For rationale behind this, see rust-lang/regex#721

Hi, and thanks for the PR! I can understand the wish to use env_logger with regex filters but without the regex perf feature, but

Normally "env_logger" is used as a dev dependency

The most common use I've seen is as a regular dependency in an application, so we definitely shouldn't just change the default feature set here.

To make this possible in a backwards-compatible manner, please adjust your PR to

• Rename the regex crate to regex1 in Cargo.toml
• Remove the perf feature from the regex dependency
• Introduce a regex Cargo feature that enables regex1/perf so all existing code that activates env_logger/regex gets the same feature set as before
• Introduce a regex_lite / regex_noperf / whatever feature that just enables the dependency, without the perf feature

We have a https://github.com/deltachat/deltachat-core-rust application, which depends on pretty_env_logger, which in turn depends on env_logger with default features. If we introduce regex_lite, then pretty_env_logger and all similar libraries will have to switch to using regex_lite, and even then with everyone using regex_lite, it is possible that a full-blown version of regex with perf features is compiled because some other dependency in the application or the application depends on it.

Is backwards compatibility worth all these complications? How likely is it that someone used env_logger and had no other dependencies that depend on regex/perf, and will notice when env_logger becomes slower and smaller?

We have a deltachat/deltachat-core-rust application, which depends on pretty_env_logger, which in turn depends on env_logger with default features.

Then make a PR to have pretty_env_logger pass through the relevant cargo features rather than always activating them. It's not a lot of work.

and even then with everyone using regex_lite, it is possible that a full-blown version of regex with perf features is compiled because some other dependency in the application or the application depends on it.

This is true even with your PR as-is.

Is backwards compatibility worth all these complications?

Yes.

• Rename the regex crate to regex1 in Cargo.toml
• Remove the perf feature from the regex dependency
• Introduce a regex Cargo feature that enables regex1/perf so all existing code that activates env_logger/regex gets the same feature set as before
• Introduce a regex_lite / regex_noperf / whatever feature that just enables the dependency, without the perf feature

By the way, I've realized that updating this PR to be backwards-compatible is probably even easier than going through that list and then replacing all uses of regex in the code by regex1 to fix the resulting compile errors. You should be able to rename it back in lib.rs with

// renamed in Cargo.toml to not conflict with the regex feature
extern crate regex1 as regex;

It appears the -Z features stabilization was merged 6 days ago 🎉

Since it doesn't look like this PR is going anywhere, I'm going to close it. Feel free to open an issue about the same thing; I'm still open to fixing this in a backwards-compatible way (though am not going to work on the fix myself).