1.95 beta regression: trait method calls in dead code in const contexts started being rejected if the impl is not `const`

[theemathas]

Regression found in the 1.95.0 beta crater run.

[INFO] [stdout] error[E0277]: the trait bound `Wrapper<u64>: const Deref` is not satisfied
[INFO] [stdout]    --> src/lib.rs:170:29
[INFO] [stdout]     |
[INFO] [stdout] 170 |         let _ : $field_ty = $unreachable_obj.$field;
[INFO] [stdout]     |                             ^^^^^^^^^^^^^^^^^^^^^^^
[INFO] [stdout] ...
[INFO] [stdout] 226 |     assert_has_field!(Point2, x :~ &'static u64);
[INFO] [stdout]     |     -------------------------------------------- in this macro invocation
[INFO] [stdout]     |
[INFO] [stdout]     = note: this error originates in the macro `assert_has_field` (in Nightly builds, run with -Z macro-backtrace for more info)
[INFO] [stdout] help: make the `impl` of trait `Deref` `const`
[INFO] [stdout]     |
[INFO] [stdout] 212 |     impl<T> const core::ops::Deref for Wrapper<T> {
[INFO] [stdout]     |             +++++
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stdout] For more information about this error, try `rustc --explain E0277`.

https://crater-reports.s3.amazonaws.com/beta-1.95-1/beta-2026-03-05/gh/JohnScience.assert_has_field/log.txt

[theemathas]

Minimized:

struct Wrapper(i32);

impl std::ops::Deref for Wrapper {
    type Target = i32;
    fn deref(&self) -> &Self::Target {
        &self.0
    }
}

#[expect(unreachable_code)]
const _: () = if false {
    let _x: Wrapper = panic!();
    let _: &i32 = &*_x;
};

[theemathas]

Bisection results:

searched toolchains b08c11df4b8373905602949a2392123a8aff6bda through 67aec36df76a7b67b71a8ee47684467e16f1847e


********************************************************************************
Regression in 7f99507f57e6c4aa0dce3daf6a13cca8cd4dd312
********************************************************************************

Attempting to search unrolled perf builds
ERROR: couldn't find perf build comment

Regressed in #149375. cc @oli-obk, @fee1-dead

[theemathas]

Notably, this code compiles both in stable and beta:

#[expect(unreachable_code)]
const _: () = if false {
    panic!();
    not_const();
};

fn not_const() {}

[theemathas]

(Edit: updated with new info that it affects all trait methods.)

Lang-nominating for a decision on what the correct behavior is.

Normally, non-const operations cannot be done inside a const context at all. However, currently (in both stable and beta), non-const operations are allowed (with an exception for trait methods, described below) in const contexts as long as those operations only happen in dead code (based on static analysis of the control flow). Is this behavior correct?

An exception to this is calling a trait method (either directly, through an operator, or implicitly). Performing this operation in dead code in const contexts is currently allowed on stable rust (1.94.0). On beta rust (1.95.0-beta.3), it is allowed only if the relevant impl is impl const Trait (including impls in the stdlib, even if the user code hasn't opted into any unstable features). Is this an acceptable regression, or should it be fixed so that the current stable behavior is preserved?