Tracking issue for --extern nounused

[ehuss]

This is a tracking issue for the nounused CLI option for --extern.

• Docs: https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/extern-options.html

This option marks the given crate so that it does not trigger unused_crate_dependencies warnings for the crate.

This is useful if the given crate might be unused, but shouldn't cause a warning.
For example, when using --extern to link with standard library crates, and the user does not explicitly use one of those crates.

The syntax is [opts ':'] name ['=' path] where opts is a comma separated list of options:

--extern nounused:foo=/path/to/libfoo.rlib

See also #98398 and #98399 which share this syntax extension.

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.

Steps

• Implementation: Add support for nounused --extern flag #96067
• Adjust documentation (see instructions on rustc-dev-guide)
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• The syntax for adding options to --extern flags was added without much bikeshedding or discussion. It may need adjusting to be more flexible for more complex options.

Implementation history

• Add support for nounused --extern flag #96067

[petrochenkov]

Stabilization blocked on "Tracking Issue --extern crate modifiers".

[est31]

I think the name is a bit hard to read. What about no_unused or allow_unused?

[jsgf]

I don't care particularly, but it should be consistent with noprelude.

Since these aren't at all user-facing, I'm not too concerned about readability so long as they make sense to someone who does have to deal with them.

[est31]

I see, the noprelude argument convinced me.

[ehuss]

#98405 is proposing to possibly use +/- syntax like -prelude or +priv.