From ca4ca599f5d805656029b435f219c088bd4ef77b Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Wed, 13 Sep 2023 20:09:05 +0000
Subject: [PATCH] Deprecate E0706

---
 compiler/rustc_ast_lowering/messages.ftl          |  6 ------
 compiler/rustc_ast_lowering/src/errors.rs         | 11 -----------
 compiler/rustc_ast_lowering/src/lib.rs            | 15 ++-------------
 .../rustc_error_codes/src/error_codes/E0706.md    |  6 ++++--
 4 files changed, 6 insertions(+), 32 deletions(-)

diff --git a/compiler/rustc_ast_lowering/messages.ftl b/compiler/rustc_ast_lowering/messages.ftl
index 8115c4b55b0af..e6c60cd180609 100644
--- a/compiler/rustc_ast_lowering/messages.ftl
+++ b/compiler/rustc_ast_lowering/messages.ftl
@@ -136,12 +136,6 @@ ast_lowering_template_modifier = template modifier
 
 ast_lowering_this_not_async = this is not `async`
 
-ast_lowering_trait_fn_async =
-    functions in traits cannot be declared `async`
-    .label = `async` because of this
-    .note = `async` trait functions are not currently supported
-    .note2 = consider using the `async-trait` crate: https://crates.io/crates/async-trait
-
 ast_lowering_underscore_expr_lhs_assign =
     in expressions, `_` can only be used on the left-hand side of an assignment
     .label = `_` not allowed here
diff --git a/compiler/rustc_ast_lowering/src/errors.rs b/compiler/rustc_ast_lowering/src/errors.rs
index a63bd4f8a0255..fe0c7d101c19c 100644
--- a/compiler/rustc_ast_lowering/src/errors.rs
+++ b/compiler/rustc_ast_lowering/src/errors.rs
@@ -354,17 +354,6 @@ pub struct InclusiveRangeWithNoEnd {
     pub span: Span,
 }
 
-#[derive(Diagnostic, Clone, Copy)]
-#[diag(ast_lowering_trait_fn_async, code = "E0706")]
-#[note]
-#[note(ast_lowering_note2)]
-pub struct TraitFnAsync {
-    #[primary_span]
-    pub fn_span: Span,
-    #[label]
-    pub span: Span,
-}
-
 #[derive(Diagnostic)]
 pub enum BadReturnTypeNotation {
     #[diag(ast_lowering_bad_return_type_notation_inputs)]
diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index d04f4e2e3b78e..9fa46181eb5eb 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -40,7 +40,7 @@
 #[macro_use]
 extern crate tracing;
 
-use crate::errors::{AssocTyParentheses, AssocTyParenthesesSub, MisplacedImplTrait, TraitFnAsync};
+use crate::errors::{AssocTyParentheses, AssocTyParenthesesSub, MisplacedImplTrait};
 
 use rustc_ast::ptr::P;
 use rustc_ast::visit;
@@ -336,13 +336,6 @@ impl FnDeclKind {
             _ => false,
         }
     }
-
-    fn async_fn_allowed(&self) -> bool {
-        match self {
-            FnDeclKind::Fn | FnDeclKind::Inherent | FnDeclKind::Impl | FnDeclKind::Trait => true,
-            _ => false,
-        }
-    }
 }
 
 #[derive(Copy, Clone)]
@@ -1797,11 +1790,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             self.lower_ty_direct(&param.ty, &itctx)
         }));
 
-        let output = if let Some((ret_id, span)) = make_ret_async {
-            if !kind.async_fn_allowed() {
-                self.tcx.sess.emit_err(TraitFnAsync { fn_span, span });
-            }
-
+        let output = if let Some((ret_id, _span)) = make_ret_async {
             let fn_def_id = self.local_def_id(fn_node_id);
             self.lower_async_fn_ret_ty(&decl.output, fn_def_id, ret_id, kind)
         } else {
diff --git a/compiler/rustc_error_codes/src/error_codes/E0706.md b/compiler/rustc_error_codes/src/error_codes/E0706.md
index fabd855a222f0..a09abb59ba8c9 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0706.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0706.md
@@ -1,8 +1,10 @@
+#### Note: this error code is no longer emitted by the compiler.
+
 `async fn`s are not yet supported in traits in Rust.
 
 Erroneous code example:
 
-```compile_fail,edition2018
+```ignore,edition2018
 trait T {
     // Neither case is currently supported.
     async fn foo() {}
@@ -13,7 +15,7 @@ trait T {
 `async fn`s return an `impl Future`, making the following two examples
 equivalent:
 
-```edition2018,ignore (example-of-desugaring-equivalence)
+```ignore,edition2018 (example-of-desugaring-equivalence)
 async fn foo() -> User {
     unimplemented!()
 }