What it does: line and column level *declaration* position information:
$ nix repl .
nix-repl> :p nixosConfigurations.micro.options.environment.systemPackages.declarationPositions
[ { column = 7; file = "/nix/store/24aj3k7fgqv3ly7qkbf98qvphasrw9nb-source/nixos/modules/config/system-path.nix"; line = 63; } ]
Use cases:
- ctags over NixOS options, as will be presented at NixCon 2023 ;)
- improving the documentation pages to go to the exact line of the
declarations.
Related work:
- https://github.com/NixOS/nixpkgs/pull/65024
This one does it for all *definitions* rather than declarations, and
it was not followed through with due to performance worries.
- https://github.com/NixOS/nixpkgs/pull/208173
The basis for this change. This change is just a rebase of that one.
I split it out to add the capability before adding users of it, in
order to simplify review. However, the ctags script in there is a
sample user of this feature.
Benchmarks: conducted by evaluating my own reasonably complex NixOS
configuration with the command:
`hyperfine -S none -w 1 -- "nix eval .#nixosConfigurations.snowflake.config.system.build.toplevel.outPath"`
```
Benchmark 1: nix eval .#nixosConfigurations.snowflake.config.system.build.toplevel.outPath
Time (mean ± σ): 8.971 s ± 0.254 s [User: 5.872 s, System: 1.388 s]
Range (min … max): 8.574 s … 9.327 s 10 runs
Benchmark 1: nix eval .#nixosConfigurations.snowflake.config.system.build.toplevel.outPath
Time (mean ± σ): 8.766 s ± 0.160 s [User: 5.873 s, System: 1.346 s]
Range (min … max): 8.496 s … 9.033 s 10 runs
```
Summary of results: it seems to be in the noise, this does not cause any
visible regression in times.
