3 files changed, 24 insertions, 2 deletions
diff --git a/compiler/rustc_parse/messages.ftl b/compiler/rustc_parse/messages.ftl
index aac8c0b3103..7c2ecf34c17 100644
--- a/compiler/rustc_parse/messages.ftl
+++ b/compiler/rustc_parse/messages.ftl
@@ -22,6 +22,8 @@ parse_associated_static_item_not_allowed = associated `static` items are not all
 
 parse_async_block_in_2015 = `async` blocks are only allowed in Rust 2018 or later
 
+parse_async_bound_modifier_in_2015 = `async` trait bounds are only allowed in Rust 2018 or later
+
 parse_async_fn_in_2015 = `async fn` is not permitted in Rust 2015
     .label = to use `async fn`, switch to Rust 2018 or later
 
diff --git a/compiler/rustc_parse/src/errors.rs b/compiler/rustc_parse/src/errors.rs
index 4e4bf9bdad9..86a64d90deb 100644
--- a/compiler/rustc_parse/src/errors.rs
+++ b/compiler/rustc_parse/src/errors.rs
@@ -1589,6 +1589,15 @@ pub(crate) struct AsyncMoveBlockIn2015 {
 }
 
 #[derive(Diagnostic)]
+#[diag(parse_async_bound_modifier_in_2015)]
+pub(crate) struct AsyncBoundModifierIn2015 {
+    #[primary_span]
+    pub span: Span,
+    #[subdiagnostic]
+    pub help: HelpUseLatestEdition,
+}
+
+#[derive(Diagnostic)]
 #[diag(parse_self_argument_pointer)]
 pub(crate) struct SelfArgumentPointer {
     #[primary_span]
diff --git a/compiler/rustc_parse/src/parser/ty.rs b/compiler/rustc_parse/src/parser/ty.rs
index 72089dc2a91..5fe54a536a7 100644
--- a/compiler/rustc_parse/src/parser/ty.rs
+++ b/compiler/rustc_parse/src/parser/ty.rs
@@ -3,8 +3,8 @@ use super::{Parser, PathStyle, TokenType};
 use crate::errors::{
     self, DynAfterMut, ExpectedFnPathFoundFnKeyword, ExpectedMutOrConstInRawPointerType,
     FnPointerCannotBeAsync, FnPointerCannotBeConst, FnPtrWithGenerics, FnPtrWithGenericsSugg,
-    InvalidDynKeyword, LifetimeAfterMut, NeedPlusAfterTraitObjectLifetime, NestedCVariadicType,
-    ReturnTypesUseThinArrow,
+    HelpUseLatestEdition, InvalidDynKeyword, LifetimeAfterMut, NeedPlusAfterTraitObjectLifetime,
+    NestedCVariadicType, ReturnTypesUseThinArrow,
 };
 use crate::{maybe_recover_from_interpolated_ty_qpath, maybe_whole};
 
@@ -882,6 +882,17 @@ impl<'a> Parser<'a> {
         let asyncness = if self.token.span.at_least_rust_2018() && self.eat_keyword(kw::Async) {
             self.sess.gated_spans.gate(sym::async_closure, self.prev_token.span);
             BoundAsyncness::Async(self.prev_token.span)
+        } else if self.may_recover()
+            && self.token.span.is_rust_2015()
+            && self.is_kw_followed_by_ident(kw::Async)
+        {
+            self.bump(); // eat `async`
+            self.dcx().emit_err(errors::AsyncBoundModifierIn2015 {
+                span: self.prev_token.span,
+                help: HelpUseLatestEdition::new(),
+            });
+            self.sess.gated_spans.gate(sym::async_closure, self.prev_token.span);
+            BoundAsyncness::Async(self.prev_token.span)
         } else {
             BoundAsyncness::Normal
         };