Why is -Zemit-thin-lto=no used?

[bjorn3]

I'm trying to figure out if anyone actually still needs to use this flag. If you are using this flag because you want to do fat LTO with -Clinker-plugin-lto, using -Clto=fat next to -Clinker-plugin-lto should be enough since rust-lang/rust#136840.

[priyasiddharth]

Currently verify-rust uses llvm-14 and hence rust-1.64. Is the flag combo backported to this rust version?

[bjorn3]

If you were to in the future update to a newer rustc version that includes rust-lang/rust#136840, do you think everything would keep working if you stop passing -Zemit-thin-lto=no? If so I think we can safely remove support for this flag from rustc without affecting you. The flag would remain available on rustc 1.64 and by the time you update to a newer rustc version without the flag, you could stop passing -Zemit-thin-lto=no.

Is the flag combo backported to this rust version?

No, we don't backport any changes to rustc versions older than the latest stable. For security fixes we generally offer a patch that distros can apply to their own older version of rustc, but we don't do any point releases for rustc versions older than latest stable.

[cuviper]

Note that the flag in question is unstable, so Rust reserves the right to remove that at any time. The question posed here is an attempt to make sure we don't remove something that's still useful though.

@priyasiddharth Would it be possible for you to check that the suggested flag combo gives you the output you'll need on current Rust compilers? (I'm hoping that's something you could see even if your entire stack isn't ready for newer LLVM.)

[priyasiddharth]

Let me check it. I will get back by end-of-week.