This was originally added in #4770 to support structured attrs in
`nix-shell` & `nix develop`: the issue was that it was somewhat awkward
to just write those files into a project directory, especially since
it'd break in case of multiple `nix-shell` invocations from the same
directory. Now the files are written to another, temporary
location when using `nix-shell`/`nix develop` and the correct path is
referenced by NIX_ATTRS_*_FILE.
In `nixpkgs`, it's now common to use these environment variables,
however we still fall back to checking to `.attrs.sh` & `.attrs.json`
since the minimum Nix version we support is 2.3.17[1] which doesn't have
this change.
This however makes implementing structured attrs support more
complicated than needed[2] and in fact we have a few places where the
check for `.attrs.sh`/`.attrs.json` isn't made, so these only break with
Nix 2.3[3].
The idea is now to
* get this into 2.3.18
* bump minver once again to 2.3.18 in nixpkgs
* remove all occurrences of `.attrs.sh`/`.attrs.json` from nixpkgs.
[1] f4bd97b8fa/lib/minver.nix
[2] https://github.com/NixOS/nixpkgs/pull/357053/files#diff-791a01ef89c157eb74d9c87ab8cbc3b81e2cf082cab70b8fec3472cd75ce860dR3-R5
[3] https://github.com/NixOS/nixpkgs/pull/357053#discussion_r1857362490
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