Add long error explaination for E0666

src/librustc_passes/error_codes.rs

@@ -552,6 +552,30 @@ trait Foo {
 ```
 "##,
 
+E0666: r##"
+`impl Trait` types cannot appear nested in the
+generic arguments of other `impl Trait` types.
+
+Example of erroneous code:
+
+```compile_fail,E0666
+trait MyGenericTrait<T> {}
+trait MyInnerTrait {}
+
+fn foo(bar: impl MyGenericTrait<impl MyInnerTrait>) {}
+```
+
+Type parameters for `impl Trait` types must be
+explicitly defined as named generic parameters:
+
+```
+trait MyGenericTrait<T> {}
+trait MyInnerTrait {}
+
+fn foo<T: MyInnerTrait>(bar: impl MyGenericTrait<T>) {}
+```
+"##,
+
 E0695: r##"
 A `break` statement without a label appeared inside a labeled block.
 
@@ -605,7 +629,6 @@ Switch to the Rust 2018 edition to use `async fn`.
 ;
     E0226, // only a single explicit lifetime bound is permitted
     E0472, // asm! is unsupported on this target
-    E0666, // nested `impl Trait` is illegal
     E0667, // `impl Trait` in projections
     E0696, // `continue` pointing to a labeled block
     E0706, // `async fn` in trait

src/test/ui/impl-trait/issues/issue-57979-deeply-nested-impl-trait-in-assoc-proj.stderr

@@ -27,3 +27,4 @@ LL |     pub fn demo(_: impl Quux<Assoc=super::Deeper<impl Foo<impl Bar>>>) { }
 
 error: aborting due to 3 previous errors
 
+For more information about this error, try `rustc --explain E0666`.

src/test/ui/impl-trait/where-allowed.stderr

@@ -272,5 +272,5 @@ LL |     type Out = impl Debug;
 
 error: aborting due to 43 previous errors
 
-Some errors have detailed explanations: E0282, E0562, E0658.
+Some errors have detailed explanations: E0282, E0562, E0658, E0666.
 For more information about an error, try `rustc --explain E0282`.

src/test/ui/nested_impl_trait.stderr

@@ -48,4 +48,5 @@ LL | fn allowed_in_ret_type() -> impl Fn() -> impl Into<u32> {
 
 error: aborting due to 6 previous errors
 
-For more information about this error, try `rustc --explain E0562`.
+Some errors have detailed explanations: E0562, E0666.
+For more information about an error, try `rustc --explain E0562`.