Error on incorrect item kind in async bound

2 files changed, 35 insertions, 9 deletions

diff --git a/compiler/rustc_ast_lowering/src/errors.rs b/compiler/rustc_ast_lowering/src/errors.rs
index 51bb8a96fad..7658dfa5d5f 100644
--- a/compiler/rustc_ast_lowering/src/errors.rs
+++ b/compiler/rustc_ast_lowering/src/errors.rs
@@ -395,3 +395,18 @@ pub(crate) struct GenericParamDefaultInBinder {
     #[primary_span]
     pub span: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(ast_lowering_async_bound_not_on_trait)]
+pub(crate) struct AsyncBoundNotOnTrait {
+    #[primary_span]
+    pub span: Span,
+    pub descr: &'static str,
+}
+
+#[derive(Diagnostic)]
+#[diag(ast_lowering_async_bound_only_for_fn_traits)]
+pub(crate) struct AsyncBoundOnlyForFnTraits {
+    #[primary_span]
+    pub span: Span,
+}
diff --git a/compiler/rustc_ast_lowering/src/path.rs b/compiler/rustc_ast_lowering/src/path.rs
index b4f1d7ce8c2..b58ac5c3dae 100644
--- a/compiler/rustc_ast_lowering/src/path.rs
+++ b/compiler/rustc_ast_lowering/src/path.rs
@@ -1,6 +1,8 @@
 use crate::ImplTraitPosition;
 
-use super::errors::{GenericTypeWithParentheses, UseAngleBrackets};
+use super::errors::{
+    AsyncBoundNotOnTrait, AsyncBoundOnlyForFnTraits, GenericTypeWithParentheses, UseAngleBrackets,
+};
 use super::ResolverAstLoweringExt;
 use super::{GenericArgsCtor, LifetimeRes, ParenthesizedGenericArgs};
 use super::{ImplTraitContext, LoweringContext, ParamMode};
@@ -42,15 +44,24 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         // When we have an `async` kw on a bound, map the trait it resolves to.
         let mut bound_modifier_allowed_features = None;
         if let Some(TraitBoundModifiers { asyncness: BoundAsyncness::Async(_), .. }) = modifiers {
-            if let Res::Def(DefKind::Trait, def_id) = res {
-                if let Some((async_def_id, features)) = self.map_trait_to_async_trait(def_id) {
-                    res = Res::Def(DefKind::Trait, async_def_id);
-                    bound_modifier_allowed_features = Some(features);
-                } else {
-                    panic!();
+            match res {
+                Res::Def(DefKind::Trait, def_id) => {
+                    if let Some((async_def_id, features)) = self.map_trait_to_async_trait(def_id) {
+                        res = Res::Def(DefKind::Trait, async_def_id);
+                        bound_modifier_allowed_features = Some(features);
+                    } else {
+                        self.dcx().emit_err(AsyncBoundOnlyForFnTraits { span: p.span });
+                    }
+                }
+                Res::Err => {
+                    // No additional error.
+                }
+                _ => {
+                    // This error isn't actually emitted AFAICT, but it's best to keep
+                    // it around in case the resolver doesn't always check the defkind
+                    // of an item or something.
+                    self.dcx().emit_err(AsyncBoundNotOnTrait { span: p.span, descr: res.descr() });
                 }
-            } else {
-                panic!();
             }
         }