1 files changed, 25 insertions, 0 deletions
diff --git a/compiler/rustc_error_codes/src/error_codes/E0666.md b/compiler/rustc_error_codes/src/error_codes/E0666.md
new file mode 100644
index 000000000..1a0dc5a52
--- /dev/null
+++ b/compiler/rustc_error_codes/src/error_codes/E0666.md
@@ -0,0 +1,25 @@
+`impl Trait` types cannot appear nested in the generic arguments of other
+`impl Trait` types.
+
+Erroneous code example:
+
+```compile_fail,E0666
+trait MyGenericTrait<T> {}
+trait MyInnerTrait {}
+
+fn foo(
+    bar: impl MyGenericTrait<impl MyInnerTrait>, // error!
+) {}
+```
+
+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>, // ok!
+) {}
+```