one-line config for using rust-lld

[skyzh]

Problem

I'm thinking of if it is possible to use rust-lld installed by rustup component add llvm-tools-preview using a one-line config like linker=rust-lld. Currently configuring it on macOS is relatively hard.

Basically what I'm doing now is like:

rustflags = [
    "-Clink-arg=-fuse-ld=/Users/skyzh/.rustup/toolchains/nightly-2022-10-16-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/bin/gcc-ld/ld64.lld",
]

Probably we can do something to make the configuration easier?

Proposed Solution

If linker = 'rust-lld' in config.toml, we can automatically generate the args for rustc by appending

 "-Clink-arg=-fuse-ld=${RUSTUP_TOOLCHAIN_PATH}/lib/rustlib/${ARCH}/bin/gcc-ld/ld64.lld"

to rustflags, so that we can easily use the rust-lld linker installed with rustup.

Notes

No response

[chirsz-ever]

Adding a -Clinker-flavor=rust-lld option for rustc may be a better design?

[kornelski]

Use of RUSTFLAGS for linker selection is problematic, because linker-specific RUSTFLAGS may require a matching value of the build.linker option, but these two options are independent and can get out of sync, breaking the build.

The build.linker option in Cargo can be overridden/shadowed by various levels of configs, env vars, and CLI arguments, but rustflags is additive. This means that if the linker is configured in two places, even if both are correct on their own, the combined result may end up with conflicting flags and break the build.

For example, if one layer of config selects mold by adding rustflags, and another config sets a target-specific linker for cross-compilation by setting build.linker, you may end up a GNU linker that is given clang/lld flags it doesn't understand.

So it'd be great if Cargo had first-class support for linker selection, allowing to select both the linker path and linker flavor together.

It would also be much simpler for users than having to deal with the multiple layers of indirection and C toolchain cruft involved in linker invocations. Linker errors are not as user-friendly as errors from Rust/Cargo, so it's best to prevent bad flags and wrong linkers being selected at the config level, sparing users from getting screenful of CLI flags dumped together with a cryptic error from a tool outside of Rust's control.

I'm not the only person having this issue: https://users.rust-lang.org/t/how-do-i-explicitly-use-the-default-rust-linker/133772

[skyzh]

thanks :)