Platform-specific linker option ignored when specified using cfg syntax

[tgnottingham]

Problem

When the following is placed in .cargo/config.toml, the linker option is respected (on an x86_64 Linux system):

[target.x86_64-unknown-linux-gnu]
linker = "clang"
rustflags = ["-C", "link-arg=-fuse-ld=lld"]

However, when the cfg syntax is used, as in the following, the linker option seems to be ignored (-C linker=clang isn't visible in cargo -vv output):

[target.'cfg(unix)']
linker = "clang"
rustflags = ["-C", "link-arg=-fuse-ld=lld"]

In either case, the rustflags option is respected.

Is the linker option being ignored here a bug?

Steps

No response

Possible Solution(s)

No response

Notes

No response

Version

cargo 1.71.1 (7f1d04c00 2023-07-29)
release: 1.71.1
commit-hash: 7f1d04c0053083b98fa50b69b6f56e339b0556a8
commit-date: 2023-07-29
host: x86_64-unknown-linux-gnu
libgit2: 1.6.4 (sys:0.17.1 vendored)
libcurl: 8.0.1-DEV (sys:0.4.61+curl-8.0.1 vendored ssl:OpenSSL/1.1.1t)
ssl: OpenSSL 1.1.1t  7 Feb 2023
os: Ubuntu 22.04 (jammy) [64-bit]

[weihanglo]

This section of [target] config lists all possible configurations and doesn't include target.<cfg>'.linker and target.<cfg>.<links>.

Delved into the history, it seems that just nobody requested for that.

• For rustflags — Add [target.'cfg(...)'] syntax for rustc(doc)flags in .cargo/config #3854
• For runner — add support for target.'cfg(..)'.runner #5959

However, the mechanisms of plain target-triple and cfg expression are different. May need some efforts to implement those configs. My guess is that they'll behave the same as target.<cfg>.runner: <triple> takes precedence over them, and errors out when multiple <cfg> of them found.

@ehuss the wise, was there any reason not supporting linker and link overridden with cfg expression? As I can tell there is a test verifying the "ignored" behavior:

cargo/tests/testsuite/tool_paths.rs

Lines 365 to 367 in 7e9de3f

	#[cargo_test]
	fn cfg_ignored_fields() {
	// Test for some ignored fields in [target.'cfg()'] tables.

[ehuss]

I don't think there is a specific reason other than nobody needed it. If we add it, I suppose it should follow the same logic as runner.

[weihanglo]

Thanks for quick reply!

Here are pointers for people wanting to jump in on adding target.<cfg>.linker feature:

• Basically as Eric said, it should follow the same logic as runner. Here is how runner find matched config:

  cargo/src/cargo/core/compiler/compilation.rs

  Lines 403 to 406 in 7e9de3f

  	fn target_runner(
  	bcx: &BuildContext<'_, '_>,
  	kind: CompileKind,
  	) -> CargoResult<Option<(PathBuf, Vec<String>)>> {

• A new linker field is required to be added to TargetCfgConfig:

  cargo/src/cargo/util/config/target.rs

  Line 12 in 7e9de3f

  pub struct TargetCfgConfig {

• TargetConfig.linker is accessible through the field itself, as well as via BuildContext::linker. We should consolidate them into one place. By copying runner's logic, perhaps adding a target_links field to Compilation struct?

---

@rustbot label -S-needs-info +S-accepted +E-easy

Assumed it is not too hard, though I might be wron.

[Rustin170506]

@rustbot claim