Generalize configuring LLD as the default linker in bootstrap

[Kobzol]

Reopen of #147157, because apparently bors can't deal with it for some reason.

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This PR generalizes the previous hardcoded logic for setting self-contained LLD to be used as the default Linker on Linux GNU targets. The changes made:

1. rust.lld now only controls whether LLD is built and included in the sysroot. If rust.lld is set to false, it will disable the default opt into LLD on x64 Linux.
2. The linker override for using rust-lld through cc can now be applied to all Linux targets. It is configured through target.<target>.default-linker-linux-override = "self-contained-lld-cc"/"off"

Here is how it behaves:

1. If you don't set any options, x64 Linux will default into default-linker-linux-override = "self-contained-lld-cc" and also rust.lld = true.
2. If you set rust.lld = false, you disable this override
3. If you set target.x86_64-unknown-linux-gnu.default-linux-linker-override = "off", you disable this override
4. If you set target.x86_64-unknown-linux-gnu.llvm-config = ..., you disable this override

Note that in contrast to what I described in #146640, I didn't bother moving rust.lld to llvm.lld. We can do that separately later, but I don't think it's worth the churn. I realized that rust is perhaps a better place for this, because it not only controls whether LLD is built, but also if it is included in the compiler sysroot. So to truly distinguish these two, we'd need both llvm.lld and rust.lld, which doesn't seem worth it. Note that rust.codegen-backends = ["llvm"] works in the same way, it tells bootstrap to build LLVM and include it in the sysroot. So it is consistent with rust.lld

Related to: #146640

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r? @ghost

[rustbot]

These commits modify compiler targets.
(See the Target Tier Policy.)

This PR modifies src/bootstrap/src/core/config.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

This PR modifies bootstrap.example.toml.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

[Kobzol]

@bors r=jieyouxu

[bors]

📌 Commit da86710 has been approved by jieyouxu

It is now in the queue for this repository.

[Zalathar]

I copy-pasted the description from the old PR, because that sort of thing tends to be handy when looking through history.

[Zalathar]

The old PR was rollup=never; should this one be too?

[Kobzol]

Bors still haven't updated the mergeability status even of this PR.. let's just try to roll it up and see what happens. It might be the only way to merge this, lol.

[jieyouxu]

Do we need to attempt a resync?

[Kobzol]

It caught up to this PR now, so I don't think so. #147569 is the only approved one right now that doesn't have the mergeability status.

[weihanglo]

#147569 is the only approved one right now that doesn't have the mergeability status.

What does that mean? Will bors pick up #147569 even if the mergeable status is missing?

[Kobzol]

You're asking questions no one knows the answer to 😆 If it gets stuck at the top of the queue, I'd consider closing that PR and reopening it again.

[weihanglo]

bors is working on it 😆
#147569 (comment)

[dpaoliello]

FYI, we started to see failures with our internal Arm64EC validation pipeline after this PR was merged:

• Previously we weren't building lld, but after this change we started to. We don't set rust.lld so I would have assumed that it would be disabled.
• The lld build doesn't use the same flags set for the LLVM build, so it was failing to link as Arm64EC since it was missing:
  

  rust/src/bootstrap/src/core/build_steps/llvm.rs

  Lines 412 to 416 in e100792

  	if target.starts_with("arm64ec") {
  	// MSVC linker requires the -machine:arm64ec flag to be passed to
  	// know it's linking as Arm64EC (vs Arm64X).
  	ldflags.exe.push(" -machine:arm64ec");
  	ldflags.shared.push(" -machine:arm64ec");

For context, this is how we are configuring and running the build (via an Arm64 VS PowerShell):

      &python .\src\bootstrap\configure.py `
        --set 'build.build-dir=build.stage0' `
        --set 'build.submodules=false' `
        --set 'llvm.download-ci-llvm=false' `
        --set 'llvm.targets=AArch64' `
        --set 'llvm.experimental-targets=' `
        --set 'llvm.link-jobs=8'
      if (!$?) { exit $LASTEXITCODE }

      &python .\x.py build `
        --stage 2 `
        --build aarch64-pc-windows-msvc `
        --host arm64ec-pc-windows-msvc `
        --target arm64ec-pc-windows-msvc `
        compiler cargo library/std rustfmt clippy rustdoc
      if (!$?) { exit $LASTEXITCODE }

[Kobzol]

Could you please share your whole bootstrap.toml?

[dpaoliello]

We don't have a checked-in bootstrap.toml, we just run configure and use that.

[Kobzol]

Ah, crap, I see the problem. LLD is enabled globally, but the override is configured per target. So if at least a single target opts into LLD, then rust.lld will be set to true. That's annoying, ugh. I'll try to figure something out.

[Kobzol]

Opened #147698.