PerfectTemplate | Empty Perfect Starter Project | HTTP library

Since your using the --pure option, nothing from your previous environment will be imported into the shell. This includes the swift binary itself. So first I would add swift to the buildInputs.

The error your getting indicates that gcc doesn't even understand the command line options being passed. I don't know anything about Swift, but after seeing that I assumed that it needs a different compiler. Sure enough, after a quick search it looks like it is llvm based. You will thus need to also add clang to your build inputs. In addition, you'll need to use mkShell instead of mkDerivation to setup the proper environment.

The former is a wrapper to the latter that is specifically designed to set up a nix-shell, and ensures the environment is handled correctly. You'll need to pass in the shellHook attribute to override the default c compiler to clang using the CC envvar. The shellHook is a way of running arbitrary commands at the start of the shell.

As a final note, the nix-shell command searches for shell.nix by default so you don't have to pass it as an argument. If shell.nix does not exist, then it will try default.nix. If neither exist, then you'll have to explicitly specify the nix expressions location.

TLDR

to put it all together: