Use RTLD_DEEPBIND for loading dylibs/proc-macros

[jonas-schievink]

This should fix #76980, by loading symbols from proc macro dylibs from the dylib itself, before falling back to symbols provided by the compiler or its dependencies.

[rust-highfive]

r? @ecstatic-morse

(rust_highfive has picked a reviewer for you, use r? to override)

[Mark-Simulacrum]

It would be good to make sure DEEPBIND is supported on other platforms as well (e.g., macOS/Windows).

Is there a chance we could engineer a test that intentionally clashes symbols with rustc? (I am fine with adding a "well known" export to rustc that we could clash with fwiw).

[cuviper]

I believe RTLD_DEEPBIND is a GNU extension -- libc only defines it under linux/gnu and uclibc/mips.

[Mark-Simulacrum]

@bors try

[bors]

⌛ Trying commit 03b326d with merge 6fa7016b655ed9c063536a8c01874ecdc82b345d...

[bors]

☀️ Try build successful - checks-actions, checks-azure
Build commit: 6fa7016b655ed9c063536a8c01874ecdc82b345d (6fa7016b655ed9c063536a8c01874ecdc82b345d)

[ecstatic-morse]

r? @Mark-Simulacrum

I don't have any domain-specific knowledge here.

[Mark-Simulacrum]

Local testing seems to indicate this doesn't fix the bug.

[jonas-schievink]

Yeah, I was using local system LLVM, which masked the bug