BasicDerivation: Add applyRewrites() method

This is the first part of rewriteDerivation() factored out into its
own method. It's not used anywhere else at the moment, but it's useful
on lazy-trees for rewriting virtual paths.

src/libstore/derivations.cc

@@ -1010,36 +1010,37 @@ void writeDerivation(Sink & out, const StoreDirConfig & store, const BasicDeriva
         out << i.first << i.second;
 }
 
-
-std::string hashPlaceholder(const OutputNameView outputName)
+void BasicDerivation::applyRewrites(const StringMap & rewrites)
 {
-    // FIXME: memoize?
-    return "/" + hashString(HashAlgorithm::SHA256, concatStrings("nix-output:", outputName)).to_string(HashFormat::Nix32, false);
-}
-
+    if (rewrites.empty()) return;
 
+    debug("rewriting the derivation");
 
-
-static void rewriteDerivation(Store & store, BasicDerivation & drv, const StringMap & rewrites)
-{
-    debug("Rewriting the derivation");
-
-    for (auto & rewrite : rewrites) {
+    for (auto & rewrite : rewrites)
         debug("rewriting %s as %s", rewrite.first, rewrite.second);
-    }
 
-    drv.builder = rewriteStrings(drv.builder, rewrites);
-    for (auto & arg : drv.args) {
+    builder = rewriteStrings(builder, rewrites);
+    for (auto & arg : args)
         arg = rewriteStrings(arg, rewrites);
-    }
 
     StringPairs newEnv;
-    for (auto & envVar : drv.env) {
+    for (auto & envVar : env) {
         auto envName = rewriteStrings(envVar.first, rewrites);
         auto envValue = rewriteStrings(envVar.second, rewrites);
         newEnv.emplace(envName, envValue);
     }
-    drv.env = newEnv;
+    env = std::move(newEnv);
+}
+
+std::string hashPlaceholder(const OutputNameView outputName)
+{
+    // FIXME: memoize?
+    return "/" + hashString(HashAlgorithm::SHA256, concatStrings("nix-output:", outputName)).to_string(HashFormat::Nix32, false);
+}
+
+static void rewriteDerivation(Store & store, BasicDerivation & drv, const StringMap & rewrites)
+{
+    drv.applyRewrites(rewrites);
 
     auto hashModulo = hashDerivationModulo(store, Derivation(drv), true);
     for (auto & [outputName, output] : drv.outputs) {

src/libstore/derivations.hh

@@ -325,6 +325,12 @@ struct BasicDerivation
 
     static std::string_view nameFromPath(const StorePath & storePath);
 
+    /**
+     * Apply string rewrites to the `env`, `args` and `builder`
+     * fields.
+     */
+    void applyRewrites(const StringMap & rewrites);
+
     bool operator == (const BasicDerivation &) const = default;
     // TODO libc++ 16 (used by darwin) missing `std::map::operator <=>`, can't do yet.
     //auto operator <=> (const BasicDerivation &) const = default;