env_logger performance, eliminating overhead of unused logging calls

[Boscop]

Quoted from here: google/tarpc#139

an interesting observation: we get ~10-20% higher throughput by removing all the debug logging calls in tarpc
things like trace! compiles to a memory load and a branch in the final code, which actually ends up mattering for common calls
I know slog (https://github.com/slog-rs/slog) has unused logging calls be completely compiled away
and also compose a bit more nicely

tarpc uses env_logger. Is is possible to keep the semantics of env_logger the same while optimizing the log calls so that unused logging calls are also completely optimized away?

[dpc]

AFAIK log has exactly same logging statement mechanism, it's just using different defaults (I'm the author of slog and I just mimicked the whole mechanism of log altogether). While slog defaults to info for release and debug for debug builds, log defaults to trace everywhere. You can just change it manually using crate features defined here: https://github.com/rust-lang-nursery/log/blob/master/Cargo.toml#L20

[sfackler]

Yep, this is exactly why those Cargo features exist.

[alexcrichton]

Sounds like this is all possible! In that case closing