Mercurial's minimum Rust version

[Raphaël Gomès]

Hi all,

Mercurial has had Rust code for a few years now. Parts of it used as 
native extensions to the Python code and others as standalone 
executables for speed. Some distros have packaged the Rust versions in 
the past and we expect more to do so in the future.

The current policy for the minimum Rust version in Mercurial is to 
follow that of Debian Stable. The idea behind the policy is to have a 
relatively... stable version that should enable most users and distros 
to use our Rust code without relying on an external toolchain like 
Rustup, and ease the integration with downstream crates like Ripgrep 
(only as an example, no effort has been made yet).

Some Mercurial contributors feel like this policy needs to change 
because it prevents us from upgrading some of our dependencies most of 
all, but also because we miss out on some niceties in the language (I am 
not aware of any major changes like async/await was before).

TLDR: does anyone care if we change our minimum Rust version policy to a 
faster moving target ? Likely Debian Testing, but that's just one idea.

Thanks,
Raphaël