Skip to content

Allow trait method paths to satisfy const Fn bounds #106210

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
merged 1 commit into from
Dec 30, 2022
Merged
Changes from all commits
Commits
File filter

Filter by extension

Filter by extension

Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
26 changes: 19 additions & 7 deletions compiler/rustc_trait_selection/src/traits/select/confirmation.rs
Original file line number Diff line number Diff line change
@@ -12,8 +12,8 @@ use rustc_index::bit_set::GrowableBitSet;
use rustc_infer::infer::InferOk;
use rustc_infer::infer::LateBoundRegionConversionTime::HigherRankedType;
use rustc_middle::ty::{
self, GenericArg, GenericArgKind, GenericParamDefKind, InternalSubsts, SubstsRef,
ToPolyTraitRef, ToPredicate, Ty, TyCtxt,
self, Binder, GenericArg, GenericArgKind, GenericParamDefKind, InternalSubsts, SubstsRef,
ToPolyTraitRef, ToPredicate, TraitRef, Ty, TyCtxt,
};
use rustc_span::def_id::DefId;

@@ -98,8 +98,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
ImplSource::Future(vtable_future)
}

FnPointerCandidate { .. } => {
let data = self.confirm_fn_pointer_candidate(obligation)?;
FnPointerCandidate { is_const } => {
let data = self.confirm_fn_pointer_candidate(obligation, is_const)?;
ImplSource::FnPointer(data)
}

@@ -597,17 +597,19 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
fn confirm_fn_pointer_candidate(
&mut self,
obligation: &TraitObligation<'tcx>,
is_const: bool,
) -> Result<ImplSourceFnPointerData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
{
debug!(?obligation, "confirm_fn_pointer_candidate");

let tcx = self.tcx();
let self_ty = self
.infcx
.shallow_resolve(obligation.self_ty().no_bound_vars())
.expect("fn pointer should not capture bound vars from predicate");
let sig = self_ty.fn_sig(self.tcx());
let sig = self_ty.fn_sig(tcx);
let trait_ref = closure_trait_ref_and_return_type(
self.tcx(),
tcx,
obligation.predicate.def_id(),
self_ty,
sig,
@@ -616,9 +618,19 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
.map_bound(|(trait_ref, _)| trait_ref);

let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
let cause = obligation.derived_cause(BuiltinDerivedObligation);

if obligation.is_const() && !is_const {
// function is a trait method
if let ty::FnDef(def_id, substs) = self_ty.kind() && let Some(trait_id) = tcx.trait_of_item(*def_id) {
let trait_ref = TraitRef::from_method(tcx, trait_id, *substs);
let poly_trait_pred = Binder::dummy(trait_ref).with_constness(ty::BoundConstness::ConstIfConst);
let obligation = Obligation::new(tcx, cause.clone(), obligation.param_env, poly_trait_pred);
nested.push(obligation);
}
}

// Confirm the `type Output: Sized;` bound that is present on `FnOnce`
let cause = obligation.derived_cause(BuiltinDerivedObligation);
let output_ty = self.infcx.replace_bound_vars_with_placeholders(sig.output());
let output_ty = normalize_with_depth_to(
self,
8 changes: 8 additions & 0 deletions compiler/rustc_trait_selection/src/traits/select/mod.rs
Original file line number Diff line number Diff line change
@@ -1374,6 +1374,14 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
FutureCandidate => {}
// FnDef where the function is const
FnPointerCandidate { is_const: true } => {}
FnPointerCandidate { is_const: false } => {
if let ty::FnDef(def_id, _) = obligation.self_ty().skip_binder().kind() && tcx.trait_of_item(*def_id).is_some() {
// Trait methods are not seen as const unless the trait is implemented as const.
// We do not filter that out in here, but nested obligations will be needed to confirm this.
} else {
continue
}
}
ConstDestructCandidate(_) => {}
_ => {
// reject all other types of candidates
18 changes: 12 additions & 6 deletions library/core/src/cmp.rs
Original file line number Diff line number Diff line change
@@ -800,9 +800,12 @@ pub trait Ord: Eq + PartialOrd<Self> {
Self: Sized,
Self: ~const Destruct,
{
// HACK(fee1-dead): go back to using `self.max_by(other, Ord::cmp)`
// when trait methods are allowed to be used when a const closure is
// expected.
#[cfg(not(bootstrap))]
{
max_by(self, other, Ord::cmp)
}

#[cfg(bootstrap)]
match self.cmp(&other) {
Ordering::Less | Ordering::Equal => other,
Ordering::Greater => self,
@@ -827,9 +830,12 @@ pub trait Ord: Eq + PartialOrd<Self> {
Self: Sized,
Self: ~const Destruct,
{
// HACK(fee1-dead): go back to using `self.min_by(other, Ord::cmp)`
// when trait methods are allowed to be used when a const closure is
// expected.
#[cfg(not(bootstrap))]
{
min_by(self, other, Ord::cmp)
}

#[cfg(bootstrap)]
match self.cmp(&other) {
Ordering::Less | Ordering::Equal => self,
Ordering::Greater => other,
Original file line number Diff line number Diff line change
@@ -0,0 +1,19 @@
#![feature(const_trait_impl)]

#[const_trait]
trait Tr {
fn a(self) -> i32;
}

impl Tr for () {
fn a(self) -> i32 { 42 }
}

const fn need_const_closure<T: ~const FnOnce(()) -> i32>(x: T) -> i32 {
x(())
}

const _: () = assert!(need_const_closure(Tr::a) == 42);
//~^ ERROR: the trait bound

fn main() {}
Original file line number Diff line number Diff line change
@@ -0,0 +1,23 @@
error[E0277]: the trait bound `(): ~const Tr` is not satisfied in `fn(()) -> i32 {<() as Tr>::a}`
--> $DIR/const-closure-trait-method-fail.rs:16:42
|
LL | const _: () = assert!(need_const_closure(Tr::a) == 42);
| ------------------ ^^^^^ within `fn(()) -> i32 {<() as Tr>::a}`, the trait `~const Tr` is not implemented for `()`
| |
| required by a bound introduced by this call
|
note: the trait `Tr` is implemented for `()`, but that implementation is not `const`
--> $DIR/const-closure-trait-method-fail.rs:16:42
|
LL | const _: () = assert!(need_const_closure(Tr::a) == 42);
| ^^^^^
= note: required because it appears within the type `fn(()) -> i32 {<() as Tr>::a}`
note: required by a bound in `need_const_closure`
--> $DIR/const-closure-trait-method-fail.rs:12:32
|
LL | const fn need_const_closure<T: ~const FnOnce(()) -> i32>(x: T) -> i32 {
| ^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `need_const_closure`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.
Original file line number Diff line number Diff line change
@@ -0,0 +1,19 @@
// check-pass
#![feature(const_trait_impl)]

#[const_trait]
trait Tr {
fn a(self) -> i32;
}

impl const Tr for () {
fn a(self) -> i32 { 42 }
}

const fn need_const_closure<T: ~const FnOnce(()) -> i32>(x: T) -> i32 {
x(())
}

const _: () = assert!(need_const_closure(Tr::a) == 42);

fn main() {}