rustdoc: Tracking issue for command-line option `--doctest-build-arg`

A command line option to pass program arguments to the underlying compiler building doctests.

Original issue motivating this feature: https://github.com/rust-lang/rust/issues/67533.

[!NOTE]

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.

A tracking issue is however not meant for large scale discussion, questions, or bug reports.
Instead, open a dedicated issue for the specific matter or post in the #t-rustdoc Zulip channel.

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Steps

• Implementation PR
• Stabilization PR

Unresolved Questions

• Should argument parsing be closer to what already exists for RUSTFLAGS (whitespace splitting and nothing else) or should it be closer to how shell parses input (take into account escaped characters and quote characters)? Or should we offer two options, one that works like RUSTFLAGS for human users and one that works like CARGO_ENCODED_RUSTLFAGS for tool usage? Update: #137096 (comment) changed the requirements and #139863 the design!
• From #139863: Even though we don't process the argument passed to --doctest-build-arg, we end up passing it via an argument file (rustc @argfile) which delimits arguments by line break (LF or CRLF, via) meaning ultimately the arguments still get split which is unfortunate. Is there are good way to resolve this (obvious but intrusive solution: smh signal to rustc not to split inside the argsfile)?

Implementation History

• https://github.com/rust-lang/rust/pull/128780
• https://github.com/rust-lang/rust/pull/139863

Start by reviewing implementation PRs 128780 and 139863, then read the unresolved questions about argument parsing and argument-file splitting. This tracking issue is complete when the design questions are resolved and a stabilization PR is implemented; no source files or tests are named here.