Allow async {} expressions in const contexts

compiler/rustc_feature/src/active.rs

@@ -650,6 +650,9 @@ declare_features! (
     /// Allows unsizing coercions in `const fn`.
     (active, const_fn_unsize, "1.53.0", Some(64992), None),
 
+    /// Allows `async {}` expressions in const contexts.
+    (active, const_async_blocks, "1.53.0", None, None),
+
     /// Allows using imported `main` function
     (active, imported_main, "1.53.0", Some(28937), None),
 

compiler/rustc_mir/src/transform/check_consts/ops.rs

@@ -141,12 +141,20 @@ impl NonConstOp for FnPtrCast {
 pub struct Generator(pub hir::GeneratorKind);
 impl NonConstOp for Generator {
     fn status_in_item(&self, _: &ConstCx<'_, '_>) -> Status {
-        Status::Forbidden
+        if let hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block) = self.0 {
+            Status::Unstable(sym::const_async_blocks)
+        } else {
+            Status::Forbidden
+        }
     }
 
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
         let msg = format!("{}s are not allowed in {}s", self.0, ccx.const_kind());
-        ccx.tcx.sess.struct_span_err(span, &msg)
+        if let hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block) = self.0 {
+            feature_err(&ccx.tcx.sess.parse_sess, sym::const_async_blocks, span, &msg)
+        } else {
+            ccx.tcx.sess.struct_span_err(span, &msg)
+        }
     }
 }
 

compiler/rustc_span/src/symbol.rs

@@ -374,6 +374,7 @@ symbols! {
         conservative_impl_trait,
         console,
         const_allocate,
+        const_async_blocks,
         const_compare_raw_pointers,
         const_constructor,
         const_eval_limit,