From a61cd86a4e0d88abfc19a13a7959d397e4dd148b Mon Sep 17 00:00:00 2001
From: Oli Scherer <github333195615777966@oli-obk.de>
Date: Wed, 15 Jan 2025 10:50:07 +0000
Subject: [PATCH] Methods of const traits are const

---
 .../src/const_eval/fn_queries.rs              | 20 +++++++++++--------
 .../src/const_eval/machine.rs                 |  5 +----
 2 files changed, 13 insertions(+), 12 deletions(-)

diff --git a/compiler/rustc_const_eval/src/const_eval/fn_queries.rs b/compiler/rustc_const_eval/src/const_eval/fn_queries.rs
index 8af17d01b0a36..35c3e3ed3150e 100644
--- a/compiler/rustc_const_eval/src/const_eval/fn_queries.rs
+++ b/compiler/rustc_const_eval/src/const_eval/fn_queries.rs
@@ -4,14 +4,18 @@ use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_middle::query::Providers;
 use rustc_middle::ty::TyCtxt;
 
-fn parent_impl_constness(tcx: TyCtxt<'_>, def_id: LocalDefId) -> hir::Constness {
+fn parent_impl_or_trait_constness(tcx: TyCtxt<'_>, def_id: LocalDefId) -> hir::Constness {
     let parent_id = tcx.local_parent(def_id);
-    if matches!(tcx.def_kind(parent_id), DefKind::Impl { .. })
-        && let Some(header) = tcx.impl_trait_header(parent_id)
-    {
-        header.constness
-    } else {
-        hir::Constness::NotConst
+    match tcx.def_kind(parent_id) {
+        DefKind::Impl { of_trait: true } => tcx.impl_trait_header(parent_id).unwrap().constness,
+        DefKind::Trait => {
+            if tcx.is_const_trait(parent_id.into()) {
+                hir::Constness::Const
+            } else {
+                hir::Constness::NotConst
+            }
+        }
+        _ => hir::Constness::NotConst,
     }
 }
 
@@ -34,7 +38,7 @@ fn constness(tcx: TyCtxt<'_>, def_id: LocalDefId) -> hir::Constness {
 
                 // If the function itself is not annotated with `const`, it may still be a `const fn`
                 // if it resides in a const trait impl.
-                parent_impl_constness(tcx, def_id)
+                parent_impl_or_trait_constness(tcx, def_id)
             } else {
                 tcx.dcx().span_bug(
                     tcx.def_span(def_id),
diff --git a/compiler/rustc_const_eval/src/const_eval/machine.rs b/compiler/rustc_const_eval/src/const_eval/machine.rs
index ba7fbb254c66d..cfdfbdb7880b2 100644
--- a/compiler/rustc_const_eval/src/const_eval/machine.rs
+++ b/compiler/rustc_const_eval/src/const_eval/machine.rs
@@ -360,10 +360,7 @@ impl<'tcx> interpret::Machine<'tcx> for CompileTimeMachine<'tcx> {
             // sensitive check here. But we can at least rule out functions that are not const at
             // all. That said, we have to allow calling functions inside a trait marked with
             // #[const_trait]. These *are* const-checked!
-            // FIXME(const_trait_impl): why does `is_const_fn` not classify them as const?
-            if (!ecx.tcx.is_const_fn(def) && !ecx.tcx.is_const_default_method(def))
-                || ecx.tcx.has_attr(def, sym::rustc_do_not_const_check)
-            {
+            if !ecx.tcx.is_const_fn(def) || ecx.tcx.has_attr(def, sym::rustc_do_not_const_check) {
                 // We certainly do *not* want to actually call the fn
                 // though, so be sure we return here.
                 throw_unsup_format!("calling non-const function `{}`", instance)