annotate rust/rhg/README.md @ 51486:0ddc34330d41

branchcache: do not accept "empty update"

This currently does not happens and it will be simpler that is remains that way.
If all update do something, we will be able to simply declare, in a later
changesets, that all update to result in a dirty branchcache.

# `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:`.