`nix formatter build` is sort of like `nix build`: it builds, links, and
prints a path to the formatter program:
$ nix formatter build
/nix/store/cb9w44vkhk2x4adfxwgdkkf5gjmm856j-treefmt/bin/treefmt
Note that unlike `nix build`, this prints the full path to the program,
not just the store path (in the example above that would be
`/nix/store/cb9w44vkhk2x4adfxwgdkkf5gjmm856j-treefmt`).
Motivation
----------
I maintain a vim plugin that automatically runs `nix fmt` on files on
save. Since `nix fmt` can be quite slow due to nix evaluation, I choose
to cache the `nix fmt `entrypoint. This was very awkward to do, see the
implementation for details:
7864607231/lua/null-ls/builtins/formatting/nix_flake_fmt.lua (L83-L110).
I recently discovered that my implementation was buggy (it didn't handle
flakes that expose a `formatter` package, such as nixpkgs), so I had to
rework the implementation:
https://github.com/nvimtools/none-ls.nvim/pull/272.
With the new `nix formatter build` command, I can delete all this akward
code, and it will be easier for other folks to build performant editor
integrations for `nix fmt`.
This refactor shouldn't change much except add a new `nix formatter run`
command. This creates space for the new `nix formatter build` command,
which I'll be introducing in the next commit.
