nitpick(non-blocking): Could maybe even move this inside of the class declaration.

Actually it can't be that, I'll say why below

nitpick(non-blocking): Would prefer to see this wrap the whole file.

I hadn't noticed this before, but there are a few of these nix::fs namespace declarations in src/nix now. I don't really understand what that's for? Just use std::filesystem so people don't have to wonder what nix::fs is.

That's because of

I hope that with more std::filesystem, most of file-system.hh goes away. However, there are a few things that aren't in there they we need, like this symlink_exists.

I put it in an fs namespace with the idea that if C++ files (not headers!) use fs:: as a shorthand for std::filesystem::, then it will nicely gell with the existing apis.

namespace nix::fs { using namespace std::filesystem; }

I do because if I do

using fs = std::filesystem;

then that fs will shadow, rather than expand, the namespace fs I put in file-system.hh.

I tried a few combinations, and the thing you commented on is the only one that will work (other than declaring symlink_exists in std::filesystem directly, which would be ill-advised.)

clangd reports for an fs::current_path identifier:

function current_path
provided by

→ path

// In namespace std::filesystem
path current_path()

This pattern also seems useful for polyfilling functions from new C++ versions.