Previously srcOnly would not work for e.g.
`srcOnly haskell.compiler.ghcjs`, as the custom `installPhase` would be
skipped if `dontInstall` is set. Consequently, we need to ensure that
the two skippable phases we need will always be executed.
Before this change `srcOnly git` gives:
duplicate derivation output 'debug', at pkgs/stdenv/generic/make-derivation.nix:270:7
This was because separateDebugInfo = true was passed on to the srcOnly
mkDerivation as well as the outputs list _including_ the debug output.
Luckily we don't need to untangle this mess since srcOnly is only
supposed to have a single output anyways.
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 b32a057425,
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
