Running benchmarks is easy (but requires a nightly compiler):

cargo bench

# In a few cases, nightly features may use different code paths:
cargo bench --features=nightly

A lot of code in Rand is performance sensitive, most of it is expected to be used in hot loops in some libraries/applications. If you change code in rand_core, in PRNG crates, or in the rngs or distributions modules (especially when an 'obvious cleanup'), make sure the benchmarks do not regress.

Please report before-and-after results for any affected benchmarks. If you are optimising something previously not benchmarked, please add new benchmarks first, then add your changes in a separate commit (to make before-and-after benchmarking easy).