From 4b2b23cc078c144a77a34711a20aee3faebbaf0e Mon Sep 17 00:00:00 2001
From: Sydney Acksman <obsidianminor@gmail.com>
Date: Sat, 26 Oct 2019 17:44:23 -0500
Subject: [PATCH 1/2] Add detailed explaination for E0666

---
 src/librustc_passes/error_codes.rs            | 25 ++++++++++++++++++-
 ...ply-nested-impl-trait-in-assoc-proj.stderr |  1 +
 src/test/ui/impl-trait/where-allowed.stderr   |  2 +-
 src/test/ui/nested_impl_trait.stderr          |  3 ++-
 4 files changed, 28 insertions(+), 3 deletions(-)

diff --git a/src/librustc_passes/error_codes.rs b/src/librustc_passes/error_codes.rs
index a2626617afec3..e409871c4e93c 100644
--- a/src/librustc_passes/error_codes.rs
+++ b/src/librustc_passes/error_codes.rs
@@ -552,6 +552,30 @@ trait Foo {
 ```
 "##,
 
+E0666: r##"
+`impl Trait` types cannot appear nested in the
+generic types 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
diff --git a/src/test/ui/impl-trait/issues/issue-57979-deeply-nested-impl-trait-in-assoc-proj.stderr b/src/test/ui/impl-trait/issues/issue-57979-deeply-nested-impl-trait-in-assoc-proj.stderr
index b9a1a4fa80a2b..2b6f15e6d3eb2 100644
--- a/src/test/ui/impl-trait/issues/issue-57979-deeply-nested-impl-trait-in-assoc-proj.stderr
+++ b/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`.
diff --git a/src/test/ui/impl-trait/where-allowed.stderr b/src/test/ui/impl-trait/where-allowed.stderr
index dfb6e6524d9a5..b01095f15a978 100644
--- a/src/test/ui/impl-trait/where-allowed.stderr
+++ b/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`.
diff --git a/src/test/ui/nested_impl_trait.stderr b/src/test/ui/nested_impl_trait.stderr
index bf853d30fab46..3749c268a68b3 100644
--- a/src/test/ui/nested_impl_trait.stderr
+++ b/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`.

From 16547f5025dd9420dc0a3b8bffd5a96f35e8df35 Mon Sep 17 00:00:00 2001
From: ObsidianMinor <ObsidianMinor@users.noreply.github.com>
Date: Sat, 26 Oct 2019 18:42:31 -0500
Subject: [PATCH 2/2] Update with word change suggestion

Co-Authored-By: varkor <github@varkor.com>
---
 src/librustc_passes/error_codes.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/librustc_passes/error_codes.rs b/src/librustc_passes/error_codes.rs
index e409871c4e93c..e22e69a069737 100644
--- a/src/librustc_passes/error_codes.rs
+++ b/src/librustc_passes/error_codes.rs
@@ -554,7 +554,7 @@ trait Foo {
 
 E0666: r##"
 `impl Trait` types cannot appear nested in the
-generic types of other `impl Trait` types.
+generic arguments of other `impl Trait` types.
 
 Example of erroneous code: