Dynamic metadata

[jeberger]

It would be nice if there was a way to specify package metadata dynamically (e.g. from a script or from build.rs).

The main reason would be to allow version to be determined automatically from the nearest VCS tag instead of needing to duplicate the version number in the tag and the Cargo.toml. I already do this when developping in Python or C (see here for a Python example).

This should probably be discussed alongside issue #4221 which addresses a similar concern about avoiding duplicate metadata.

[carols10cents]

Similarly as #4221, I don't think this is something we're going to support in cargo itself since it could be implemented completely in an external script.