`x test --tests library/core` does more work than it needs to

[jyn514]

Summary

Bootstrap builds rustdoc to run unit tests for core, even though it doesn't need to.

I suspect this is a regression from #153143 (or I guess not really a regression since --tests didn't exist before ...).

Command used

x test --tests library/core

Expected behaviour

Bootstrap builds stage 1 rustc/libtest, then runs unit tests for core.

Actual behaviour

Building bootstrap
Building stage1 compiler artifacts (stage0 -> stage1, aarch64-apple-darwin)
Creating a sysroot for stage1 compiler (use `rustup toolchain link 'name' build/host/stage1`)
Building stage1 library artifacts (stage1 -> stage1, aarch64-apple-darwin)
Building stage1 rustdoc_tool_binary (stage0 -> stage1, aarch64-apple-darwin)
Testing stage1 {core} (aarch64-apple-darwin)
     Running tests/lib.rs (build/aarch64-apple-darwin/stage1-std/aarch64-apple-darwin/dist/build/coretests/a88ee02e242ca77e/out/coretests-a88ee02e242ca77e)

Bootstrap configuration (bootstrap.toml)

profile = "compiler"
change-id = 154587

[rust]
deny-warnings = false

Operating system

macOS Tahoe 26.4.1 (25E253)

HEAD

Forked from 4429659

Additional context

Build Log

<log>

[Kobzol]

From bootstrap's code it looks like we always build stage 1 rustdoc when we run stage 1 tests, so I don't think that it's a recent regression (though it is something that would regress after the stage0 redesign). This happens here, which is on a different level of granularity than where we select which kinds of tests will run.