ICE: `thir_body queried for type_const` (via cfg_attr)

[Keith-Cancel]

I found this with the following code:

#![allow(incomplete_features)]
#![feature(min_generic_const_args)]

pub trait Foo {
    #[cfg_attr(feature = "some_feature", type_const)]
    const N: usize;
}

impl Foo for u32 {
    #[cfg_attr(feature = "some_feature", type_const)]
    const N: usize = 99;
}

fn main() {}

If one gets rid of the cfg_attr to just #[type_const] builds just fine. Also no issue if the feature flag is off.

Build Command:
rustc test.rs --cfg 'feature="some_feature"'

rustc-ice-2026-01-17T22_27_14-1367133.txt

@rustbot label +F-min_generic_const_args +C-bug +I-ICE +T-compiler +requires-debug-assertions

[Keith-Cancel]

Oh also ICE's on a non-debug build.

Just less helpful output:

note: no errors encountered even though delayed bugs were created

note: those delayed bugs will now be shown as internal compiler errors

error: internal compiler error: cannot call const_of_item on a non-type_const
  --> test.rs:11:5
   |
11 |     const N: usize = 99;
   |     ^^^^^^^^^^^^^^
   |
note: delayed at /rustc-dev/f6a07efc81e4b5a9166884f50c2cb1686009c3fd/compiler/rustc_hir_analysis/src/collect.rs:1547:31 - disabled backtrace
  --> test.rs:11:5
   |
11 |     const N: usize = 99;
   |     ^^^^^^^^^^^^^^

[mu001999]

I think this is because cfg_attr has not been expanded when parsing:

rust/compiler/rustc_parse/src/parser/item.rs

Lines 1512 to 1516 in cd7ad97

	if attr::contains_name(attrs, sym::type_const) {
	Some(ConstItemRhs::TypeConst(self.parse_const_arg()?))
	} else {
	Some(ConstItemRhs::Body(self.parse_expr()?))
	}

If we continue to use attr to mark type_const, I think the way to fix this is to deny such code.

And maybe we can suggest like the following, but I personally think this is less common:

#[cfg(feature = "some_feature")]
#[type_const]
const N: usize = 99;

#[cfg(not(feature = "some_feature"))]
cosnt N: usize = 99;

[mu001999]

And reduced (not need --cfg 'feature="some_feature"'):

#![allow(incomplete_features)]
#![feature(min_generic_const_args)]

pub trait Foo {
    #[cfg_attr(feature = "some_feature", type_const)]
    const N: usize;
}

impl Foo for u32 {
    #[cfg_attr(feature = "some_feature", type_const)]
    const N: usize = 99;
}

fn main() {}

[mu001999]

@rustbot claim

[mu001999]

@rustbot release-assignment