fix: Add -Zjson-target-spec on Cargo calls where needed

[ChayimFriedman2]

Fixes #21821.

CC @ShoyuVanilla assuming you know - do we need to gate this only for affected Cargo versions? If so, how?

[ShoyuVanilla]

Yeah, the older versions without that flag will error with it. For the later, I guess we could just do it based on the toolchain version. Would that be hard because of some quirks in our codebase? (I can't check whether it's hard or not since I'm outside rn. Sry 😅 )

[Veykril]

We already have the sysroot here so we should have the toolchain version queried already somewhere

[ShoyuVanilla]

As Lukas said we should the toolchain version available there, I guess you could compare the toolchain version like the following lines:

rust-analyzer/crates/project-model/src/cargo_config_file.rs

Lines 171 to 173 in 803fb51

	let usage = if *toolchain_version >= MINIMUM_TOOLCHAIN_VERSION_SUPPORTING_LOCKFILE_PATH_ENV {
	LockfileUsage::WithEnvVar
	} else if *toolchain_version >= MINIMUM_TOOLCHAIN_VERSION_SUPPORTING_LOCKFILE_PATH_FLAG {

[MavenRain]

Hi @ChayimFriedman2, I've dropped #22435 in favour of this PR. Two notes that may help unstick it:

1. The cargo metadata invocation in crates/project-model/src/cargo_workspace.rs:698-702 also passes .json target specs (via --filter-platform) and needs the same fix. It's not covered in this PR. The user-reported error in Custom targets triplets loaded from json now require "-Z json-target-spec" #21821 surfaces from that path.

2. For the version gate, -Zjson-target-spec was added in Add -Z json-target-spec cargo#16557, milestoned 1.95.0 (merged 2026-01-25). The existing pattern at build_dependencies.rs:521-530 (COMP_TIME_DEPS_MIN_TOOLCHAIN_VERSION) works as precedent: toolchain.is_some_and(|v| *v >= MIN_VERSION_FOR_JSON_TARGET_SPEC).

I'd be happy to push a commit to this branch that adds the metadata site and version gating if you'd like, or leave it for you. Either is fine.

[ChayimFriedman2]

r? @ShoyuVanilla

But feel free to re-assign.

[ShoyuVanilla]

I'm a bit uncertain on whether we should make this default when the toolchain_version.is_none(), as it will fail when the toolchain is empty but actually 1.95.0 =<, like in flychecks with InvocationStrategy::Once.
But as you have written #23012 this would be okay

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