From 026322c34b2ae68af15b59df69a121209c8364f1 Mon Sep 17 00:00:00 2001
From: Michael Howell <michael@notriddle.com>
Date: Mon, 30 Aug 2021 22:11:25 -0700
Subject: fix(rustc_typeck): produce better errors for dyn auto trait

Fixes #85026
---
 compiler/rustc_error_codes/src/error_codes.rs      |  1 +
 .../rustc_error_codes/src/error_codes/E0785.md     | 30 ++++++++++++++++++++++
 2 files changed, 31 insertions(+)
 create mode 100644 compiler/rustc_error_codes/src/error_codes/E0785.md

(limited to 'compiler/rustc_error_codes')

diff --git a/compiler/rustc_error_codes/src/error_codes.rs b/compiler/rustc_error_codes/src/error_codes.rs
index c2e62328cb1..45d91c2047d 100644
--- a/compiler/rustc_error_codes/src/error_codes.rs
+++ b/compiler/rustc_error_codes/src/error_codes.rs
@@ -480,6 +480,7 @@ E0781: include_str!("./error_codes/E0781.md"),
 E0782: include_str!("./error_codes/E0782.md"),
 E0783: include_str!("./error_codes/E0783.md"),
 E0784: include_str!("./error_codes/E0784.md"),
+E0785: include_str!("./error_codes/E0785.md"),
 ;
 //  E0006, // merged with E0005
 //  E0008, // cannot bind by-move into a pattern guard
diff --git a/compiler/rustc_error_codes/src/error_codes/E0785.md b/compiler/rustc_error_codes/src/error_codes/E0785.md
new file mode 100644
index 00000000000..b8a03cd109d
--- /dev/null
+++ b/compiler/rustc_error_codes/src/error_codes/E0785.md
@@ -0,0 +1,30 @@
+An inherent `impl` was written on a dyn auto trait.
+
+Erroneous code example:
+
+```compile_fail,E0785
+#![feature(auto_traits)]
+
+auto trait AutoTrait {}
+
+impl dyn AutoTrait {}
+```
+
+Dyn objects allow any number of auto traits, plus at most one non-auto trait.
+The non-auto trait becomes the "principal trait".
+
+When checking if an impl on a dyn trait is coherent, the principal trait is
+normally the only one considered. Since the erroneous code has no principal
+trait, it cannot be implemented at all.
+
+Working example:
+
+```
+#![feature(auto_traits)]
+
+trait PrincipalTrait {}
+
+auto trait AutoTrait {}
+
+impl dyn (PrincipalTrait + AutoTrait + Send) {}
+```
-- 
cgit 1.4.1-3-g733a5