Function withhold_nix 

pub fn withhold_nix(
    target: &Utf8Path,
    nix: bool,
    recorded: Option<&Manifest>,
    entries: &mut Vec<Entry>,
) -> Result<Vec<Withheld>, RkError>

Drop the Nix destinations this target cannot take from a projection, naming each with its reason.

The one judgment every landing verb shares, so a preview, an apply, an upgrade, and an adoption all withhold identically: an unsupported crate shape withholds the whole capability, and a flake pair of the target’s own withholds the pair and the workflow while the seeded package expression still lands.

Errors

Any read failure from the pair check other than absence.