* String interpolation. Expressions like

"--with-freetype2-library=" + freetype + "/lib"

  can now be written as

    "--with-freetype2-library=${freetype}/lib"

  An arbitrary expression can be enclosed within ${...}, not just
  identifiers.

* Escaping in string literals: \n, \r, \t interpreted as in C, any
  other character following \ is interpreted as-is.
  
* Newlines are now allowed in string literals.

src/libexpr/eval.cc

@@ -231,7 +231,7 @@ static ATerm concatStrings(EvalState & state, const ATermVector & args)
 {
     ATermList context = ATempty;
     ostringstream s;
-    bool isPath;
+    bool isPath = false;
 
     for (ATermVector::const_iterator i = args.begin(); i != args.end(); ++i) {
         bool isPath2;
@@ -449,6 +449,14 @@ Expr evalExpr2(EvalState & state, Expr e)
         return makeList(ATconcat(l1, l2));
     }
 
+    /* String concatenation. */
+    ATermList es;
+    if (matchConcatStrings(e, es)) {
+        ATermVector args;
+        for (ATermIterator i(es); i; ++i) args.push_back(*i);
+        return concatStrings(state, args);
+    }
+
     /* Barf. */
     throw badTerm("invalid expression", e);
 }