Is your feature request related to a problem? Please describe.

nix-shell doesn't currently provide bash completion for new commands that provide their own completion scripts.
NixOS/nixpkgs#103501 aims to change that, but can't support completions in nix-shell -p because that command adds to the wrong attribute, buildInputs instead of nativeBuildInputs.

Describe the solution you'd like

nix-shell should set nativeBuildInputs (one way or another).

This aligns with having strictDeps by default (which is not a requirement though). StrictDeps enforces desirable properties like outputs not retaining references to build tools. It also prevents mistakes that break cross-compilation.

We can pick from these designs or think of something better.

principled scheme

-p sets both and emits a warning
--tool adds to nativeBuildInputs
--input adds to buildInputs

minimal scheme

-p sets nativeBuildInputs
--input adds to buildInputs

With the minimal scheme we don't have to update all documentation to change -ps to --tools, but it will silently break some nix-shell invocations.

compatible scheme

Like the principled scheme but without the warning. Does nothing to help towards strictDeps.

Describe alternatives you've considered

Also add buildInputs to XDG_DATA_DIRS, but this "non-strictDeps" behavior is risky, potentially affecting any build in Nixpkgs.

Additional context