[Feature Request] Support mercurial repositories as dependencies

[troplin]

It would be great if I could specify dependencies as mercurial repositories, similar to how git dependencies already work:

[dependencies]
mydependency = { hg = "https://bitbucket.org/troplin/mydependency" }

[alexcrichton]

I'd love to have this as well! This would basically mean copying what src/cargo/sources/git currently does into src/cargo/sources/hg. It would be stellar if there's a hg-as-a-library implementation (like libgit2) but falling back onto an hg command line seems reasonable for now.

[markand]

Mercurial is meant to be used by command line anyway. Its command line output is intended to be the same version over versions because we have a strict backward compatibility about it.

[Ppjet6]

Started looking into this as part of the impl days in Paris.

Locally I defined a new Kind::Hg(HgReference) in src/cargo/core/source/source_id.rs, and also created a (almost empty, for now) hg module in src/cargo/sources as suggested.
Before this gets merged (if ever), I'd prefer to use the same struct for the reference.

Also there seem to be quite a few mentions of git2 in src/cargo/sources/remote and I still need to figure out the relation with src/cargo/sources/git.

There are also a few things that can be abstracted away from this git module.

[matklad]

@Ppjet6: feel free to ask me in person about this: I am at the impl days as well!

[epage]

See also #12102