Some bit twiddling to realize FOD output path calculation uses the NAR representation of the contents, at least when not interacting with directories.

fod_calculations.go

package main

import (
	"encoding/hex"
	"fmt"

	nixhash "github.com/nix-community/go-nix/pkg/hash"
	"github.com/nix-community/go-nix/pkg/nixbase32"
)

func main() {
	// $ nix-build -A third_party.nixpkgs.helix.src
	// /nix/store/ppv6f5hbjiyylzmzlhyap0np0hnvgvrm-source

	// $ curl http://cache.nixos.org/ppv6f5hbjiyylzmzlhyap0np0hnvgvrm.narinfo
	// StorePath: /nix/store/ppv6f5hbjiyylzmzlhyap0np0hnvgvrm-source
	// URL: nar/0bx02akdwj2dx6bivpaacwbyfavdqfgl7hdz1a2cck60jbqz7rbv.nar.xz
	// Compression: xz
	// FileHash: sha256:0bx02akdwj2dx6bivpaacwbyfavdqfgl7hdz1a2cck60jbqz7rbv
	// FileSize: 75367028
	// NarHash: sha256:09lg4f80s1vybsw2nl8jgl1jqzvvqvfk8pdmg7gdw54y299j3856
	// NarSize: 653811776
	// References:
	// Deriver: 9pja3bj31rwwdphxqqmvqgfxw78zzm12-source.drv
	// Sig: cache.nixos.org-1:iLR0UvlHKLRSYRQLE6YjnIKJ1Tk7TgF906FLM0JrR0LVgvyDjVhXkgkYtV7fbRU7C/6M6r7CUaiLgf8tf6aoBw==
	// CA: fixed:r:sha256:09lg4f80s1vybsw2nl8jgl1jqzvvqvfk8pdmg7gdw54y299j3856

	// $ nix-instantiate -A third_party.nixpkgs.helix.src
	// /nix/store/scyjy4l5xy52scmr59m8lnfl5k12yjhj-source.drv
	// $ nix show-derivation /nix/store/scyjy4l5xy52scmr59m8lnfl5k12yjhj-source.drv
	// says:
	//
	// "outputs": {
	//   "out": {
	//     "path": "/nix/store/ppv6f5hbjiyylzmzlhyap0np0hnvgvrm-source",
	//     "hashAlgo": "r:sha256",
	//     "hash": "a6a02153129e14dede79b55d34ddc67b7f2c037d12512bb85e7e070d90238f26"
	//   }
	// },

	// Let's figure out how this "a6a02153129e14dede79b55d34ddc67b7f2c037d12512bb85e7e070d90238f26" was produced

	s := "a6a02153129e14dede79b55d34ddc67b7f2c037d12512bb85e7e070d90238f26"

	fmt.Printf("hash as encoding/hex: %v\n", s)

	dgst, err := hex.DecodeString(s)
	if err != nil {
		panic(err)
	}
	fmt.Printf("encoded: %x\n", dgst)

	compressed := nixhash.CompressHash(dgst, 32)
	fmt.Printf("compressed: %x\n", compressed)

	fmt.Printf("compressed nixbase32: %v\n", nixbase32.EncodeToString(compressed))

	// does output 09lg4f80s1vybsw2nl8jgl1jqzvvqvfk8pdmg7gdw54y299j3856

	// … which is what NarHash in the output above says

	// So yeah, fixed output derivations hash the NAR representation of a File/
	// Directory/…, and this means we need to produce a NAR in memory to end up with
	// the same hashing.
}