Treat trait fns marked with the attr as const

compiler/rustc_mir/src/const_eval/fn_queries.rs

@@ -3,7 +3,7 @@
 use rustc_middle::hir::map::blocks::FnLikeNode;
 use rustc_middle::ty::query::Providers;
 use rustc_middle::ty::TyCtxt;
-use rustc_span::symbol::Symbol;
+use rustc_span::symbol::{sym, Symbol};
 use rustc_target::spec::abi::Abi;
 
 /// Whether the `def_id` counts as const fn in your current crate, considering all active
@@ -60,6 +60,9 @@ fn is_const_fn_raw(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
             return true;
         }
 
+        if tcx.has_attr(def_id, sym::default_method_body_is_const) {
+            return true;
+        }
         // If the function itself is not annotated with `const`, it may still be a `const fn`
         // if it resides in a const trait impl.
         is_parent_const_impl_raw(tcx, hir_id)

src/test/ui/rfc-2632-const-trait-impl/const-default-method-bodies.rs

@@ -0,0 +1,32 @@
+// TODO fix this test
+
+#![feature(const_trait_impl)]
+#![allow(incomplete_features)]
+
+trait ConstDefaultFn: Sized {
+    fn b(self);
+
+    #[default_method_body_is_const]
+    fn a(self) {
+        self.b();
+    }
+}
+
+struct NonConstImpl;
+struct ConstImpl;
+
+impl ConstDefaultFn for NonConstImpl {
+    fn b(self) {}
+}
+
+impl const ConstDefaultFn for ConstImpl {
+    fn b(self) {}
+}
+
+const fn test() {
+    NonConstImpl.a();
+    //~^ ERROR calls in constant functions are limited to constant functions, tuple structs and tuple variants
+    ConstImpl.a();
+}
+
+fn main() {}