Add long error explanation for E0667

Author: GuillaumeGomez

compiler/rustc_error_codes/src/error_codes.rs

@@ -394,6 +394,7 @@ E0663: include_str!("./error_codes/E0663.md"),
 E0664: include_str!("./error_codes/E0664.md"),
 E0665: include_str!("./error_codes/E0665.md"),
 E0666: include_str!("./error_codes/E0666.md"),
+E0667: include_str!("./error_codes/E0667.md"),
 E0668: include_str!("./error_codes/E0668.md"),
 E0669: include_str!("./error_codes/E0669.md"),
 E0670: include_str!("./error_codes/E0670.md"),
@@ -633,7 +634,6 @@ E0787: include_str!("./error_codes/E0787.md"),
            // attribute
     E0640, // infer outlives requirements
 //  E0645, // trait aliases not finished
-    E0667, // `impl Trait` in projections
 //  E0694, // an unknown tool name found in scoped attributes
 //  E0702, // replaced with a generic attribute input check
 //  E0707, // multiple elided lifetimes used in arguments of `async fn`

compiler/rustc_error_codes/src/error_codes/E0667.md

@@ -0,0 +1,18 @@
+`impl Trait` is not allowed in path parameters.
+
+Erroneous code example:
+
+```compile_fail,E0667
+fn some_fn(mut x: impl Iterator) -> <impl Iterator>::Item { // error!
+    x.next().unwrap()
+}
+```
+
+You cannot use `impl Trait` in path parameters. If you want something
+equivalent, you can do this instead:
+
+```
+fn some_fn<T: Iterator>(mut x: T) -> T::Item { // ok!
+    x.next().unwrap()
+}
+```