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:

• on-unsupported governs the behavior of rhg when it encounters something
  that it does not support but “full” hg possibly does.
  This can be in configuration, on the command line, or in a repository.

  • abort, the default value, makes rhg print a message to stderr
    to explain what is not supported, then terminate with a 252 exit code.
  • abort-silent makes it terminate with the same exit code,
    but without printing anything.
  • fallback makes it silently call a (presumably Python-based) hg
    subprocess with the same command-line parameters.
    The rhg.fallback-executable configuration must be set.

• fallback-executable: path to the executable to run in a sub-process
  when falling back to a Python implementation of Mercurial.

• allowed-extensions: a list of extension names that rhg can ignore.

  Mercurial extensions can modify the behavior of existing hg sub-commands,
  including those that rhg otherwise supports.
  Because it cannot load Python extensions, finding them
  enabled in configuration is considered “unsupported” (see above).
  A few exceptions are made for extensions that rhg does know about,
  with the Rust implementation duplicating their behavior.

  This configuration makes additional exceptions: rhg will proceed even if
  those extensions are enabled.

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:.