dupStringWithLen -> makeImmutableString

Refactor the `size == 0` logic into a new helper function that
replaces dupStringWithLen.
The name had to change, because unlike a `dup`-function, it does
not always allocate a new string.

src/libexpr/eval.cc

@@ -63,9 +63,15 @@ static char * dupString(const char * s)
 }
 
 
-static char * dupStringWithLen(const char * s, size_t size)
+// When there's no need to write to the string, we can optimize away empty
+// string allocations.
+// This function handles makeImmutableStringWithLen(null, 0) by returning the
+// empty string.
+static const char * makeImmutableStringWithLen(const char * s, size_t size)
 {
     char * t;
+    if (size == 0)
+        return "";
 #if HAVE_BOEHMGC
     t = GC_STRNDUP(s, size);
 #else
@@ -75,6 +81,10 @@ static char * dupStringWithLen(const char * s, size_t size)
     return t;
 }
 
+static inline const char * makeImmutableString(std::string_view s) {
+    return makeImmutableStringWithLen(s.data(), s.size());
+}
+
 
 RootValue allocRootValue(Value * v)
 {
@@ -805,13 +815,7 @@ LocalNoInline(void addErrorTrace(Error & e, const Pos & pos, const char * s, con
 
 void Value::mkString(std::string_view s)
 {
-    if (s.size() == 0) {
+    mkString(makeImmutableString(s));
-        // s.data() may not be valid and we don't need to allocate.
-        mkString("");
-    }
-    else {
-        mkString(dupStringWithLen(s.data(), s.size()));
-    }
 }
 
 
@@ -842,14 +846,7 @@ void Value::mkStringMove(const char * s, const PathSet & context)
 
 void Value::mkPath(std::string_view s)
 {
-    if (s.size() == 0) {
+    mkPath(makeImmutableString(s));
-        // Pathological, but better than crashing in dupStringWithLen, as
-        // s.data() may be null.
-        mkPath("");
-    }
-    else {
-        mkPath(dupStringWithLen(s.data(), s.size()));
-    }
 }