Switch x64 windows msvc target to use lld by default.

[crlf0710]

cc #71520 .

[rust-highfive]

r? @estebank

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[est31]

Should this be added to 1.50? I believe due to the many stabilizations it will be a base MSRV for a long time. Maybe LLD causes regressions/bugs so maybe it will be switched back in a future release. If 1.50.0 included lld though one would have to deal with it for a long time.

[est31]

As an alternative, one could add it, but make it conditional on the nightly channel. The stable and beta releases would still use the default setting.

[crlf0710]

r? @Mark-Simulacrum

[bombless]

Is it still x86_64_pc_windows_msvc or should it be x86_64_pc_windows_none or something?

[mati865]

You still need MSVC toolchain for it to build, target vendor shouldn't be changed here.

[est31]

There is a proposal for a pure rust target in the todo list in #58713

[Mark-Simulacrum]

r? @petrochenkov

[petrochenkov]

@crlf0710
Have you tested this?
In the target spec you need to change base.linker_flavor at least + rust-lld is not found in PATH for some reason if it's taken from the target spec, so you may have to adjust some PATH setting or linker search code in link.rs (or perhaps rust-lld is just not shipped for MSVC targets).

[crlf0710]

Thanks for the review! Unfortunately this seems to need a larger change than i thought, and i'm not very confident on getting the cross-compilation stuff correct... I'll just close this PR before i sort it out. Everybody is welcome to pick this up.

Related zulip stream: https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/unblocked.3A.20using.20lld.20by.20default.20for.20x64.20msvc.20windows