rust/rhg/README.md

rust: move `filter_map_results` to public util

This is a useful general-purpose function.

It will be used in the next changesets.

# `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:`.