TypeId doesn't match itself in generic function.

[rodrimati1992]

I tried this code:
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=57bb53139f14a69326e005ea9d9f7cd7

#![feature(const_type_id)]

use std::any::TypeId;

pub struct GetTypeId<T>(T);

impl<T: 'static> GetTypeId<T> {
    pub const VALUE: TypeId = TypeId::of::<T>();
}

const fn check_type_id<T: 'static>() -> bool {
    matches!(GetTypeId::<T>::VALUE, GetTypeId::<T>::VALUE)
}

fn main() {
    assert!(check_type_id::<usize>());
}

I expected the code to run without errors

Instead, the assertion failed.

Meta

Compiler version: 1.46.0-nightly (2020-07-01 f781bab)

[nbdd0121]

#![feature(const_type_id)]
#![feature(core_intrinsics)]

pub struct GetTypeId<T>(T);

impl<T: 'static> GetTypeId<T> {
    pub const VALUE: u64 = std::intrinsics::type_id::<T>();
}

const fn check_type_id<T: 'static>() -> bool {
    matches!(GetTypeId::<T>::VALUE, GetTypeId::<T>::VALUE)
}

fn main() {
    assert!(check_type_id::<usize>());
}

Seems something wrong with the intrinsics evaluation.

[nbdd0121]

I think the issue is that TyKind::Param is in place when we try to hash Ty. I am not very familiar with the code here but I think this might be evaluated before monomorphization which causes this issue.

[nbdd0121]

Another example showing this is pre-monomorphization:

#![feature(const_type_id)]
#![feature(core_intrinsics)]

struct X([u8; std::intrinsics::type_id::<Self>() as usize]);

fn main() {
    let _ = std::mem::MaybeUninit::<X>::uninit();
}

rustc happily compiles this, but I would expect something like cycle detected when const-evaluating + checking X::0::{{constant}}#0.

[sportzer]

I was trying to do something somewhat similar with const type_id when I filed #72602. There seem to be some inconsistencies in what combinations of generics and patterns are allowed, which might be contributing to the problem.

[RalfJung]

I have no idea what is going on, but it certainly looks odd... Cc @rust-lang/wg-const-eval @eddyb @nikomatsakis

[oli-obk]

rust/src/librustc_mir/interpret/intrinsics.rs

Line 71 in cd1a46d

sym::type_id => ConstValue::from_u64(tcx.type_id_hash(tp_ty)),

needs to monomorphize the type before evaluating it I'd guess (an assert checking for needs_subst would make sense there, too I'd think).

[eddyb]

@oli-obk I wonder if instead of needs_subst we should more explicitly check for "type or const params", but I think it's mostly about aesthetics thanks to miri working with lifetime-erased types.

You said "assert" but I think the type is already monomorphized and what we need to do is return TooGeneric if it's not monomorphic. Same for type_name, actually - any reflection that doesn't rely on layout_of will have this problem.

[oli-obk]

oh right, this may be an evaluation happening too early, so TooGeneric is totally reasonable to emit here. I'm always wary of emitting TooGeneric.