Teach typeck/borrowck/solvers how to deal with async closures

compiler/rustc_ast_lowering/src/expr.rs

@@ -1,3 +1,5 @@
+use std::assert_matches::assert_matches;
+
 use super::errors::{
     AsyncCoroutinesNotSupported, AwaitOnlyInAsyncFnAndBlocks, BaseExpressionDoubleDot,
     ClosureCannotBeStatic, CoroutineTooManyParameters,
@@ -1027,6 +1029,12 @@ impl<'hir> LoweringContext<'_, 'hir> {
     ) -> hir::ExprKind<'hir> {
         let (binder_clause, generic_params) = self.lower_closure_binder(binder);
 
+        assert_matches!(
+            coroutine_kind,
+            CoroutineKind::Async { .. },
+            "only async closures are supported currently"
+        );
+
         let body = self.with_new_scopes(fn_decl_span, |this| {
             let inner_decl =
                 FnDecl { inputs: decl.inputs.clone(), output: FnRetTy::Default(fn_decl_span) };