Issue description

substituteAll is a bash function and a Nix function in pkgs.

The bash function is bad because

• it takes variables from the environment, which is too easily affected, and often the definition of those values is through mkDerivation arguments, in which case it is not clear that any relation exists between the definition and the usage.
  • example: nixos/system: Support pre-activated images #237040 (comment)
• the variables tend to be hard to grep. For one, the @ signs are omitted anywhere but in the file
  • this is a --subst-var complaint as well

The Nix function is even worse. Nix has semi-structured data, so the expectations are a bit higher. Variables passed to the pkgs.substituteAll functions all get substituted into the file, right? ... WRONG! Those variables are poured carelessly into mkDerivation, which assigns special meaning to them, and even ignores and replaces some, like system.

Steps to reproduce

• Use hostPlatform.system in nixos-generate-config #228133 (comment)

Technical details

substituteAll would be useful if it wasn't such a hack. Maybe something could be achieved with structuredAttrs, so that the variables aren't taken from the environment, but from a separate, safe, json file.

There's also this. Not sure if I want to know more about the horrors inside.

• substituteAll should fail if you pass in uppercase variables #28086