Improve a TAIT error and add an error code plus documentation

21 files changed, 145 insertions, 82 deletions

diff --git a/compiler/rustc_borrowck/src/region_infer/opaque_types.rs b/compiler/rustc_borrowck/src/region_infer/opaque_types.rs
index c6e42336ef8..9d8812b7eea 100644
--- a/compiler/rustc_borrowck/src/region_infer/opaque_types.rs
+++ b/compiler/rustc_borrowck/src/region_infer/opaque_types.rs
@@ -12,6 +12,8 @@ use rustc_span::Span;
 use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt as _;
 use rustc_trait_selection::traits::ObligationCtxt;
 
+use crate::session_diagnostics::NonGenericOpaqueTypeParam;
+
 use super::RegionInferenceContext;
 
 impl<'tcx> RegionInferenceContext<'tcx> {
@@ -389,17 +391,13 @@ fn check_opaque_type_parameter_valid(
         } else {
             // Prevent `fn foo() -> Foo<u32>` from being defining.
             let opaque_param = opaque_generics.param_at(i, tcx);
-            tcx.sess
-                .struct_span_err(span, "non-defining opaque type use in defining scope")
-                .span_note(
-                    tcx.def_span(opaque_param.def_id),
-                    &format!(
-                        "used non-generic {} `{}` for generic parameter",
-                        opaque_param.kind.descr(),
-                        arg,
-                    ),
-                )
-                .emit();
+            let kind = opaque_param.kind.descr();
+            tcx.sess.emit_err(NonGenericOpaqueTypeParam {
+                ty: arg,
+                kind,
+                span,
+                param_span: tcx.def_span(opaque_param.def_id),
+            });
             return false;
         }
     }
diff --git a/compiler/rustc_borrowck/src/session_diagnostics.rs b/compiler/rustc_borrowck/src/session_diagnostics.rs
index 577332c0744..23acf159240 100644
--- a/compiler/rustc_borrowck/src/session_diagnostics.rs
+++ b/compiler/rustc_borrowck/src/session_diagnostics.rs
@@ -1,6 +1,6 @@
 use rustc_errors::{IntoDiagnosticArg, MultiSpan};
 use rustc_macros::{Diagnostic, LintDiagnostic, Subdiagnostic};
-use rustc_middle::ty::Ty;
+use rustc_middle::ty::{GenericArg, Ty};
 use rustc_span::Span;
 
 use crate::diagnostics::RegionName;
@@ -240,3 +240,14 @@ pub(crate) struct MoveBorrow<'a> {
     #[label]
     pub borrow_span: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(borrowck_opaque_type_non_generic_param, code = "E0792")]
+pub(crate) struct NonGenericOpaqueTypeParam<'a, 'tcx> {
+    pub ty: GenericArg<'tcx>,
+    pub kind: &'a str,
+    #[primary_span]
+    pub span: Span,
+    #[label]
+    pub param_span: Span,
+}
diff --git a/compiler/rustc_error_codes/src/error_codes.rs b/compiler/rustc_error_codes/src/error_codes.rs
index 24258974bb9..5c77448f908 100644
--- a/compiler/rustc_error_codes/src/error_codes.rs
+++ b/compiler/rustc_error_codes/src/error_codes.rs
@@ -508,6 +508,7 @@ E0787: include_str!("./error_codes/E0787.md"),
 E0788: include_str!("./error_codes/E0788.md"),
 E0790: include_str!("./error_codes/E0790.md"),
 E0791: include_str!("./error_codes/E0791.md"),
+E0792: include_str!("./error_codes/E0792.md"),
 ;
 //  E0006, // merged with E0005
 //  E0008, // cannot bind by-move into a pattern guard
diff --git a/compiler/rustc_error_codes/src/error_codes/E0792.md b/compiler/rustc_error_codes/src/error_codes/E0792.md
new file mode 100644
index 00000000000..bad2b5abfe4
--- /dev/null
+++ b/compiler/rustc_error_codes/src/error_codes/E0792.md
@@ -0,0 +1,60 @@
+A type alias impl trait can only have its hidden type assigned
+when used fully generically (and within their defining scope).
+This means
+
+```compile_fail,E0792
+#![feature(type_alias_impl_trait)]
+
+type Foo<T> = impl std::fmt::Debug;
+
+fn foo() -> Foo<u32> {
+    5u32
+}
+```
+
+is not accepted. If it were accepted, one could create unsound situations like
+
+```compile_fail,E0792
+#![feature(type_alias_impl_trait)]
+
+type Foo<T> = impl Default;
+
+fn foo() -> Foo<u32> {
+    5u32
+}
+
+fn main() {
+    let x = Foo::<&'static mut String>::default();
+}
+```
+
+
+Instead you need to make the function generic:
+
+```
+#![feature(type_alias_impl_trait)]
+
+type Foo<T> = impl std::fmt::Debug;
+
+fn foo<U>() -> Foo<U> {
+    5u32
+}
+```
+
+This means that no matter the generic parameter to `foo`,
+the hidden type will always be `u32`.
+If you want to link the generic parameter to the hidden type,
+you can do that, too:
+
+
+```
+#![feature(type_alias_impl_trait)]
+
+use std::fmt::Debug;
+
+type Foo<T: Debug> = impl Debug;
+
+fn foo<U: Debug>() -> Foo<U> {
+    Vec::<U>::new()
+}
+```
diff --git a/compiler/rustc_error_messages/locales/en-US/borrowck.ftl b/compiler/rustc_error_messages/locales/en-US/borrowck.ftl
index 2cd4733220e..9e4332c4283 100644
--- a/compiler/rustc_error_messages/locales/en-US/borrowck.ftl
+++ b/compiler/rustc_error_messages/locales/en-US/borrowck.ftl
@@ -120,3 +120,7 @@ borrowck_cannot_move_when_borrowed =
         [value] value
         *[other] {$value_place}
     } occurs here
+
+borrowck_opaque_type_non_generic_param =
+    expected generic {$kind} parameter, found `{$ty}`
+    .label = this generic parameter must be used with a generic {$kind} parameter
diff --git a/compiler/rustc_middle/src/ty/diagnostics.rs b/compiler/rustc_middle/src/ty/diagnostics.rs
index 8c22df7395f..771b63f59c1 100644
--- a/compiler/rustc_middle/src/ty/diagnostics.rs
+++ b/compiler/rustc_middle/src/ty/diagnostics.rs
@@ -17,7 +17,7 @@ use rustc_type_ir::sty::TyKind::*;
 
 impl<'tcx> IntoDiagnosticArg for Ty<'tcx> {
     fn into_diagnostic_arg(self) -> DiagnosticArgValue<'static> {
-        format!("{}", self).into_diagnostic_arg()
+        self.to_string().into_diagnostic_arg()
     }
 }
 
diff --git a/compiler/rustc_middle/src/ty/subst.rs b/compiler/rustc_middle/src/ty/subst.rs
index 8f764011d0a..75a8dda3157 100644
--- a/compiler/rustc_middle/src/ty/subst.rs
+++ b/compiler/rustc_middle/src/ty/subst.rs
@@ -7,6 +7,7 @@ use crate::ty::visit::{TypeVisitable, TypeVisitor};
 use crate::ty::{self, Lift, List, ParamConst, Ty, TyCtxt};
 
 use rustc_data_structures::intern::Interned;
+use rustc_errors::{DiagnosticArgValue, IntoDiagnosticArg};
 use rustc_hir::def_id::DefId;
 use rustc_macros::HashStable;
 use rustc_serialize::{self, Decodable, Encodable};
@@ -36,6 +37,12 @@ pub struct GenericArg<'tcx> {
     marker: PhantomData<(Ty<'tcx>, ty::Region<'tcx>, ty::Const<'tcx>)>,
 }
 
+impl<'tcx> IntoDiagnosticArg for GenericArg<'tcx> {
+    fn into_diagnostic_arg(self) -> DiagnosticArgValue<'static> {
+        self.to_string().into_diagnostic_arg()
+    }
+}
+
 const TAG_MASK: usize = 0b11;
 const TYPE_TAG: usize = 0b00;
 const REGION_TAG: usize = 0b01;
diff --git a/tests/ui/type-alias-impl-trait/bound_reduction2.rs b/tests/ui/type-alias-impl-trait/bound_reduction2.rs
index 4d2890b5de5..0bcc9e002ca 100644
--- a/tests/ui/type-alias-impl-trait/bound_reduction2.rs
+++ b/tests/ui/type-alias-impl-trait/bound_reduction2.rs
@@ -14,5 +14,5 @@ impl<W> Trait<W> for () {}
 
 fn foo_desugared<T: TraitWithAssoc>(_: T) -> Foo<T::Assoc> {
     ()
-    //~^ ERROR non-defining opaque type use
+    //~^ ERROR expected generic type parameter, found `<T as TraitWithAssoc>::Assoc`
 }
diff --git a/tests/ui/type-alias-impl-trait/bound_reduction2.stderr b/tests/ui/type-alias-impl-trait/bound_reduction2.stderr
index c405b1f6af2..3c259bd9e97 100644
--- a/tests/ui/type-alias-impl-trait/bound_reduction2.stderr
+++ b/tests/ui/type-alias-impl-trait/bound_reduction2.stderr
@@ -1,14 +1,12 @@
-error: non-defining opaque type use in defining scope
+error[E0792]: expected generic type parameter, found `<T as TraitWithAssoc>::Assoc`
   --> $DIR/bound_reduction2.rs:16:5
    |
+LL | type Foo<V> = impl Trait<V>;
+   |          - this generic parameter must be used with a generic type parameter
+...
 LL |     ()
    |     ^^
-   |
-note: used non-generic type `<T as TraitWithAssoc>::Assoc` for generic parameter
-  --> $DIR/bound_reduction2.rs:9:10
-   |
-LL | type Foo<V> = impl Trait<V>;
-   |          ^
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0792`.
diff --git a/tests/ui/type-alias-impl-trait/generic_nondefining_use.rs b/tests/ui/type-alias-impl-trait/generic_nondefining_use.rs
index f39741a6a62..f5045d382aa 100644
--- a/tests/ui/type-alias-impl-trait/generic_nondefining_use.rs
+++ b/tests/ui/type-alias-impl-trait/generic_nondefining_use.rs
@@ -10,12 +10,11 @@ type OneLifetime<'a> = impl Debug;
 
 type OneConst<const X: usize> = impl Debug;
 
-
 // Not defining uses, because they doesn't define *all* possible generics.
 
 fn concrete_ty() -> OneTy<u32> {
     5u32
-    //~^ ERROR non-defining opaque type use in defining scope
+    //~^ ERROR expected generic type parameter, found `u32`
 }
 
 fn concrete_lifetime() -> OneLifetime<'static> {
@@ -25,5 +24,5 @@ fn concrete_lifetime() -> OneLifetime<'static> {
 
 fn concrete_const() -> OneConst<{ 123 }> {
     7u32
-    //~^ ERROR non-defining opaque type use in defining scope
+    //~^ ERROR expected generic constant parameter, found `123`
 }
diff --git a/tests/ui/type-alias-impl-trait/generic_nondefining_use.stderr b/tests/ui/type-alias-impl-trait/generic_nondefining_use.stderr
index e7565525ad3..564648630b1 100644
--- a/tests/ui/type-alias-impl-trait/generic_nondefining_use.stderr
+++ b/tests/ui/type-alias-impl-trait/generic_nondefining_use.stderr
@@ -1,17 +1,14 @@
-error: non-defining opaque type use in defining scope
-  --> $DIR/generic_nondefining_use.rs:17:5
+error[E0792]: expected generic type parameter, found `u32`
+  --> $DIR/generic_nondefining_use.rs:16:5
    |
+LL | type OneTy<T> = impl Debug;
+   |            - this generic parameter must be used with a generic type parameter
+...
 LL |     5u32
    |     ^^^^
-   |
-note: used non-generic type `u32` for generic parameter
-  --> $DIR/generic_nondefining_use.rs:7:12
-   |
-LL | type OneTy<T> = impl Debug;
-   |            ^
 
 error: non-defining opaque type use in defining scope
-  --> $DIR/generic_nondefining_use.rs:22:5
+  --> $DIR/generic_nondefining_use.rs:21:5
    |
 LL | type OneLifetime<'a> = impl Debug;
    |                  -- cannot use static lifetime; use a bound lifetime instead or remove the lifetime parameter from the opaque type
@@ -19,17 +16,15 @@ LL | type OneLifetime<'a> = impl Debug;
 LL |     6u32
    |     ^^^^
 
-error: non-defining opaque type use in defining scope
-  --> $DIR/generic_nondefining_use.rs:27:5
+error[E0792]: expected generic constant parameter, found `123`
+  --> $DIR/generic_nondefining_use.rs:26:5
    |
+LL | type OneConst<const X: usize> = impl Debug;
+   |               -------------- this generic parameter must be used with a generic constant parameter
+...
 LL |     7u32
    |     ^^^^
-   |
-note: used non-generic constant `123` for generic parameter
-  --> $DIR/generic_nondefining_use.rs:11:15
-   |
-LL | type OneConst<const X: usize> = impl Debug;
-   |               ^^^^^^^^^^^^^^
 
 error: aborting due to 3 previous errors
 
+For more information about this error, try `rustc --explain E0792`.
diff --git a/tests/ui/type-alias-impl-trait/generic_type_does_not_live_long_enough.rs b/tests/ui/type-alias-impl-trait/generic_type_does_not_live_long_enough.rs
index cb90776472b..d3e169a70d3 100644
--- a/tests/ui/type-alias-impl-trait/generic_type_does_not_live_long_enough.rs
+++ b/tests/ui/type-alias-impl-trait/generic_type_does_not_live_long_enough.rs
@@ -4,7 +4,7 @@ fn main() {
     let y = 42;
     let x = wrong_generic(&y);
     let z: i32 = x;
-    //~^ ERROR non-defining opaque type use
+    //~^ ERROR expected generic type parameter, found `&'static i32
 }
 
 type WrongGeneric<T> = impl 'static;
diff --git a/tests/ui/type-alias-impl-trait/generic_type_does_not_live_long_enough.stderr b/tests/ui/type-alias-impl-trait/generic_type_does_not_live_long_enough.stderr
index ba583241a69..19115fd2866 100644
--- a/tests/ui/type-alias-impl-trait/generic_type_does_not_live_long_enough.stderr
+++ b/tests/ui/type-alias-impl-trait/generic_type_does_not_live_long_enough.stderr
@@ -4,17 +4,14 @@ error: at least one trait must be specified
 LL | type WrongGeneric<T> = impl 'static;
    |                        ^^^^^^^^^^^^
 
-error: non-defining opaque type use in defining scope
+error[E0792]: expected generic type parameter, found `&'static i32`
   --> $DIR/generic_type_does_not_live_long_enough.rs:6:18
    |
 LL |     let z: i32 = x;
    |                  ^
-   |
-note: used non-generic type `&'static i32` for generic parameter
-  --> $DIR/generic_type_does_not_live_long_enough.rs:10:19
-   |
+...
 LL | type WrongGeneric<T> = impl 'static;
-   |                   ^
+   |                   - this generic parameter must be used with a generic type parameter
 
 error[E0310]: the parameter type `T` may not live long enough
   --> $DIR/generic_type_does_not_live_long_enough.rs:14:5
@@ -29,4 +26,5 @@ LL | fn wrong_generic<T: 'static>(t: T) -> WrongGeneric<T> {
 
 error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0310`.
+Some errors have detailed explanations: E0310, E0792.
+For more information about an error, try `rustc --explain E0310`.
diff --git a/tests/ui/type-alias-impl-trait/issue-60564.rs b/tests/ui/type-alias-impl-trait/issue-60564.rs
index 4fc7679311a..c2f4c370807 100644
--- a/tests/ui/type-alias-impl-trait/issue-60564.rs
+++ b/tests/ui/type-alias-impl-trait/issue-60564.rs
@@ -18,7 +18,7 @@ where
     type BitsIter = IterBitsIter<T, E, u8>;
     fn iter_bits(self, n: u8) -> Self::BitsIter {
         (0u8..n).rev().map(move |shift| ((self >> T::from(shift)) & T::from(1)).try_into().unwrap())
-        //~^ ERROR non-defining opaque type use in defining scope
+        //~^ ERROR expected generic type parameter, found `u8`
     }
 }
 
diff --git a/tests/ui/type-alias-impl-trait/issue-60564.stderr b/tests/ui/type-alias-impl-trait/issue-60564.stderr
index bbc93657be3..f8fdb004d09 100644
--- a/tests/ui/type-alias-impl-trait/issue-60564.stderr
+++ b/tests/ui/type-alias-impl-trait/issue-60564.stderr
@@ -1,14 +1,12 @@
-error: non-defining opaque type use in defining scope
+error[E0792]: expected generic type parameter, found `u8`
   --> $DIR/issue-60564.rs:20:9
    |
+LL | type IterBitsIter<T, E, I> = impl std::iter::Iterator<Item = I>;
+   |                         - this generic parameter must be used with a generic type parameter
+...
 LL |         (0u8..n).rev().map(move |shift| ((self >> T::from(shift)) & T::from(1)).try_into().unwrap())
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-note: used non-generic type `u8` for generic parameter
-  --> $DIR/issue-60564.rs:8:25
-   |
-LL | type IterBitsIter<T, E, I> = impl std::iter::Iterator<Item = I>;
-   |                         ^
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0792`.
diff --git a/tests/ui/type-alias-impl-trait/issue-68368-non-defining-use-2.rs b/tests/ui/type-alias-impl-trait/issue-68368-non-defining-use-2.rs
index 5223fb1c702..5e0a82a7286 100644
--- a/tests/ui/type-alias-impl-trait/issue-68368-non-defining-use-2.rs
+++ b/tests/ui/type-alias-impl-trait/issue-68368-non-defining-use-2.rs
@@ -7,7 +7,7 @@ trait Trait<T> {}
 type Alias<'a, U> = impl Trait<U>;
 
 fn f<'a>() -> Alias<'a, ()> {}
-//~^ ERROR non-defining opaque type use in defining scope
+//~^ ERROR expected generic type parameter, found `()`
 
 fn main() {}
 
diff --git a/tests/ui/type-alias-impl-trait/issue-68368-non-defining-use-2.stderr b/tests/ui/type-alias-impl-trait/issue-68368-non-defining-use-2.stderr
index 7fb9a0c410e..271743a4010 100644
--- a/tests/ui/type-alias-impl-trait/issue-68368-non-defining-use-2.stderr
+++ b/tests/ui/type-alias-impl-trait/issue-68368-non-defining-use-2.stderr
@@ -1,14 +1,12 @@
-error: non-defining opaque type use in defining scope
+error[E0792]: expected generic type parameter, found `()`
   --> $DIR/issue-68368-non-defining-use-2.rs:9:29
    |
+LL | type Alias<'a, U> = impl Trait<U>;
+   |                - this generic parameter must be used with a generic type parameter
+LL |
 LL | fn f<'a>() -> Alias<'a, ()> {}
    |                             ^^
-   |
-note: used non-generic type `()` for generic parameter
-  --> $DIR/issue-68368-non-defining-use-2.rs:7:16
-   |
-LL | type Alias<'a, U> = impl Trait<U>;
-   |                ^
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0792`.
diff --git a/tests/ui/type-alias-impl-trait/issue-68368-non-defining-use.rs b/tests/ui/type-alias-impl-trait/issue-68368-non-defining-use.rs
index b50462bf237..3b32260c96f 100644
--- a/tests/ui/type-alias-impl-trait/issue-68368-non-defining-use.rs
+++ b/tests/ui/type-alias-impl-trait/issue-68368-non-defining-use.rs
@@ -7,7 +7,7 @@ trait Trait<T> {}
 type Alias<'a, U> = impl Trait<U>;
 
 fn f<'a>() -> Alias<'a, ()> {}
-//~^ ERROR non-defining opaque type use in defining scope
+//~^ ERROR expected generic type parameter, found `()`
 
 fn main() {}
 
diff --git a/tests/ui/type-alias-impl-trait/issue-68368-non-defining-use.stderr b/tests/ui/type-alias-impl-trait/issue-68368-non-defining-use.stderr
index 8059621b61a..4d9a8d6eef9 100644
--- a/tests/ui/type-alias-impl-trait/issue-68368-non-defining-use.stderr
+++ b/tests/ui/type-alias-impl-trait/issue-68368-non-defining-use.stderr
@@ -1,14 +1,12 @@
-error: non-defining opaque type use in defining scope
+error[E0792]: expected generic type parameter, found `()`
   --> $DIR/issue-68368-non-defining-use.rs:9:29
    |
+LL | type Alias<'a, U> = impl Trait<U>;
+   |                - this generic parameter must be used with a generic type parameter
+LL |
 LL | fn f<'a>() -> Alias<'a, ()> {}
    |                             ^^
-   |
-note: used non-generic type `()` for generic parameter
-  --> $DIR/issue-68368-non-defining-use.rs:7:16
-   |
-LL | type Alias<'a, U> = impl Trait<U>;
-   |                ^
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0792`.
diff --git a/tests/ui/type-alias-impl-trait/issue-69136-inner-lifetime-resolve-error.rs b/tests/ui/type-alias-impl-trait/issue-69136-inner-lifetime-resolve-error.rs
index 428454bc048..7657fe2fb1a 100644
--- a/tests/ui/type-alias-impl-trait/issue-69136-inner-lifetime-resolve-error.rs
+++ b/tests/ui/type-alias-impl-trait/issue-69136-inner-lifetime-resolve-error.rs
@@ -18,6 +18,6 @@ type Return<A> = impl WithAssoc<A, AssocType = impl SomeTrait + 'a>;
 //~^ ERROR use of undeclared lifetime name `'a`
 
 fn my_fun() -> Return<()> {}
-//~^ ERROR non-defining opaque type use in defining scope
+//~^ ERROR expected generic type parameter, found `()`
 
 fn main() {}
diff --git a/tests/ui/type-alias-impl-trait/issue-69136-inner-lifetime-resolve-error.stderr b/tests/ui/type-alias-impl-trait/issue-69136-inner-lifetime-resolve-error.stderr
index 7b50c8af26e..d1250786d93 100644
--- a/tests/ui/type-alias-impl-trait/issue-69136-inner-lifetime-resolve-error.stderr
+++ b/tests/ui/type-alias-impl-trait/issue-69136-inner-lifetime-resolve-error.stderr
@@ -14,18 +14,16 @@ help: consider introducing lifetime `'a` here
 LL | type Return<'a, A> = impl WithAssoc<A, AssocType = impl SomeTrait + 'a>;
    |             +++
 
-error: non-defining opaque type use in defining scope
+error[E0792]: expected generic type parameter, found `()`
   --> $DIR/issue-69136-inner-lifetime-resolve-error.rs:20:27
    |
+LL | type Return<A> = impl WithAssoc<A, AssocType = impl SomeTrait + 'a>;
+   |             - this generic parameter must be used with a generic type parameter
+...
 LL | fn my_fun() -> Return<()> {}
    |                           ^^
-   |
-note: used non-generic type `()` for generic parameter
-  --> $DIR/issue-69136-inner-lifetime-resolve-error.rs:17:13
-   |
-LL | type Return<A> = impl WithAssoc<A, AssocType = impl SomeTrait + 'a>;
-   |             ^
 
 error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0261`.
+Some errors have detailed explanations: E0261, E0792.
+For more information about an error, try `rustc --explain E0261`.