Description
Proposal
Problem statement
#231 was recently accepted as a way to check if two types are the same in const
contexts, providing a new TypeId::matches
. However, this API suffers from a few problems:
- Its goal is to be useful at compile time, but it relies on being able to construct
TypeId
s. Whether or not we can provide a safeconst TypeId::of
is not well known at this time; if it is possible, it will likely be a while off. (Tracking Issue forconst fn
type_id
rust#77125, Collisions in type_id rust#10389) - If we eventually get a const
PartialEq
,TypeId::matches
will become a duplicate of a more natural API.
These two problems seem to provide significant barriers to eventual stabiliztion of TypeId::matches
Motivating examples or use cases
Most of the use cases for const type equality checks is comparing types that are available either as a generic or as a concrete type, rather than unknown TypeId
s. This is currently true by default since const TypeId::of
is not available, but seems likely to be the case more generally.
One use case of checking types for equality is to provide downcasting behavior:
use core::ptr;
const fn const_downcast<T, Expected>(value: &T) -> Option<&Expected> {
if /* magic expression */ {
Some( unsafe { &*ptr::from_ref(value).cast::<Expected>() })
} else {
None
}
}
#231 provides a more complete example.
Solution sketch
Rather than provide an interface that relies on TypeId
s, provide a standalone function that takes the types as generic parameters:
// core::any
/// Return `true` if `T` and `U` are the same type.
const fn is_same_type<T: 'static, U: 'static>() -> bool {
// or intrinsic call
TypeId::of::<T>().t == TypeId::of::<U>().t
}
Usage:
use core::{any, ptr};
const fn const_downcast<T, Expected>(value: &T) -> Option<&Expected> {
if any::is_same_type::<T, Expected>() {
Some( unsafe { &*ptr::from_ref(value).cast::<Expected>() })
} else {
None
}
}
This can be implemented using TypeId
(as in this example), but it could also be implemented as an intrinsic. The intrinsic option allows hooking into compiler logic to provide the check, meaning const stability is not blocked on const TypeId
uncertainties.
Usage is also simpler, which means it serves a simplification purpose even after const PartialEq
.
Alternatives
- A lang feature that allows comparing types directly such as
T == i32
. - The status quo
matches
from ACP: Add const fn TypeId::matches for comparing type ids in consts #231.
Links and related work
- C++ provides
std::is_same
. - Zig has
@typeInfo
for reflection, the result of which can be compared.
What happens now?
This issue contains an API change proposal (or ACP) and is part of the libs-api team feature lifecycle. Once this issue is filed, the libs-api team will review open proposals as capability becomes available. Current response times do not have a clear estimate, but may be up to several months.
Possible responses
The libs team may respond in various different ways. First, the team will consider the problem (this doesn't require any concrete solution or alternatives to have been proposed):
- We think this problem seems worth solving, and the standard library might be the right place to solve it.
- We think that this probably doesn't belong in the standard library.
Second, if there's a concrete solution:
- We think this specific solution looks roughly right, approved, you or someone else should implement this. (Further review will still happen on the subsequent implementation PR.)
- We're not sure this is the right solution, and the alternatives or other materials don't give us enough information to be sure about that. Here are some questions we have that aren't answered, or rough ideas about alternatives we'd want to see discussed.