`__rust_alloc` can no longer be used to provide a custom allocator

[anforowicz]

As discussed in #127173 (comment) Chromium’s update of the Rust toolchain is blocked, because of how __rust_alloc and similar symbols are expected to be mangled going forward. I have opened rust-lang/compiler-team#858 for the long-term, officially-supported solution for __rust_alloc, but in the meantime @bjorn3 suggested authoring and merging a temporary exception for mangling of the allocator symbols.

IIUC Chromium currently depends on the spelling of the following symbols:

• __rust_no_alloc_shim_is_unstable
• __rust_alloc
• __rust_dealloc
• __rust_realloc
• __rust_alloc_zeroed
• __rust_alloc_error_handler
• __rust_alloc_error_handler_should_panic

/cc @chbaker0, @alanzhao1, @zmodem from Chromium

[chbaker0]

Seems like #86844 is related

[apiraino]

Assigning priority (discussion on Zulip).

@rustbot label -I-prioritize +P-critical

[anforowicz]

IIUC Chromium has been able to successfully apply a workaround that has been discussed in https://rust-lang.zulipchat.com/#narrow/channel/233931-t-compiler.2Fmajor-changes/topic/.60-Zemit-code-for-final-artifact-to-link.60.20.E2.80.A6.20compiler-team.23858/with/510973829. So this issue can probably closed at this point.