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A-const-evalArea: Constant evaluation, covers all const contexts (static, const fn, ...)Area: Constant evaluation, covers all const contexts (static, const fn, ...)C-bugCategory: This is a bug.Category: This is a bug.P-highHigh priorityHigh priorityT-compilerRelevant to the compiler team, which will review and decide on the PR/issue.Relevant to the compiler team, which will review and decide on the PR/issue.regression-from-stable-to-stablePerformance or correctness regression from one stable version to another.Performance or correctness regression from one stable version to another.
Description
It looks like this was broken in the release of 1.32.0 (and is still broken, even on nightly) (checked on https://rust.godbolt.org/). The following example program:
union Uninit {
_never_use: *const u8,
uninit: (),
}
const UNINIT: Uninit = Uninit { uninit: () };
fn main() {
let _ = UNINIT; // This line isn't actually necessary.
}
Fails to compile:
$ rustc --edition=2018 t.rs
error[E0080]: it is undefined behavior to use this value
--> t.rs:6:1
|
6 | const UNINIT: Uninit = Uninit { uninit: () };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempted to read undefined bytes
|
= note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
error: aborting due to previous error
For more information about this error, try `rustc --explain E0080`.
Interestingly, making the union
#[repr(C)]
fixes it.
My Rust version:
$ rustc --version
rustc 1.35.0-nightly (e68bf8ae1 2019-03-11)
I know union
field accesses aren't stable, but I don't think this qualifies as a field access. And yes, I'm aware of MaybeUninit
.
cc @RalfJung
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A-const-evalArea: Constant evaluation, covers all const contexts (static, const fn, ...)Area: Constant evaluation, covers all const contexts (static, const fn, ...)C-bugCategory: This is a bug.Category: This is a bug.P-highHigh priorityHigh priorityT-compilerRelevant to the compiler team, which will review and decide on the PR/issue.Relevant to the compiler team, which will review and decide on the PR/issue.regression-from-stable-to-stablePerformance or correctness regression from one stable version to another.Performance or correctness regression from one stable version to another.