Implement -Zgcc-ld=lld stabilization MCP by lqd · Pull Request #96401 · rust-lang/rust (original) (raw)

This PR is a draft implementation of MCP #510 to stabilize -Zgcc-ld=lld as -Clink-self-contained=linker -Clinker-flavor=gcc:lld.

The changes to the -Clink-self-contained and -Clinker-flavor flags described below are tentative, currently protected by -Z unstable-options to gather feedback on nightly, and subject to FCP (either in this PR or when they are truly stabilized when removing the -Z unstable-options check).

1. -C link-self-contained

Today, -C link-self-contained is a tristate bool, whose default value is determined by the current target spec. The on and off values force the rust-provided CRT objects to be linked.

The way to choose the target-defined behavior is to omit the flag, there is no value one can use.

Proposed behavior: to model using a rust-provided linker (like rust-lld), a new value -C link-self-contained=linker is added. This flag becomes a bit more complex, controlling two facets: the CRT linking, and the use of a rustup linker.

In order to not change stable behavior:

• -C link-self-contained=y still targets only the CRT linking (although it can be argued that this could opt in to both)
• -C link-self-contained=n disables both
• -C link-self-contained=linker turns on the linker facet, but keeps the default CRT facet
• a value reifying the current default, to make it explicit for the future: -C link-self-contained=auto. If linker ever becomes a default, this would allow to turn off the linker facet while still keeping the target-defined CRT-linking behavior. Otherwise, the fact that this value is only available when the flag is absent will prevent picking it explicitly.
• depending on whether we'd want -C link-self-contained=y to mean a single or both facets, a value like -C link-self-contained=crt would be useful if linker ever became the default: this would allow to turn off the linker facet while opting into the CRT-linking.
• turning both on with -C link-self-contained=all. We had discussed e.g. -C link-self-contained=crt,linker, but without a way or need to turn off CRT-linking while enabling the linker (a use-case -C link-self-contained=linker would serve most of the time, as the CRT-linking is only enabled in a very limited number or targets) it doesn't seem that useful to specify both as crt,linker. If we need a way to do so, then -C link-self-contained=crt,linker and e.g. -C link-self-contained=no-crt,linker could be used.

2. -C linker-flavor

Today, -C linker-flavor is a list of flavor mappings (LinkerFlavors) that target specs can use.

Proposed behavior: these stable values are retained, but a new extensible one is added for the gcc flavor: gcc:*. The gcc: prefix would allow to specify a linker for cc to use, much like one would add a -fuse-ld link-arg today: -C linker-flavor=gcc:lld would be a thin wrapper over that, and pass -fuse-ld=lld to the cc process.

3. -C linker-flavor=gcc:lld -C link-self-contained=linker

This is the combination that allows using rust-lld, by providing either -fuse-ld or -B with the sysroot path containing the lld-wrappers that will launch rust-lld.

Opening as draft for discussion about:

• the naming of the options and structures, for the two flags
• the new behavior of -Clink-self-contained described above: the individual and aggregate opt-ins and opt-outs, making sure that stable behavior is preserved, and that there are adequate options to keep the stable behavior if linker becomes a default one day
• the zulip thread mentioned opportunities for cleanups but I'm not 100% sure what they are, but they can probably be done in follow-up PRs.

and because:

• I wonder what should we do on windows gnu ? Since IIUC it supports a gcc linker-flavor (and rust-lld already with -C linker=rust-lld). Maybe nothing in particular
• and how/if we need to mention the unstable values in the --help text or wait for when we'll drop the -Z unstable-options requirement ?
• I'm not sure about some of the run-make tests, maybe some need to be more constrained (e.g. linux only), and would love feedback about them

r? @ghost