Feature Requst: Add regex support to test filters

[TyOverby]

Proposal

I'd like to filter my tests based on a more complex pattern than just substring.

The current behavior of rust tests is that when a pattern is specified, then only tests that contain that pattern as a substring are run.

cargo test -- foo will run only tests that contain the substring foo.

I propose to amend this to allow for a regex syntax.

cargo test -- /regex/ which will run tests that match the regex regex.

This is "technically" a breaking change because it's possible that people are already using filters with slashes in them, but since test names can't contain slashes these invocations would always be running 0 tests.

Misc

• Let me know if this should be an RFC. The feature seems pretty small, but I'd understand if it should be more speced out.
• I'd be happy to implement this if people are interested.

[ririsoft]

Hello,

I came to this issue while trying to run only one test in a test suite with several tests sharing the same base name. Coming from Go I wanted to run a test with foo$ for instance.

However reading this comment on Cargo, I discovered the --exact option that perfectly suite my needs.

While I believe this feature request would be a plus, I recognize that the substring feature is enough for me so far.

[kennytm]

This is "technically" a breaking change because it's possible that people are already using filters with slashes in them,

We could make it truly backward-compatible by introducing a --regex flag like the existing --exact flag (#38181)

cargo test -Z unstable-options --regex 'foo$'

One potential issue is that this makes libtest depends on regex.

[Jasonoro]

This looks like useful functionality, and I would like to take a stab at trying to implement it. Using a --regex flag seems like a good idea.

@kennytm You said that making libtest depend on regex might be an issue. Do you have any idea on who to ping to find out if this is a big issue or not? The dev guide just has a TODO there, and while the regex crate is approved I'd like to get some clarification on that.

@rustbot claim

[Jasonoro]

This is currently blocked on rust-lang/regex#476, due to the fact that regex requires std which isn't available when building libtest. See also the following zulipchat conversation.

[Jasonoro]

Some more discussion has been started in this conversation. I'll update the issue when a conclusion has been reached.

[RalfJung]

/filter/ seems nice because it lets one mix regex and non-regex filters.

due to the fact that regex requires std which isn't available when building libtest

libtest itself depends on std, so it should be okay for its dependencies to depend on std.

[RalfJung]

I think all that needs to happen on the regex side is that it needs to depend on rustc-std-workspace-std; see https://github.com/rust-lang/rust/tree/master/library/rustc-std-workspace-core for how to use that dependency.

This should probably be enabled by a rustc-dep-of-std feature similar to libc.

[RalfJung]

However, adding a new dependency to the standard library is still a big deal. regex is approved for use in the compiler but that does not mean it can be used in the standard library. I am not sure what the process for that looks like.