libstore: remove impure-drvs feature

no documentation seems to have existed for this feature.

Change-Id: I3ec8afd9aeedbff1cc5edabc9074df33dae7d357

lix/libstore/derivations.cc

@@ -1166,8 +1166,6 @@ try {
 }
 
 
-const Hash impureOutputHash = hashString(HashType::SHA256, "impure");
-
 JSON DerivationOutput::toJSON(
     const Store & store, std::string_view drvName, OutputNameView outputName) const
 {

lix/libstore/derivations.hh

@@ -472,6 +472,4 @@ void writeDerivation(Sink & out, const Store & store, const BasicDerivation & dr
  */
 std::string hashPlaceholder(const OutputNameView outputName);
 
-extern const Hash impureOutputHash;
-
 }

lix/libutil/config.cc

@@ -363,8 +363,6 @@ template<> ExperimentalFeatures BaseSetting<ExperimentalFeatures>::parse(const s
             } else if (*thisXpFeature == Xp::DynamicDerivations) {
                 static std::once_flag warned;
                 warnDeprecated294(warned, s);
-            } else if (*thisXpFeature == Xp::ImpureDerivations) {
-                throw Error("impure derivations are no longer supported");
             }
             res = res | thisXpFeature.value();
         } else

lix/libutil/experimental-features/impure-derivations.md

@@ -1,28 +0,0 @@
----
-name: impure-derivations
-internalName: ImpureDerivations
----
-Allow derivations to produce non-fixed outputs by setting the
-`__impure` derivation attribute to `true`. An impure derivation can
-have differing outputs each time it is built.
-
-Example:
-
-```
-derivation {
-  name = "impure";
-  builder = /bin/sh;
-  __impure = true; # mark this derivation as impure
-  args = [ "-c" "read -n 10 random < /dev/random; echo $random > $out" ];
-  system = builtins.currentSystem;
-}
-```
-
-Each time this derivation is built, it can produce a different
-output (as the builder outputs random bytes to `$out`).  Impure
-derivations also have access to the network, and only fixed-output
-or other impure derivations can rely on impure derivations. Finally,
-an impure derivation cannot also be
-[content-addressed](#xp-feature-ca-derivations).
-
-This is a more explicit alternative to using [`builtins.currentTime`](@docroot@/language/builtin-constants.md#builtins-currentTime).

lix/libutil/meson.build

@@ -147,7 +147,6 @@ experimental_feature_definitions = files(
   'experimental-features/dynamic-derivations.md',
   'experimental-features/fetch-closure.md',
   'experimental-features/flakes.md',
-  'experimental-features/impure-derivations.md',
   'experimental-features/lix-custom-sub-commands.md',
   'experimental-features/nix-command.md',
   'experimental-features/no-url-literals.md',