Cargo should warn when workspace "exclude" does not match anything

[matthiasbeyer]

Problem

I just found that a workspace configuration with

[workspace]
members = ["crates/*"]
exclude = ["crates/excluded"]

Does not warn when excluded does not exist.

Proposed Solution

Cargo should warn when a member of the workspace.exclude array does not match anything.

Notes

Had a look at open issues and this seems partially related to #4377 but I did not find any other open issue that explicitly notes what I wrote above. Maybe I missed it of course 😆

[epage]

Could you update this to match the “New lint" template? I'm assuming this wouldn't qualify as a hard coded diagnostic and would need to be implemented as a lint.

Also, warning is a Solution. Lack of it is not Problem. Be sure to reframe the Problem statement in terms of end user impact.

[raushan728]

Lint name: workspace_exclude_no_match

What it does

Warns when an exclude pattern does not match any package in the workspace.

Why is this bad?

The user might think a crate is excluded but it isn't, leading to accidental inclusion in workspace builds.

Example

[workspace]
members = ["crates/*"]
exclude = ["crates/excluded"]  # no crate matches this

No warning currently.

Drawbacks

Could warn if a glob is intentionally empty for future use; might be noisy in some edge cases. But it's a warning, not an error.

Alternatives

Keep silent (current behavior). Make it a hard error (too strict).

[raushan728]

@rustbot claim

[weihanglo]

I think workspace.exclude is kinda broken and annoying enough that we might want to think about #6745 more before adding a lint.
The lint itself may work but I am afraid it would push people towards #6745 and find more things not working well.

[raushan728]

Just to tie things together for a design decision:

Problem (user impact): A stale or mistyped exclude pattern silently does nothing. The user assumes a crate is excluded, but it gets built wasted debugging, wrong workspace behavior.

Re. #6745 as discussed on the PR (#17143) , this lint only fires when a pattern matched zero packages. It doesn't touch the nested-path exclusion logic. That bug exists independently; the lint won't worsen it.