rust/hg-core/src/re2/mod.rs

rust-re2: add wrapper for calling Re2 from Rust

This assumes that Re2 is installed following Google's guide. I am not sure
how we want to integrate it in the project, but I think a follow-up patch would
be more appropriate for such work.
As it stands, *not* having Re2 installed results in a compilation error, which
is a problem as it breaks install compatibility. Hence, this is gated behind
a non-default `with-re2` compilation feature.

Differential Revision: https://phab.mercurial-scm.org/D7910

/// re2 module
///
/// The Python implementation of Mercurial uses the Re2 regex engine when
/// possible and if the bindings are installed, falling back to Python's `re`
/// in case of unsupported syntax (Re2 is a non-backtracking engine).
///
/// Using it from Rust is not ideal. We need C++ bindings, a C++ compiler,
/// Re2 needs to be installed... why not just use the `regex` crate?
///
/// Using Re2 from the Rust implementation guarantees backwards compatibility.
/// We know it will work out of the box without needing to figure out the
/// subtle differences in syntax. For example, `regex` currently does not
/// support empty alternations (regex like `a||b`) which happens more often
/// than we might think. Old benchmarks also showed worse performance from
/// regex than with Re2, but the methodology and results were lost, so take
/// this with a grain of salt.
///
/// The idea is to use Re2 for now as a temporary phase and then investigate
/// how much work would be needed to use `regex`.
mod re2;
pub use re2::Re2;