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Methods of const traits are const #135541

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Jan 16, 2025
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Methods of const traits are const #135541

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20 changes: 12 additions & 8 deletions compiler/rustc_const_eval/src/const_eval/fn_queries.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -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,
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This catch all is a bit sad, but since a parent could be anything that a fn is in, it could be a module, another fn, anything really.

}
}

Expand All @@ -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),
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5 changes: 1 addition & 4 deletions compiler/rustc_const_eval/src/const_eval/machine.rs
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Expand Up @@ -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)
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