D7952: rust: add a README

[gracinet (Georges Racinet)]

gracinet added a comment.


  My general feeling is that I'd like to deprecate `HGWITHRUSTEXT`.
  
  We now have `python setup.py --rust [build|install|etc]` and the coincidental `make local PURE=--rust` (works because `$PURE`, while meant to be for `--pure` is inserted blindly at the right place in the `setup.py` command.
  
  Maybe this is a good time to decide of a better make variable ? EXTENSION_TYPE ? COMPILE_OPTIONS ? BUILD_OPTIONS ?
  
  It's obviously a good thing to have official instructions, thank you Valentin for raising the subject.

INLINE COMMENTS

> pulkit wrote in README.rst:25
> IIRC, it's now possible to use `HGMODULEPOLICY=rust`. @gracinet that's possible now right?

`HGMODULEPOLICY` is for runtime, it can be `rust+c` or `rust+c-allow`. 
In the first form, not having the Rust extension is a failure, in the second, there's a fallback.

REPOSITORY
  rHG Mercurial

CHANGES SINCE LAST ACTION
  https://phab.mercurial-scm.org/D7952/new/

REVISION DETAIL
  https://phab.mercurial-scm.org/D7952

To: valentin.gatienbaron, #hg-reviewers, Alphare
Cc: pulkit, gracinet, Alphare, mjpieters, mercurial-devel