Error on incorrect item kind in async bound

compiler/rustc_ast_lowering/messages.ftl

@@ -11,6 +11,12 @@ ast_lowering_argument = argument
 ast_lowering_assoc_ty_parentheses =
     parenthesized generic arguments cannot be used in associated type constraints
 
+ast_lowering_async_bound_not_on_trait =
+    `async` bound modifier only allowed on trait, not `{$descr}`
+
+ast_lowering_async_bound_only_for_fn_traits =
+    `async` bound modifier only allowed on `Fn`/`FnMut`/`FnOnce` traits
+
 ast_lowering_async_coroutines_not_supported =
     `async` coroutines are not yet supported
 

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,
+}

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!();
             }
         }
 

tests/ui/async-await/async-fn/not-a-trait.rs

@@ -0,0 +1,13 @@
+// edition:2018
+
+#![feature(async_closure)]
+
+struct S;
+
+fn test(x: impl async S) {}
+//~^ ERROR expected trait, found struct `S`
+
+fn missing(x: impl async Missing) {}
+//~^ ERROR cannot find trait `Missing` in this scope
+
+fn main() {}

tests/ui/async-await/async-fn/not-a-trait.stderr

@@ -0,0 +1,16 @@
+error[E0404]: expected trait, found struct `S`
+  --> $DIR/not-a-trait.rs:7:23
+   |
+LL | fn test(x: impl async S) {}
+   |                       ^ not a trait
+
+error[E0405]: cannot find trait `Missing` in this scope
+  --> $DIR/not-a-trait.rs:10:26
+   |
+LL | fn missing(x: impl async Missing) {}
+   |                          ^^^^^^^ not found in this scope
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0404, E0405.
+For more information about an error, try `rustc --explain E0404`.

tests/ui/async-await/async-fn/wrong-trait.rs

@@ -0,0 +1,10 @@
+// edition:2018
+
+#![feature(async_closure)]
+
+trait Foo {}
+
+fn test(x: impl async Foo) {}
+//~^ ERROR `async` bound modifier only allowed on `Fn`/`FnMut`/`FnOnce` traits
+
+fn main() {}

tests/ui/async-await/async-fn/wrong-trait.stderr

@@ -0,0 +1,8 @@
+error: `async` bound modifier only allowed on `Fn`/`FnMut`/`FnOnce` traits
+  --> $DIR/wrong-trait.rs:7:23
+   |
+LL | fn test(x: impl async Foo) {}
+   |                       ^^^
+
+error: aborting due to 1 previous error
+