8 @@
-error[E0284]: type annotations needed: cannot satisfy `<dyn Object<U, Output = T> as Object<U>>::Output normalizes-to _`
+error[E0284]: type annotations needed: cannot normalize `<dyn Object<U, Output = T> as Object<U>>::Output`
  --> $DIR/indirect-impl-for-trait-obj-coherence.rs:25:41
  |
 LL | foo::<dyn Object<U, Output = T>, U>(x)
- | ^ cannot satisfy `<dyn Object<U, Output = T> as Object<U>>::Output normalizes-to _`
+ | ^ cannot normalize `<dyn Object<U, Output = T> as Object<U>>::Output`
 
 error: aborting due to 1 previous error
 

tests/ui/coherence/indirect-impl-for-trait-obj-coherence.rs

@@ -23,7 +23,7 @@ fn foo<T: ?Sized, U>(x: <T as Object<U>>::Output) -> U {
 #[allow(dead_code)]
 fn transmute<T, U>(x: T) -> U {
  foo::<dyn Object<U, Output = T>, U>(x)
- //[next]~^ ERROR type annotations needed: cannot satisfy `<dyn Object<U, Output = T> as Object<U>>::Output normalizes-to _`
+ //[next]~^ ERROR type annotations needed: cannot normalize `<dyn Object<U, Output = T> as Object<U>>::Output`
 }
 
 fn main() {}

tests/ui/coherence/occurs-check/associated-type.next.stderr

@@ -16,11 +16,11 @@ LL | | for<'a> *const T: ToUnit<'a>,
  |
  = note: this behavior recently changed as a result of a bug fix; see rust-lang/rust#56105 for details
 
-error[E0284]: type annotations needed: cannot satisfy `<for<'a> fn(&'a (), ()) as Overlap<for<'a> fn(&'a (), ())>>::Assoc normalizes-to _`
+error[E0284]: type annotations needed: cannot normalize `<for<'a> fn(&'a (), ()) as Overlap<for<'a> fn(&'a (), ())>>::Assoc`
  --> $DIR/associated-type.rs:44:59
  |
 LL | foo::<for<'a> fn(&'a (), ()), for<'a> fn(&'a (), ())>(3usize);
- | ^^^^^^ cannot satisfy `<for<'a> fn(&'a (), ()) as Overlap<for<'a> fn(&'a (), ())>>::Assoc normalizes-to _`
+ | ^^^^^^ cannot normalize `<for<'a> fn(&'a (), ()) as Overlap<for<'a> fn(&'a (), ())>>::Assoc`
 
 error: aborting due to 2 previous errors
 

tests/ui/coherence/occurs-check/associated-type.rs

@@ -42,5 +42,5 @@ fn foo<T: Overlap<U>, U>(x: T::Assoc) -> T::Assoc {
 
 fn main() {
  foo::<for<'a> fn(&'a (), ()), for<'a> fn(&'a (), ())>(3usize);
- //[next]~^ ERROR: cannot satisfy
+ //[next]~^ ERROR: cannot normalize
 }

tests/ui/traits/next-solver/canonical/const-region-infer-to-static-in-binder.stderr

@@ -1,8 +1,8 @@
-error[E0284]: type annotations needed: cannot satisfy `X::{constant#0} normalizes-to _`
+error[E0284]: type annotations needed: cannot normalize `X::{constant#0}`
  --> $DIR/const-region-infer-to-static-in-binder.rs:4:10
  |
 LL | struct X<const FN: fn() = { || {} }>;
- | ^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot satisfy `X::{constant#0} normalizes-to _`
+ | ^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot normalize `X::{constant#0}`
 
 error: using function pointers as const generic parameters is forbidden
  --> $DIR/const-region-infer-to-static-in-binder.rs:4:20

tests/ui/traits/next-solver/specialization-transmute.stderr

@@ -10,11 +10,11 @@ LL | #![feature(specialization)]
 
 error: cannot normalize `<T as Default>::Id: '_`
 
-error[E0284]: type annotations needed: cannot satisfy `<T as Default>::Id normalizes-to _`
+error[E0284]: type annotations needed: cannot normalize `<T as Default>::Id`
  --> $DIR/specialization-transmute.rs:15:23
  |
 LL | fn intu(&self) -> &Self::Id {
- | ^^^^^^^^^ cannot satisfy `<T as Default>::Id normalizes-to _`
+ | ^^^^^^^^^ cannot normalize `<T as Default>::Id`
 
 error[E0284]: type annotations needed: cannot satisfy `<T as Default>::Id normalizes-to T`
  --> $DIR/specialization-transmute.rs:17:9