Make the error for opaque types that have no hidden types a bit informative

compiler/rustc_typeck/src/collect/type_of.rs

@@ -724,7 +724,12 @@ fn find_opaque_ty_constraints(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Ty<'_> {
         Some((_, ty)) => ty,
         None => {
             let span = tcx.def_span(def_id);
-            tcx.sess.span_err(span, "could not find defining uses");
+            let name = tcx.item_name(tcx.parent(def_id.to_def_id()).unwrap());
+            let label = format!(
+                "`{}` must be used in combination with a concrete type within the same module",
+                name
+            );
+            tcx.sess.struct_span_err(span, "unconstrained opaque type").note(&label).emit();
             tcx.ty_error()
         }
     }

src/test/ui/generic-associated-types/issue-87258_a.rs

@@ -16,7 +16,7 @@ pub trait Trait2 {
 
 impl<'c, S: Trait2> Trait2 for &'c mut S {
     type FooFuture<'a> = impl Trait1;
-    //~^ ERROR could not find defining uses
+    //~^ ERROR unconstrained opaque type
     fn foo<'a>() -> Self::FooFuture<'a> {
         Struct(unimplemented!())
     }

src/test/ui/generic-associated-types/issue-87258_a.stderr

@@ -1,8 +1,10 @@
-error: could not find defining uses
+error: unconstrained opaque type
   --> $DIR/issue-87258_a.rs:18:26
    |
 LL |     type FooFuture<'a> = impl Trait1;
    |                          ^^^^^^^^^^^
+   |
+   = note: `FooFuture` must be used in combination with a concrete type within the same module
 
 error: aborting due to previous error
 

src/test/ui/generic-associated-types/issue-87258_b.rs

@@ -15,7 +15,7 @@ pub trait Trait2 {
 }
 
 type Helper<'xenon, 'yttrium, KABOOM: Trait2> = impl Trait1;
-//~^ ERROR could not find defining uses
+//~^ ERROR unconstrained opaque type
 
 impl<'c, S: Trait2> Trait2 for &'c mut S {
     type FooFuture<'a> = Helper<'c, 'a, S>;

src/test/ui/generic-associated-types/issue-87258_b.stderr

@@ -1,8 +1,10 @@
-error: could not find defining uses
+error: unconstrained opaque type
   --> $DIR/issue-87258_b.rs:17:49
    |
 LL | type Helper<'xenon, 'yttrium, KABOOM: Trait2> = impl Trait1;
    |                                                 ^^^^^^^^^^^
+   |
+   = note: `Helper` must be used in combination with a concrete type within the same module
 
 error: aborting due to previous error
 

src/test/ui/generic-associated-types/issue-88595.rs

@@ -18,7 +18,7 @@ struct C;
 impl<'a> A<'a> for C {
     type B<'b> = impl Clone;
     //~^ ERROR: lifetime bound not satisfied
-    //~| ERROR: could not find defining uses
+    //~| ERROR: unconstrained opaque type
 
     fn a(&'a self) -> Self::B<'a> {} //~ ERROR: non-defining opaque type use in defining scope
 }

src/test/ui/generic-associated-types/issue-88595.stderr

@@ -29,11 +29,13 @@ LL | impl<'a> A<'a> for C {
 LL |     type B<'b> = impl Clone;
    |            ^^
 
-error: could not find defining uses
+error: unconstrained opaque type
   --> $DIR/issue-88595.rs:19:18
    |
 LL |     type B<'b> = impl Clone;
    |                  ^^^^^^^^^^
+   |
+   = note: `B` must be used in combination with a concrete type within the same module
 
 error: aborting due to 3 previous errors
 

src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle.rs

@@ -2,7 +2,7 @@
 
 mod a {
     type Foo = impl PartialEq<(Foo, i32)>;
-    //~^ ERROR could not find defining uses
+    //~^ ERROR unconstrained opaque type
 
     struct Bar;
 
@@ -15,7 +15,7 @@ mod a {
 
 mod b {
     type Foo = impl PartialEq<(Foo, i32)>;
-    //~^ ERROR could not find defining uses
+    //~^ ERROR unconstrained opaque type
 
     struct Bar;
 

src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle.stderr

@@ -1,14 +1,18 @@
-error: could not find defining uses
+error: unconstrained opaque type
   --> $DIR/recursive-type-alias-impl-trait-declaration-too-subtle.rs:4:16
    |
 LL |     type Foo = impl PartialEq<(Foo, i32)>;
    |                ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `Foo` must be used in combination with a concrete type within the same module
 
-error: could not find defining uses
+error: unconstrained opaque type
   --> $DIR/recursive-type-alias-impl-trait-declaration-too-subtle.rs:17:16
    |
 LL |     type Foo = impl PartialEq<(Foo, i32)>;
    |                ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `Foo` must be used in combination with a concrete type within the same module
 
 error: aborting due to 2 previous errors
 

src/test/ui/impl-trait/two_tait_defining_each_other2.rs

@@ -1,7 +1,7 @@
 #![feature(type_alias_impl_trait)]
 
 type A = impl Foo;
-//~^ ERROR could not find defining uses
+//~^ ERROR unconstrained opaque type
 type B = impl Foo;
 
 trait Foo {}

src/test/ui/impl-trait/two_tait_defining_each_other2.stderr

@@ -1,8 +1,10 @@
-error: could not find defining uses
+error: unconstrained opaque type
   --> $DIR/two_tait_defining_each_other2.rs:3:10
    |
 LL | type A = impl Foo;
    |          ^^^^^^^^
+   |
+   = note: `A` must be used in combination with a concrete type within the same module
 
 error: aborting due to previous error
 

src/test/ui/lint/inline-trait-and-foreign-items.rs

@@ -23,7 +23,7 @@ impl Trait for () {
     type T = Self;
 
     #[inline] //~ ERROR attribute should be applied to function or closure
-    type U = impl Trait; //~ ERROR could not find defining uses
+    type U = impl Trait; //~ ERROR unconstrained opaque type
 }
 
 extern "C" {

src/test/ui/lint/inline-trait-and-foreign-items.stderr

@@ -61,11 +61,13 @@ LL |     #[inline]
 LL |     type T;
    |     ------- not a function or closure
 
-error: could not find defining uses
+error: unconstrained opaque type
   --> $DIR/inline-trait-and-foreign-items.rs:26:14
    |
 LL |     type U = impl Trait;
    |              ^^^^^^^^^^
+   |
+   = note: `U` must be used in combination with a concrete type within the same module
 
 error: aborting due to 6 previous errors; 2 warnings emitted
 

src/test/ui/never_type/impl_trait_fallback3.rs

@@ -7,7 +7,7 @@ trait T {
 }
 
 type Foo = impl T;
-//~^ ERROR could not find defining uses
+//~^ ERROR unconstrained opaque type
 
 fn a() -> Foo {
     // This is not a defining use, it doesn't actually constrain the opaque type.

src/test/ui/never_type/impl_trait_fallback3.stderr

@@ -1,8 +1,10 @@
-error: could not find defining uses
+error: unconstrained opaque type
   --> $DIR/impl_trait_fallback3.rs:9:12
    |
 LL | type Foo = impl T;
    |            ^^^^^^
+   |
+   = note: `Foo` must be used in combination with a concrete type within the same module
 
 error: aborting due to previous error
 

src/test/ui/save-analysis/issue-68621.rs

@@ -11,7 +11,7 @@ trait Service {
 struct Struct;
 
 impl Service for Struct {
-    type Future = impl Trait; //~ ERROR: could not find defining uses
+    type Future = impl Trait; //~ ERROR: unconstrained opaque type
 }
 
 fn main() {}

src/test/ui/save-analysis/issue-68621.stderr

@@ -1,8 +1,10 @@
-error: could not find defining uses
+error: unconstrained opaque type
   --> $DIR/issue-68621.rs:14:19
    |
 LL |     type Future = impl Trait;
    |                   ^^^^^^^^^^
+   |
+   = note: `Future` must be used in combination with a concrete type within the same module
 
 error: aborting due to previous error
 

src/test/ui/type-alias-impl-trait/bound_reduction2.rs

@@ -7,7 +7,7 @@ trait TraitWithAssoc {
 }
 
 type Foo<V> = impl Trait<V>;
-//~^ ERROR could not find defining uses
+//~^ ERROR unconstrained opaque type
 
 trait Trait<U> {}
 

src/test/ui/type-alias-impl-trait/bound_reduction2.stderr

@@ -10,11 +10,13 @@ note: used non-generic type `<T as TraitWithAssoc>::Assoc` for generic parameter
 LL | type Foo<V> = impl Trait<V>;
    |          ^
 
-error: could not find defining uses
+error: unconstrained opaque type
   --> $DIR/bound_reduction2.rs:9:15
    |
 LL | type Foo<V> = impl Trait<V>;
    |               ^^^^^^^^^^^^^
+   |
+   = note: `Foo` must be used in combination with a concrete type within the same module
 
 error: aborting due to 2 previous errors
 

src/test/ui/type-alias-impl-trait/bounds-are-checked.rs

@@ -4,7 +4,7 @@
 #![feature(type_alias_impl_trait)]
 
 type X<'a> = impl Into<&'static str> + From<&'a str>;
-//~^ ERROR could not find defining uses
+//~^ ERROR unconstrained opaque type
 
 fn f<'a: 'static>(t: &'a str) -> X<'a> {
     //~^ WARNING unnecessary lifetime parameter

src/test/ui/type-alias-impl-trait/bounds-are-checked.stderr

@@ -15,11 +15,13 @@ LL | type X<'a> = impl Into<&'static str> + From<&'a str>;
 LL |     t
    |     ^
 
-error: could not find defining uses
+error: unconstrained opaque type
   --> $DIR/bounds-are-checked.rs:6:14
    |
 LL | type X<'a> = impl Into<&'static str> + From<&'a str>;
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `X` must be used in combination with a concrete type within the same module
 
 error: aborting due to 2 previous errors; 1 warning emitted
 

src/test/ui/type-alias-impl-trait/declared_but_never_defined.rs

@@ -3,4 +3,4 @@
 fn main() {}
 
 // declared but never defined
-type Bar = impl std::fmt::Debug; //~ ERROR could not find defining uses
+type Bar = impl std::fmt::Debug; //~ ERROR unconstrained opaque type

src/test/ui/type-alias-impl-trait/declared_but_never_defined.stderr

@@ -1,8 +1,10 @@
-error: could not find defining uses
+error: unconstrained opaque type
   --> $DIR/declared_but_never_defined.rs:6:12
    |
 LL | type Bar = impl std::fmt::Debug;
    |            ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `Bar` must be used in combination with a concrete type within the same module
 
 error: aborting due to previous error
 

src/test/ui/type-alias-impl-trait/declared_but_not_defined_in_scope.rs

@@ -4,7 +4,7 @@ fn main() {}
 
 mod boo {
     // declared in module but not defined inside of it
-    pub type Boo = impl ::std::fmt::Debug; //~ ERROR could not find defining uses
+    pub type Boo = impl ::std::fmt::Debug; //~ ERROR unconstrained opaque type
 }
 
 fn bomp() -> boo::Boo {

src/test/ui/type-alias-impl-trait/declared_but_not_defined_in_scope.stderr

@@ -1,8 +1,10 @@
-error: could not find defining uses
+error: unconstrained opaque type
   --> $DIR/declared_but_not_defined_in_scope.rs:7:20
    |
 LL |     pub type Boo = impl ::std::fmt::Debug;
    |                    ^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `Boo` must be used in combination with a concrete type within the same module
 
 error[E0308]: mismatched types
   --> $DIR/declared_but_not_defined_in_scope.rs:11:5

src/test/ui/type-alias-impl-trait/generic_duplicate_lifetime_param.rs

@@ -3,7 +3,7 @@
 fn main() {}
 
 type Two<'a, 'b> = impl std::fmt::Debug;
-//~^ ERROR could not find defining uses
+//~^ ERROR unconstrained opaque type
 
 fn one<'a>(t: &'a ()) -> Two<'a, 'a> {
     t

src/test/ui/type-alias-impl-trait/generic_duplicate_lifetime_param.stderr

@@ -10,11 +10,13 @@ note: lifetime used multiple times
 LL | type Two<'a, 'b> = impl std::fmt::Debug;
    |          ^^  ^^
 
-error: could not find defining uses
+error: unconstrained opaque type
   --> $DIR/generic_duplicate_lifetime_param.rs:5:20
    |
 LL | type Two<'a, 'b> = impl std::fmt::Debug;
    |                    ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `Two` must be used in combination with a concrete type within the same module
 
 error: aborting due to 2 previous errors
 

src/test/ui/type-alias-impl-trait/generic_duplicate_param_use.rs

@@ -6,11 +6,11 @@ fn main() {}
 
 // test that unused generic parameters are ok
 type TwoTys<T, U> = impl Debug;
-//~^ ERROR could not find defining uses
+//~^ ERROR unconstrained opaque type
 type TwoLifetimes<'a, 'b> = impl Debug;
-//~^ ERROR could not find defining uses
+//~^ ERROR unconstrained opaque type
 type TwoConsts<const X: usize, const Y: usize> = impl Debug;
-//~^ ERROR could not find defining uses
+//~^ ERROR unconstrained opaque type
 
 fn one_ty<T: Debug>(t: T) -> TwoTys<T, T> {
     t

src/test/ui/type-alias-impl-trait/generic_duplicate_param_use.stderr

@@ -10,11 +10,13 @@ note: type used multiple times
 LL | type TwoTys<T, U> = impl Debug;
    |             ^  ^
 
-error: could not find defining uses
+error: unconstrained opaque type
   --> $DIR/generic_duplicate_param_use.rs:8:21
    |
 LL | type TwoTys<T, U> = impl Debug;
    |                     ^^^^^^^^^^
+   |
+   = note: `TwoTys` must be used in combination with a concrete type within the same module
 
 error: non-defining opaque type use in defining scope
   --> $DIR/generic_duplicate_param_use.rs:21:5
@@ -28,11 +30,13 @@ note: lifetime used multiple times
 LL | type TwoLifetimes<'a, 'b> = impl Debug;
    |                   ^^  ^^
 
-error: could not find defining uses
+error: unconstrained opaque type
   --> $DIR/generic_duplicate_param_use.rs:10:29
    |
 LL | type TwoLifetimes<'a, 'b> = impl Debug;
    |                             ^^^^^^^^^^
+   |
+   = note: `TwoLifetimes` must be used in combination with a concrete type within the same module
 
 error: non-defining opaque type use in defining scope
   --> $DIR/generic_duplicate_param_use.rs:26:5
@@ -46,11 +50,13 @@ note: constant used multiple times
 LL | type TwoConsts<const X: usize, const Y: usize> = impl Debug;
    |                      ^               ^
 
-error: could not find defining uses
+error: unconstrained opaque type
   --> $DIR/generic_duplicate_param_use.rs:12:50
    |
 LL | type TwoConsts<const X: usize, const Y: usize> = impl Debug;
    |                                                  ^^^^^^^^^^
+   |
+   = note: `TwoConsts` must be used in combination with a concrete type within the same module
 
 error: aborting due to 6 previous errors
 

src/test/ui/type-alias-impl-trait/generic_nondefining_use.rs

@@ -5,11 +5,11 @@ use std::fmt::Debug;
 fn main() {}
 
 type OneTy<T> = impl Debug;
-//~^ ERROR could not find defining uses
+//~^ ERROR unconstrained opaque type
 type OneLifetime<'a> = impl Debug;
-//~^ ERROR could not find defining uses
+//~^ ERROR unconstrained opaque type
 type OneConst<const X: usize> = impl Debug;
-//~^ ERROR could not find defining uses
+//~^ ERROR unconstrained opaque type
 
 // Not defining uses, because they doesn't define *all* possible generics.
 

src/test/ui/type-alias-impl-trait/generic_nondefining_use.stderr

@@ -10,11 +10,13 @@ note: used non-generic type `u32` for generic parameter
 LL | type OneTy<T> = impl Debug;
    |            ^
 
-error: could not find defining uses
+error: unconstrained opaque type
   --> $DIR/generic_nondefining_use.rs:7:17
    |
 LL | type OneTy<T> = impl Debug;
    |                 ^^^^^^^^^^
+   |
+   = note: `OneTy` must be used in combination with a concrete type within the same module
 
 error: non-defining opaque type use in defining scope
   --> $DIR/generic_nondefining_use.rs:22:5
@@ -25,11 +27,13 @@ LL | type OneLifetime<'a> = impl Debug;
 LL |     6u32
    |     ^^^^
 
-error: could not find defining uses
+error: unconstrained opaque type
   --> $DIR/generic_nondefining_use.rs:9:24
    |
 LL | type OneLifetime<'a> = impl Debug;
    |                        ^^^^^^^^^^
+   |
+   = note: `OneLifetime` must be used in combination with a concrete type within the same module
 
 error: non-defining opaque type use in defining scope
   --> $DIR/generic_nondefining_use.rs:27:5
@@ -43,11 +47,13 @@ note: used non-generic constant `123_usize` for generic parameter
 LL | type OneConst<const X: usize> = impl Debug;
    |                     ^
 
-error: could not find defining uses
+error: unconstrained opaque type
   --> $DIR/generic_nondefining_use.rs:11:33
    |
 LL | type OneConst<const X: usize> = impl Debug;
    |                                 ^^^^^^^^^^
+   |
+   = note: `OneConst` must be used in combination with a concrete type within the same module
 
 error: aborting due to 6 previous errors
 

src/test/ui/type-alias-impl-trait/issue-60564.rs

@@ -6,7 +6,7 @@ trait IterBits {
 }
 
 type IterBitsIter<T, E, I> = impl std::iter::Iterator<Item = I>;
-//~^ ERROR could not find defining uses
+//~^ ERROR unconstrained opaque type
 
 impl<T: Copy, E> IterBits for T
 where

src/test/ui/type-alias-impl-trait/issue-60564.stderr

@@ -10,11 +10,13 @@ note: used non-generic type `u8` for generic parameter
 LL | type IterBitsIter<T, E, I> = impl std::iter::Iterator<Item = I>;
    |                         ^
 
-error: could not find defining uses
+error: unconstrained opaque type
   --> $DIR/issue-60564.rs:8:30
    |
 LL | type IterBitsIter<T, E, I> = impl std::iter::Iterator<Item = I>;
    |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `IterBitsIter` must be used in combination with a concrete type within the same module
 
 error: aborting due to 2 previous errors
 

src/test/ui/type-alias-impl-trait/issue-68368-non-defining-use-2.rs

@@ -5,7 +5,7 @@
 #![feature(type_alias_impl_trait)]
 trait Trait<T> {}
 type Alias<'a, U> = impl Trait<U>;
-//~^ ERROR could not find defining uses
+//~^ ERROR unconstrained opaque type
 fn f<'a>() -> Alias<'a, ()> {}
 //~^ ERROR non-defining opaque type use in defining scope
 

src/test/ui/type-alias-impl-trait/issue-68368-non-defining-use-2.stderr

@@ -10,11 +10,13 @@ note: used non-generic type `()` for generic parameter
 LL | type Alias<'a, U> = impl Trait<U>;
    |                ^
 
-error: could not find defining uses
+error: unconstrained opaque type
   --> $DIR/issue-68368-non-defining-use-2.rs:7:21
    |
 LL | type Alias<'a, U> = impl Trait<U>;
    |                     ^^^^^^^^^^^^^
+   |
+   = note: `Alias` must be used in combination with a concrete type within the same module
 
 error: aborting due to 2 previous errors
 

src/test/ui/type-alias-impl-trait/issue-68368-non-defining-use.rs

@@ -5,7 +5,7 @@
 #![feature(type_alias_impl_trait)]
 trait Trait<T> {}
 type Alias<'a, U> = impl Trait<U>;
-//~^ ERROR could not find defining uses
+//~^ ERROR unconstrained opaque type
 fn f<'a>() -> Alias<'a, ()> {}
 //~^ ERROR non-defining opaque type use in defining scope
 

src/test/ui/type-alias-impl-trait/issue-68368-non-defining-use.stderr

@@ -10,11 +10,13 @@ note: used non-generic type `()` for generic parameter
 LL | type Alias<'a, U> = impl Trait<U>;
    |                ^
 
-error: could not find defining uses
+error: unconstrained opaque type
   --> $DIR/issue-68368-non-defining-use.rs:7:21
    |
 LL | type Alias<'a, U> = impl Trait<U>;
    |                     ^^^^^^^^^^^^^
+   |
+   = note: `Alias` must be used in combination with a concrete type within the same module
 
 error: aborting due to 2 previous errors
 

src/test/ui/type-alias-impl-trait/nested-tait-inference3.rs

@@ -4,7 +4,7 @@
 use std::fmt::Debug;
 
 type FooX = impl Debug;
-//~^ could not find defining uses
+//~^ unconstrained opaque type
 
 trait Foo<A> { }
 

src/test/ui/type-alias-impl-trait/nested-tait-inference3.stderr

@@ -1,8 +1,10 @@
-error: could not find defining uses
+error: unconstrained opaque type
   --> $DIR/nested-tait-inference3.rs:6:13
    |
 LL | type FooX = impl Debug;
    |             ^^^^^^^^^^
+   |
+   = note: `FooX` must be used in combination with a concrete type within the same module
 
 error: aborting due to previous error
 

src/test/ui/type-alias-impl-trait/no_inferrable_concrete_type.rs

@@ -1,9 +1,9 @@
 // Issue 52985: user code provides no use case that allows a type alias `impl Trait`
-// We now emit a 'could not find defining uses' error
+// We now emit a 'unconstrained opaque type' error
 
 #![feature(type_alias_impl_trait)]
 
-type Foo = impl Copy; //~ could not find defining uses
+type Foo = impl Copy; //~ unconstrained opaque type
 
 // make compiler happy about using 'Foo'
 fn bar(x: Foo) -> Foo {

src/test/ui/type-alias-impl-trait/no_inferrable_concrete_type.stderr

@@ -1,8 +1,10 @@
-error: could not find defining uses
+error: unconstrained opaque type
   --> $DIR/no_inferrable_concrete_type.rs:6:12
    |
 LL | type Foo = impl Copy;
    |            ^^^^^^^^^
+   |
+   = note: `Foo` must be used in combination with a concrete type within the same module
 
 error: aborting due to previous error
 

src/test/ui/type-alias-impl-trait/type-alias-impl-trait-with-cycle-error.rs

@@ -1,7 +1,7 @@
 #![feature(type_alias_impl_trait)]
 
 type Foo = impl Fn() -> Foo;
-//~^ ERROR: could not find defining uses
+//~^ ERROR: unconstrained opaque type
 
 fn crash(x: Foo) -> Foo {
     x

src/test/ui/type-alias-impl-trait/type-alias-impl-trait-with-cycle-error.stderr

@@ -1,8 +1,10 @@
-error: could not find defining uses
+error: unconstrained opaque type
   --> $DIR/type-alias-impl-trait-with-cycle-error.rs:3:12
    |
 LL | type Foo = impl Fn() -> Foo;
    |            ^^^^^^^^^^^^^^^^
+   |
+   = note: `Foo` must be used in combination with a concrete type within the same module
 
 error: aborting due to previous error
 

src/test/ui/type-alias-impl-trait/type-alias-impl-trait-with-cycle-error2.rs

@@ -5,7 +5,7 @@ pub trait Bar<T> {
 }
 
 type Foo = impl Bar<Foo, Item = Foo>;
-//~^ ERROR: could not find defining uses
+//~^ ERROR: unconstrained opaque type
 
 fn crash(x: Foo) -> Foo {
     x

src/test/ui/type-alias-impl-trait/type-alias-impl-trait-with-cycle-error2.stderr

@@ -1,8 +1,10 @@
-error: could not find defining uses
+error: unconstrained opaque type
   --> $DIR/type-alias-impl-trait-with-cycle-error2.rs:7:12
    |
 LL | type Foo = impl Bar<Foo, Item = Foo>;
    |            ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `Foo` must be used in combination with a concrete type within the same module
 
 error: aborting due to previous error