codegen: skip stores for entirely-uninit constant aggregate fields

[glandium]

MIR GVN (since #147827) propagates MaybeUninit::uninit() as const <uninit> in aggregate constructions. Without this fix, codegen would emit a memcpy from an [N x i8] undef global for each such field, which LLVM materializes as zero-initialization.

This mirrors the existing all_bytes_uninit skip already present for Rvalue::Use (added in #147827) into the Rvalue::Aggregate field loop.

Fixes: #157743

[rustbot]

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[jackh726]

Uhh...not code I'm familiar with. Let's see if a reroll helps.

@rustbot reroll

[mu001999]

@rustbot reroll

[chenyukang]

@rustbot reroll

[oli-obk]

do you have capacity to try this out as a general fix?

View changes since this review

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[rustbot]

rustc_codegen_gcc is developed in its own repository. If possible, consider making this change to rust-lang/rustc_codegen_gcc instead.

cc @antoyo, @GuillaumeGomez

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[rust-log-analyzer]

The job pr-check-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

failures:

---- net::tcp::tests::connect_timeout_error stdout ----

thread 'net::tcp::tests::connect_timeout_error' (50824) panicked at library/std/src/net/tcp/tests.rs:66:53:
called `Result::unwrap()` on an `Err` value: Os { code: 98, kind: AddrInUse, message: "Address already in use" }
stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::result::unwrap_failed