rhg

The rhg executable implements a subset of the functionnality of hg
using only Rust, to avoid the startup cost of a Python interpreter.
This subset is initially small but grows over time as rhg is improved.
When fallback to the Python implementation is configured (see below),
rhg aims to be a drop-in replacement for hg that should behave the same,
except that some commands run faster.

Building

To compile rhg, either run cargo build --release from this rust/rhg/
directory, or run make build-rhg from the repository root.
The executable can then be found at rust/target/release/rhg.

Mercurial configuration

rhg reads Mercurial configuration from the usual sources:
the user’s ~/.hgrc, a repository’s .hg/hgrc, command line --config, etc.
It has some specific configuration in the [rhg] section.

See hg help config.rhg for details.

Installation and configuration example

For example, to install rhg as hg for the current user with fallback to
the system-wide install of Mercurial, and allow it to run even though the
rebase and absorb extensions are enabled, on a Unix-like platform:

• Build rhg (see above)
• Make sure the ~/.local/bin exists and is in $PATH
• From the repository root, make a symbolic link with
  ln -s rust/target/release/rhg ~/.local/bin/hg
• Configure ~/.hgrc with:

[rhg]
on-unsupported = fallback
fallback-executable = /usr/bin/hg
allowed-extensions = rebase, absorb

• Check that the output of running
  hg notarealsubcommand
  starts with hg: unknown command, which indicates fallback.

• Check that the output of running
  hg notarealsubcommand --config rhg.on-unsupported=abort
  starts with unsupported feature:.