The only reason to pass build inputs is to extend the unpackPhase with
custom unpack commands. Eg: add "unrar" to unpack rar sources. And those
should really be passed as native build inputs. Why? Because
nativeBuildInputs is for dependencies that are used at build time but
will not propagate as runtime dependencies. And also, cross-compilation.
Previously, callPackage would try and fill the arguments such as `name`
and `src` which would cause problems if those existed as top-level
attributes. This also makes it clearer what part is the function
signature.
Then document the derivation inline in the code to explain the ellipsis
and various use-cases.
This reverts commit b32a057425b337b37db41d3b4a67d39657477c05,
which breaks even the most straightforward uses of srcOnly:
nix-repl> srcOnly guile
error: anonymous function at /home/src/nixpkgs/pkgs/build-support/src-only/default.nix:1:1 called with unexpected argument 'drvPath', at /home/src/nixpkgs/lib/customisation.nix:69:16
nix-repl> srcOnly hello
error: anonymous function at /home/src/nixpkgs/pkgs/build-support/src-only/default.nix:1:1 called with unexpected argument 'drvPath', at /home/src/nixpkgs/lib/customisation.nix:69:16
Link: https://github.com/NixOS/nixpkgs/pull/80903#issuecomment-617172927
