nix-repl> foo = "qwe"

nix-repl> pkgs.substituteAll { src = ''asd @foo@ asd''; }
error: The path name 'asd @foo@ asd' is invalid: the ' ' character is invalid. Path names are alphanumeric and can include the symbols +-._?= and must not begin with a period. Note: If 'asd @foo@ asd' is a source file and you cannot rename it on disk, builtins.path { name = ... } can be used to give it an alternative name.
«derivation
nix-repl>