1df69bc9db
Remove rustc's notion of "preferred" alignment AKA `__alignof` In PR rust-lang/rust#90877 T-lang decided not to remove `intrinsics::pref_align_of`. However, the intrinsic and its supporting code 1. is a nightly feature, so can be removed at compiler/libs discretion 2. requires considerable effort in the compiler to support, as it necessarily complicates every single site reasoning about alignment 3. has been justified based on relevance to codegen, but it is only a requirement for C++ (not C, not Rust) stack frame layout for AIX[^1], in ways Rust would not consider even with increased C++ interop 4. is only used by rustc to overalign some globals, not correctness[^2] 5. can be adequately replaced by other rules for globals, as it mostly affects alignments for a few types under 16 bytes of alignment 6. has only one clear beneficiary: automating C -> Rust translation for GNU extensions like `__alignof`[^3] 7. such code was likely intended to be `alignof` or `_Alignof`, because the GNU extension is a "false friend" of the C keyword, which makes the choice to support such a mapping very questionable 8. makes it easy to do incorrect codegen in the compiler by its mere presence as usual Rust rules of alignment (e.g. `size == align * N`) do not hold with preferred alignment[^4] Despite an automated translation tool like c2rust using it, we have made multiple attempts to find a crate that actually has been committed to public repositories, like GitHub or crates.io, using such translated code. We have found none. While it is possible someone privately uses this intrinsic, it seems unlikely, and it is behind a feature gate that will warn about using the internal features of rustc. The implementation is clearly damaging the code quality of the compiler. Thus it is within the compiler team's purview to simply rip it out. If T-lang wishes to have this intrinsic restored for c2rust's benefit, it would have to use a radically different implementation that somehow does not cause internal incorrectness. Until then, remove the intrinsic and its supporting code, as one tool and an ill-considered GCC extension cannot justify risking correctness. Because we touch a fair amount of the compiler to change this at all, and unfortunately the duplication of AbiAndPrefAlign is deep-rooted, we keep an "AbiAlign" type which we can wean code off later. [^1]: https://github.com/rust-lang/rust/issues/91971#issuecomment-2451330704 [^2]: as viewable in the code altered by this PR [^3]: c2rust: https://github.com/immunant/c2rust/blame/3b1ec86b9b0cf363adfd3178cc45a891a970eef2/c2rust-transpile/src/translator/mod.rs#L3175 [^4]: https://github.com/rust-lang/rustc_codegen_cranelift/issues/1560 |
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