lix/lix/libexpr/builtins/unsafeDiscardStringContext.md

name, args

name	args
unsafeDiscardStringContext	s

Returns a copy of the string s with all string context associated with s removed. Since string context is used for dependency tracking the returned string will also have no dependencies on store objects, even when the original string s had such dependencies.

This function is mainly useful when discarding dependencies is explicitly required, e.g. to produce a string listing all inputs of a derivation without propagating these inputs as dependencies into all users of the listing. For example, the derivation dep in the following example will pull hello into its closure despite never using it while nodep will not:

dep = runCommand "dep" {
  inherit hello;
} "echo hello is at $hello >$out";

nodep = runCommand "nodep" {
  hello = builtins.unsafeDiscardStringContext hello;
} "echo hello is at $hello >$out";

This behavior also makes this function unsafe: if s contains the path of a store object that is not present in the store then any use of s in a derivation tree will attempt to realize that path in the store, but no use of unsafeDiscardStringContext s will. This can lead to derivation outputs that refer to paths that were never created.

Lix cannot determine whether such reference are safe or not and must pass this obligation to the user.