Remove `[root]` section from `Cargo.lock` file

[matklad]

Cargo.lock includes [root] section which is no longer used for anything, because it became obsolete when Cargo workspaces were implemented.

About a year ago, support for rootless lockfiles was implemented: 5e644df.

However, by default Cargo still adds an arbitrary root to be compatible with older Cargos. Given that we have a year's worth of new Cargos, I think we can safely switch to rootless lockfiles.

This code should be removed:

cargo/src/cargo/ops/lockfile.rs

Lines 45 to 49 in d7e3b7f

	let use_root_key = if let Ok(ref orig) = orig {
	!orig.starts_with("[[package]]")
	} else {
	true
	};

However, we should make sure that Cargo is still able to read a lockfile with a root section (there's a test for this).

[matklad]

@alexcrichton do you also think that a year is a long enough grace period? To be clear, new Cargos will always read the old lockfiles, but a Cargo from a year ago will fail to read the new lockfile.

[alexcrichton]

Whoa I had no idea that we had support for this! In that case I think we're probably in a pretty good situation. Although I think we may be missing one piece to flip the switch.

Right now while an older Cargo will parse a root-missing lockfile, will it also generate one? The problem we just want to avoid is that if you build with different verisons of Cargo the lock file oscillates between two different formats. If older Cargo though generates a lock file without [root] when reading a lock file without [root], sounds great!

[matklad]

If older Cargo though generates a lock file without [root] when reading a lock file without [root], sounds great!

This is exactly what was implemented back then, yeah!

[raytung]

@matklad Mind if I give this a go? :)

[matklad]

@raytung sure, it's yours :)

[tamird]

Fixed in #4571?

[matklad]

Indeed!