This has several advantages:
1. It takes up less space on disk in-between builds in the nix store.
2. It uses less space in the binary cache for vendor derivation packages.
3. It uses less network traffic downloading from the binary cache.
4. It plays nicely with hashed mirrors like tarballs.nixos.org, which only
substitute --flat hashes on single files (not recursive directory hashes).
5. It's consistent with how simple `fetchurl` src derivations work.
6. It provides a stronger abstraction between input src-package and output
package, e.g., it's harder to accidentally depend on the src derivation at
runtime by referencing something like `${src}/etc/index.html`. Likewise, in
the store it's harder to get confused with something that is just there as a
build-time dependency vs. a runtime dependency, since the build-time
src dependencies are tarred up.
Disadvantages are:
1. It takes slightly longer to untar at the start of a build.
As currently implemented, this attaches the compacted vendor.tar.gz feature as a
rider on `verifyCargoDeps`, since both of them are relatively newly implemented
behavior that change the `cargoSha256`.
If this PR is accepted, I will push forward the remaining rust packages with a
series of treewide PRs to update the `cargoSha256`s.
It was previously referencing `$bin`, but this package no longer produces a
`bin` output, just `out` and `share`. Updated to make the comparison check a bit
more robust.
Also updated to avoid direct dependency on the `$src` directory out of the nix
store, instead using the processed src setup in the unpackPhase. This provides a
cleaner abstraction between the build/install phase and the input src phase, and
avoids an unnecessary dependency on whether the source disted tarball comes from
`fetchFromGitHub` (in which case it's an unpacked directory) or something like
`fetchurl`. In either case, stdenv is responsible for processing the input `src`
and setting up a clean build dir for us, so we should use that.
This produces an equivalent directory tree, except that the vim plugin is no
longer broken.
