Update syntax in existing tests

18 files changed, 85 insertions, 65 deletions

diff --git a/src/test/ui/impl-trait/associated-existential-type-generic-trait.rs b/src/test/ui/impl-trait/associated-existential-type-generic-trait.rs
index 130de9ccf62..6c7c46b0e3d 100644
--- a/src/test/ui/impl-trait/associated-existential-type-generic-trait.rs
+++ b/src/test/ui/impl-trait/associated-existential-type-generic-trait.rs
@@ -1,4 +1,4 @@
-#![feature(existential_type)]
+#![feature(type_alias_impl_trait)]
 // build-pass (FIXME(62277): could be check-pass?)
 
 trait Bar {}
@@ -11,7 +11,7 @@ trait Foo<T> {
 }
 
 impl<W> Foo<W> for i32 {
-    existential type Assoc: Bar;
+    type Assoc = impl Bar;
     fn foo(w: W) -> Self::Assoc {
         Dummy(w)
     }
@@ -21,7 +21,7 @@ struct NonGeneric;
 impl Bar for NonGeneric {}
 
 impl<W> Foo<W> for u32 {
-    existential type Assoc: Bar;
+    type Assoc = impl Bar;
     fn foo(_: W) -> Self::Assoc {
         NonGeneric
     }
diff --git a/src/test/ui/impl-trait/associated-existential-type-trivial.rs b/src/test/ui/impl-trait/associated-existential-type-trivial.rs
index 0e83b4084b6..cdda341cad8 100644
--- a/src/test/ui/impl-trait/associated-existential-type-trivial.rs
+++ b/src/test/ui/impl-trait/associated-existential-type-trivial.rs
@@ -1,4 +1,4 @@
-#![feature(existential_type)]
+#![feature(type_alias_impl_trait)]
 // build-pass (FIXME(62277): could be check-pass?)
 
 trait Bar {}
@@ -11,7 +11,7 @@ trait Foo {
 }
 
 impl Foo for i32 {
-    existential type Assoc: Bar;
+    type Assoc = impl Bar;
     fn foo() -> Self::Assoc {
         Dummy
     }
diff --git a/src/test/ui/impl-trait/associated-existential-type.rs b/src/test/ui/impl-trait/associated-existential-type.rs
index 58966aefe1c..d0661d66f4b 100644
--- a/src/test/ui/impl-trait/associated-existential-type.rs
+++ b/src/test/ui/impl-trait/associated-existential-type.rs
@@ -1,4 +1,4 @@
-#![feature(existential_type)]
+#![feature(type_alias_impl_trait)]
 // build-pass (FIXME(62277): could be check-pass?)
 
 trait Bar {}
@@ -12,7 +12,7 @@ trait Foo {
 }
 
 impl Foo for i32 {
-    existential type Assoc: Bar;
+    type Assoc = impl Bar;
     fn foo() -> Self::Assoc {
         Dummy
     }
diff --git a/src/test/ui/impl-trait/bound-normalization-fail.rs b/src/test/ui/impl-trait/bound-normalization-fail.rs
index 476ae62fa0f..c33261bfd09 100644
--- a/src/test/ui/impl-trait/bound-normalization-fail.rs
+++ b/src/test/ui/impl-trait/bound-normalization-fail.rs
@@ -2,7 +2,6 @@
 // edition:2018
 
 #![feature(async_await)]
-#![feature(existential_type)]
 #![feature(impl_trait_in_bindings)]
 //~^ WARNING the feature `impl_trait_in_bindings` is incomplete
 
diff --git a/src/test/ui/impl-trait/bound-normalization-fail.stderr b/src/test/ui/impl-trait/bound-normalization-fail.stderr
index 24a687491e5..aa306a7e08a 100644
--- a/src/test/ui/impl-trait/bound-normalization-fail.stderr
+++ b/src/test/ui/impl-trait/bound-normalization-fail.stderr
@@ -1,5 +1,5 @@
 warning: the feature `impl_trait_in_bindings` is incomplete and may cause the compiler to crash
-  --> $DIR/bound-normalization-fail.rs:6:12
+  --> $DIR/bound-normalization-fail.rs:5:12
    |
 LL | #![feature(impl_trait_in_bindings)]
    |            ^^^^^^^^^^^^^^^^^^^^^^
@@ -7,7 +7,7 @@ LL | #![feature(impl_trait_in_bindings)]
    = note: `#[warn(incomplete_features)]` on by default
 
 error[E0271]: type mismatch resolving `<Foo<()> as FooLike>::Output == <T as impl_trait::Trait>::Assoc`
-  --> $DIR/bound-normalization-fail.rs:30:32
+  --> $DIR/bound-normalization-fail.rs:29:32
    |
 LL |     fn foo_fail<T: Trait>() -> impl FooLike<Output=T::Assoc> {
    |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected (), found associated type
@@ -17,7 +17,7 @@ LL |     fn foo_fail<T: Trait>() -> impl FooLike<Output=T::Assoc> {
    = note: the return type of a function must have a statically known size
 
 error[E0271]: type mismatch resolving `<Foo<()> as FooLike>::Output == <T as lifetimes::Trait<'static>>::Assoc`
-  --> $DIR/bound-normalization-fail.rs:47:41
+  --> $DIR/bound-normalization-fail.rs:46:41
    |
 LL |     fn foo2_fail<'a, T: Trait<'a>>() -> impl FooLike<Output=T::Assoc> {
    |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected (), found associated type
diff --git a/src/test/ui/impl-trait/bound-normalization-pass.rs b/src/test/ui/impl-trait/bound-normalization-pass.rs
index 3fdd7c0ecb6..33348340bce 100644
--- a/src/test/ui/impl-trait/bound-normalization-pass.rs
+++ b/src/test/ui/impl-trait/bound-normalization-pass.rs
@@ -2,7 +2,7 @@
 // edition:2018
 
 #![feature(async_await)]
-#![feature(existential_type)]
+#![feature(type_alias_impl_trait)]
 #![feature(impl_trait_in_bindings)]
 //~^ WARNING the feature `impl_trait_in_bindings` is incomplete
 
@@ -99,7 +99,7 @@ mod existential_types {
         type Out = u8;
     }
 
-    existential type Ex: Trait<Out = <() as Implemented>::Assoc>;
+    type Ex = impl Trait<Out = <() as Implemented>::Assoc>;
 
     fn define() -> Ex {
         ()
diff --git a/src/test/ui/impl-trait/existential_type_in_fn_body.rs b/src/test/ui/impl-trait/existential_type_in_fn_body.rs
index 32fc4a97ef4..91be4efd56a 100644
--- a/src/test/ui/impl-trait/existential_type_in_fn_body.rs
+++ b/src/test/ui/impl-trait/existential_type_in_fn_body.rs
@@ -1,11 +1,11 @@
 // build-pass (FIXME(62277): could be check-pass?)
 
-#![feature(existential_type)]
+#![feature(type_alias_impl_trait)]
 
 use std::fmt::Debug;
 
 fn main() {
-    existential type Existential: Debug;
+    type Existential = impl Debug;
 
     fn f() -> Existential {}
     println!("{:?}", f());
diff --git a/src/test/ui/impl-trait/issue-55872-1.rs b/src/test/ui/impl-trait/issue-55872-1.rs
index 5095b26f8a4..c127e4cef49 100644
--- a/src/test/ui/impl-trait/issue-55872-1.rs
+++ b/src/test/ui/impl-trait/issue-55872-1.rs
@@ -1,5 +1,5 @@
 // ignore-tidy-linelength
-#![feature(existential_type)]
+#![feature(type_alias_impl_trait)]
 
 pub trait Bar
 {
@@ -9,7 +9,7 @@ pub trait Bar
 }
 
 impl<S: Default> Bar for S {
-    existential type E: Copy;
+    type E = impl Copy;
     //~^ ERROR the trait bound `S: std::marker::Copy` is not satisfied in `(S, T)` [E0277]
     //~^^ ERROR the trait bound `T: std::marker::Copy` is not satisfied in `(S, T)` [E0277]
 
diff --git a/src/test/ui/impl-trait/issue-55872-1.stderr b/src/test/ui/impl-trait/issue-55872-1.stderr
index 04b4d2d4a50..8a429626a5b 100644
--- a/src/test/ui/impl-trait/issue-55872-1.stderr
+++ b/src/test/ui/impl-trait/issue-55872-1.stderr
@@ -1,8 +1,8 @@
 error[E0277]: the trait bound `S: std::marker::Copy` is not satisfied in `(S, T)`
   --> $DIR/issue-55872-1.rs:12:5
    |
-LL |     existential type E: Copy;
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^ within `(S, T)`, the trait `std::marker::Copy` is not implemented for `S`
+LL |     type E = impl Copy;
+   |     ^^^^^^^^^^^^^^^^^^^ within `(S, T)`, the trait `std::marker::Copy` is not implemented for `S`
    |
    = help: consider adding a `where S: std::marker::Copy` bound
    = note: required because it appears within the type `(S, T)`
@@ -11,8 +11,8 @@ LL |     existential type E: Copy;
 error[E0277]: the trait bound `T: std::marker::Copy` is not satisfied in `(S, T)`
   --> $DIR/issue-55872-1.rs:12:5
    |
-LL |     existential type E: Copy;
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^ within `(S, T)`, the trait `std::marker::Copy` is not implemented for `T`
+LL |     type E = impl Copy;
+   |     ^^^^^^^^^^^^^^^^^^^ within `(S, T)`, the trait `std::marker::Copy` is not implemented for `T`
    |
    = help: consider adding a `where T: std::marker::Copy` bound
    = note: required because it appears within the type `(S, T)`
diff --git a/src/test/ui/impl-trait/issue-55872-2.rs b/src/test/ui/impl-trait/issue-55872-2.rs
index 2bdeb14bdc3..d7432bda839 100644
--- a/src/test/ui/impl-trait/issue-55872-2.rs
+++ b/src/test/ui/impl-trait/issue-55872-2.rs
@@ -1,6 +1,6 @@
 // edition:2018
 // ignore-tidy-linelength
-#![feature(async_await, existential_type)]
+#![feature(async_await, type_alias_impl_trait)]
 
 pub trait Bar {
     type E: Copy;
@@ -9,7 +9,7 @@ pub trait Bar {
 }
 
 impl<S> Bar for S {
-    existential type E: Copy;
+    type E = impl Copy;
     //~^ ERROR the trait bound `impl std::future::Future: std::marker::Copy` is not satisfied [E0277]
     fn foo<T>() -> Self::E {
     //~^ ERROR type parameter `T` is part of concrete type but not used in parameter list for existential type
diff --git a/src/test/ui/impl-trait/issue-55872-2.stderr b/src/test/ui/impl-trait/issue-55872-2.stderr
index 2505a82ee23..c9cc178d3f8 100644
--- a/src/test/ui/impl-trait/issue-55872-2.stderr
+++ b/src/test/ui/impl-trait/issue-55872-2.stderr
@@ -1,8 +1,8 @@
 error[E0277]: the trait bound `impl std::future::Future: std::marker::Copy` is not satisfied
   --> $DIR/issue-55872-2.rs:12:5
    |
-LL |     existential type E: Copy;
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::marker::Copy` is not implemented for `impl std::future::Future`
+LL |     type E = impl Copy;
+   |     ^^^^^^^^^^^^^^^^^^^ the trait `std::marker::Copy` is not implemented for `impl std::future::Future`
    |
    = note: the return type of a function must have a statically known size
 
diff --git a/src/test/ui/impl-trait/issue-55872.rs b/src/test/ui/impl-trait/issue-55872.rs
index 95604545c37..bf2c5c82208 100644
--- a/src/test/ui/impl-trait/issue-55872.rs
+++ b/src/test/ui/impl-trait/issue-55872.rs
@@ -1,5 +1,5 @@
 // ignore-tidy-linelength
-#![feature(existential_type)]
+#![feature(type_alias_impl_trait)]
 
 pub trait Bar {
     type E: Copy;
@@ -8,7 +8,7 @@ pub trait Bar {
 }
 
 impl<S> Bar for S {
-    existential type E: Copy;
+    type E = impl Copy;
 
     fn foo<T>() -> Self::E {
     //~^ ERROR type parameter `T` is part of concrete type but not used in parameter list for existential type
diff --git a/src/test/ui/impl-trait/issues/issue-53457.rs b/src/test/ui/impl-trait/issues/issue-53457.rs
index 3f6a4fb2787..de8c579743f 100644
--- a/src/test/ui/impl-trait/issues/issue-53457.rs
+++ b/src/test/ui/impl-trait/issues/issue-53457.rs
@@ -1,8 +1,8 @@
 // run-pass
 
-#![feature(existential_type)]
+#![feature(type_alias_impl_trait)]
 
-existential type X: Clone;
+type X = impl Clone;
 
 fn bar<F: Fn(&i32) + Clone>(f: F) -> F {
     f
diff --git a/src/test/ui/impl-trait/multiple-lifetimes/error-handling.rs b/src/test/ui/impl-trait/multiple-lifetimes/error-handling.rs
index 61e858ee02d..b226cf058aa 100644
--- a/src/test/ui/impl-trait/multiple-lifetimes/error-handling.rs
+++ b/src/test/ui/impl-trait/multiple-lifetimes/error-handling.rs
@@ -1,14 +1,14 @@
 // compile-flags:-Zborrowck=mir
 
 #![feature(member_constraints)]
-#![feature(existential_type)]
+#![feature(type_alias_impl_trait)]
 
 #[derive(Clone)]
 struct CopyIfEq<T, U>(T, U);
 
 impl<T: Copy> Copy for CopyIfEq<T, T> {}
 
-existential type E<'a, 'b>: Sized;
+type E<'a, 'b> = impl Sized;
 
 fn foo<'a, 'b, 'c>(x: &'static i32, mut y: &'a i32) -> E<'b, 'c> {
     //~^ ERROR lifetime may not live long enough
diff --git a/src/test/ui/impl-trait/multiple-lifetimes/error-handling.stderr b/src/test/ui/impl-trait/multiple-lifetimes/error-handling.stderr
index b59dfbe9f2a..f27e6ff44a6 100644
--- a/src/test/ui/impl-trait/multiple-lifetimes/error-handling.stderr
+++ b/src/test/ui/impl-trait/multiple-lifetimes/error-handling.stderr
@@ -5,7 +5,7 @@ LL | fn foo<'a, 'b, 'c>(x: &'static i32, mut y: &'a i32) -> E<'b, 'c> {
    |        -- lifetime `'a` defined here                   ^^^^^^^^^ opaque type requires that `'a` must outlive `'static`
 help: to allow this `impl Trait` to capture borrowed data with lifetime `'a`, add `'a` as a constraint
    |
-LL | existential type E<'a, 'b>: Sized; + 'a
+LL | type E<'a, 'b> = impl Sized; + 'a
    |
 
 error: aborting due to previous error
diff --git a/src/test/ui/impl-trait/multiple-lifetimes/ordinary-bounds-pick-original-existential.rs b/src/test/ui/impl-trait/multiple-lifetimes/ordinary-bounds-pick-original-existential.rs
index 43915e05e88..ae44b371f4f 100644
--- a/src/test/ui/impl-trait/multiple-lifetimes/ordinary-bounds-pick-original-existential.rs
+++ b/src/test/ui/impl-trait/multiple-lifetimes/ordinary-bounds-pick-original-existential.rs
@@ -4,7 +4,7 @@
 //[mir]compile-flags: -Z borrowck=mir
 
 #![feature(member_constraints)]
-#![feature(existential_type)]
+#![feature(type_alias_impl_trait)]
 
 trait Trait<'a, 'b> { }
 impl<T> Trait<'_, '_> for T { }
@@ -12,7 +12,7 @@ impl<T> Trait<'_, '_> for T { }
 // Here we wind up selecting `'a` and `'b` in the hidden type because
 // those are the types that appear in the original values.
 
-existential type Foo<'a, 'b>: Trait<'a, 'b>;
+type Foo<'a, 'b> = impl Trait<'a, 'b>;
 
 fn upper_bounds<'a, 'b>(a: &'a u8, b: &'b u8) -> Foo<'a, 'b> {
     // In this simple case, you have a hidden type `(&'0 u8, &'1 u8)` and constraints like
diff --git a/src/test/ui/impl-trait/where-allowed.rs b/src/test/ui/impl-trait/where-allowed.rs
index 31bc9f449a0..5b6105421fe 100644
--- a/src/test/ui/impl-trait/where-allowed.rs
+++ b/src/test/ui/impl-trait/where-allowed.rs
@@ -120,7 +120,8 @@ trait DummyTrait {
 }
 impl DummyTrait for () {
     type Out = impl Debug;
-    //~^ ERROR `impl Trait` not allowed outside of function and inherent method return types
+    //~^ ERROR existential types are unstable
+    //~^^ ERROR could not find defining uses
 
     fn in_trait_impl_parameter(_: impl Debug) { }
     // Allowed
@@ -155,7 +156,8 @@ extern "C" fn in_extern_fn_return() -> impl Debug {
 }
 
 type InTypeAlias<R> = impl Debug;
-//~^ ERROR `impl Trait` not allowed outside of function and inherent method return types
+//~^ ERROR existential types are unstable
+//~^^ ERROR could not find defining uses
 
 type InReturnInTypeAlias<R> = fn() -> impl Debug;
 //~^ ERROR `impl Trait` not allowed outside of function and inherent method return types
diff --git a/src/test/ui/impl-trait/where-allowed.stderr b/src/test/ui/impl-trait/where-allowed.stderr
index 9895b028a4e..08f456199e9 100644
--- a/src/test/ui/impl-trait/where-allowed.stderr
+++ b/src/test/ui/impl-trait/where-allowed.stderr
@@ -16,6 +16,24 @@ LL | fn in_impl_Fn_parameter_in_return() -> &'static impl Fn(impl Debug) { panic
    |                                                 |       nested `impl Trait` here
    |                                                 outer `impl Trait`
 
+error[E0658]: existential types are unstable
+  --> $DIR/where-allowed.rs:122:5
+   |
+LL |     type Out = impl Debug;
+   |     ^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/63063
+   = help: add `#![feature(type_alias_impl_trait)]` to the crate attributes to enable
+
+error[E0658]: existential types are unstable
+  --> $DIR/where-allowed.rs:158:1
+   |
+LL | type InTypeAlias<R> = impl Debug;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/63063
+   = help: add `#![feature(type_alias_impl_trait)]` to the crate attributes to enable
+
 error[E0562]: `impl Trait` not allowed outside of function and inherent method return types
   --> $DIR/where-allowed.rs:18:40
    |
@@ -137,97 +155,85 @@ LL |     fn in_return() -> impl Debug;
    |                       ^^^^^^^^^^
 
 error[E0562]: `impl Trait` not allowed outside of function and inherent method return types
-  --> $DIR/where-allowed.rs:122:16
-   |
-LL |     type Out = impl Debug;
-   |                ^^^^^^^^^^
-
-error[E0562]: `impl Trait` not allowed outside of function and inherent method return types
-  --> $DIR/where-allowed.rs:128:34
+  --> $DIR/where-allowed.rs:129:34
    |
 LL |     fn in_trait_impl_return() -> impl Debug { () }
    |                                  ^^^^^^^^^^
 
 error[E0562]: `impl Trait` not allowed outside of function and inherent method return types
-  --> $DIR/where-allowed.rs:141:33
+  --> $DIR/where-allowed.rs:142:33
    |
 LL |     fn in_foreign_parameters(_: impl Debug);
    |                                 ^^^^^^^^^^
 
 error[E0562]: `impl Trait` not allowed outside of function and inherent method return types
-  --> $DIR/where-allowed.rs:144:31
+  --> $DIR/where-allowed.rs:145:31
    |
 LL |     fn in_foreign_return() -> impl Debug;
    |                               ^^^^^^^^^^
 
 error[E0562]: `impl Trait` not allowed outside of function and inherent method return types
-  --> $DIR/where-allowed.rs:157:23
-   |
-LL | type InTypeAlias<R> = impl Debug;
-   |                       ^^^^^^^^^^
-
-error[E0562]: `impl Trait` not allowed outside of function and inherent method return types
-  --> $DIR/where-allowed.rs:160:39
+  --> $DIR/where-allowed.rs:162:39
    |
 LL | type InReturnInTypeAlias<R> = fn() -> impl Debug;
    |                                       ^^^^^^^^^^
 
 error[E0562]: `impl Trait` not allowed outside of function and inherent method return types
-  --> $DIR/where-allowed.rs:164:16
+  --> $DIR/where-allowed.rs:166:16
    |
 LL | impl PartialEq<impl Debug> for () {
    |                ^^^^^^^^^^
 
 error[E0562]: `impl Trait` not allowed outside of function and inherent method return types
-  --> $DIR/where-allowed.rs:169:24
+  --> $DIR/where-allowed.rs:171:24
    |
 LL | impl PartialEq<()> for impl Debug {
    |                        ^^^^^^^^^^
 
 error[E0562]: `impl Trait` not allowed outside of function and inherent method return types
-  --> $DIR/where-allowed.rs:174:6
+  --> $DIR/where-allowed.rs:176:6
    |
 LL | impl impl Debug {
    |      ^^^^^^^^^^
 
 error[E0562]: `impl Trait` not allowed outside of function and inherent method return types
-  --> $DIR/where-allowed.rs:180:24
+  --> $DIR/where-allowed.rs:182:24
    |
 LL | impl InInherentImplAdt<impl Debug> {
    |                        ^^^^^^^^^^
 
 error[E0562]: `impl Trait` not allowed outside of function and inherent method return types
-  --> $DIR/where-allowed.rs:186:11
+  --> $DIR/where-allowed.rs:188:11
    |
 LL |     where impl Debug: Debug
    |           ^^^^^^^^^^
 
 error[E0562]: `impl Trait` not allowed outside of function and inherent method return types
-  --> $DIR/where-allowed.rs:193:15
+  --> $DIR/where-allowed.rs:195:15
    |
 LL |     where Vec<impl Debug>: Debug
    |               ^^^^^^^^^^
 
 error[E0562]: `impl Trait` not allowed outside of function and inherent method return types
-  --> $DIR/where-allowed.rs:200:24
+  --> $DIR/where-allowed.rs:202:24
    |
 LL |     where T: PartialEq<impl Debug>
    |                        ^^^^^^^^^^
 
 error[E0562]: `impl Trait` not allowed outside of function and inherent method return types
-  --> $DIR/where-allowed.rs:207:17
+  --> $DIR/where-allowed.rs:209:17
    |
 LL |     where T: Fn(impl Debug)
    |                 ^^^^^^^^^^
 
 error[E0562]: `impl Trait` not allowed outside of function and inherent method return types
-  --> $DIR/where-allowed.rs:214:22
+  --> $DIR/where-allowed.rs:216:22
    |
 LL |     where T: Fn() -> impl Debug
    |                      ^^^^^^^^^^
 
 error[E0562]: `impl Trait` not allowed outside of function and inherent method return types
-  --> $DIR/where-allowed.rs:220:29
+  --> $DIR/where-allowed.rs:222:29
    |
 LL |     let _in_local_variable: impl Fn() = || {};
    |                             ^^^^^^^^^
@@ -235,11 +241,24 @@ LL |     let _in_local_variable: impl Fn() = || {};
    = help: add `#![feature(impl_trait_in_bindings)]` to the crate attributes to enable
 
 error[E0562]: `impl Trait` not allowed outside of function and inherent method return types
-  --> $DIR/where-allowed.rs:222:46
+  --> $DIR/where-allowed.rs:224:46
    |
 LL |     let _in_return_in_local_variable = || -> impl Fn() { || {} };
    |                                              ^^^^^^^^^
 
-error: aborting due to 39 previous errors
+error: could not find defining uses
+  --> $DIR/where-allowed.rs:158:1
+   |
+LL | type InTypeAlias<R> = impl Debug;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: could not find defining uses
+  --> $DIR/where-allowed.rs:122:5
+   |
+LL |     type Out = impl Debug;
+   |     ^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 41 previous errors
 
-For more information about this error, try `rustc --explain E0562`.
+Some errors have detailed explanations: E0562, E0658.
+For more information about an error, try `rustc --explain E0562`.