Tracking Issue for profile-rustflags

Summary

Original issue: #7878
Implementation: #10217
Documentation: https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#profile-rustflags-option

The profile-rustflags feature allows setting RUSTFLAGS per-profile.

Status update: https://github.com/rust-lang/cargo/issues/10271#issuecomment-3258932042

For per-target-profiles, see #4897

Unresolved Issues

• Are we ok with rustflags that can be set in Cargo.toml
• How should things interact with rustdoc? If there is a flag that is required to correctly build a project, then cargo test may fail if they aren't passed to rustdoc. However, rustdoc doesn't accept all of rustc's flags. Adding a rustdocflags option seems to be adding more unwanted complexity, though.
• This introduces potential backwards-compatibility hazards. For example, when Cargo adds new features or changes the way it invokes rustc, that can interfere with pre-existing flags. Is documenting this sufficient?
• What should the behavior be with build scripts and proc-macros? Those have historically been a pain point with rustflags. Perhaps there should be a default build-override.rustflags = [].
• Profile rustflags are not considered as part of TargetInfo, which impacts feature resolution and other things.

Future Extensions

No response

About tracking issues

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 about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

Start by reviewing implementation issue #10217 and the unstable Cargo documentation linked in the issue. Then examine the unresolved questions about rustdoc, build scripts, proc-macros, compatibility, and TargetInfo. Done means the tracking questions have clear decisions and the profile-rustflags behavior and documentation are complete.