Make "${./path} ..." evaluate to a string, not a path

Wacky string coercion semantics caused expressions like

  exec = "${./my-script} params...";

to evaluate to a path (‘/path/my-script params’), because
anti-quotations are desuged to string concatenation:

  exec = ./my-script + " params...";

By constrast, adding a space at the start would yield a string as
expected:

  exec = " ${./my-script} params...";

Now the first example also evaluates to a string.

src/libexpr/eval.cc

@@ -966,7 +966,7 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
            since paths are copied when they are used in a derivation),
            and none of the strings are allowed to have contexts. */
         if (first) {
-            isPath = vStr.type == tPath;
+            isPath = !forceString && vStr.type == tPath;
             first = false;
         }
 

src/libexpr/nixexpr.hh

@@ -277,8 +277,10 @@ MakeBinOp(OpConcatLists, "++")
 
 struct ExprConcatStrings : Expr
 {
+    bool forceString;
     vector<Expr *> * es;
-    ExprConcatStrings(vector<Expr *> * es) : es(es) { };
+    ExprConcatStrings(bool forceString, vector<Expr *> * es)
+        : forceString(forceString), es(es) { };
     COMMON_METHODS
 };
 

src/libexpr/parser.y

@@ -203,7 +203,7 @@ static Expr * stripIndentation(SymbolTable & symbols, vector<Expr *> & es)
         es2->push_back(new ExprString(symbols.create(s2)));
     }
 
-    return es2->size() == 1 ? (*es2)[0] : new ExprConcatStrings(es2);
+    return es2->size() == 1 ? (*es2)[0] : new ExprConcatStrings(true, es2);
 }
 
 
@@ -318,7 +318,7 @@ expr_op
     { vector<Expr *> * l = new vector<Expr *>;
       l->push_back($1);
       l->push_back($3);
-      $$ = new ExprConcatStrings(l);
+      $$ = new ExprConcatStrings(false, l);
     }
   | expr_op CONCAT expr_op { $$ = new ExprOpConcatLists($1, $3); }
   | expr_app
@@ -349,7 +349,7 @@ expr_simple
       /* For efficiency, and to simplify parse trees a bit. */
       if ($2->empty()) $$ = new ExprString(data->symbols.create(""));
       else if ($2->size() == 1) $$ = $2->front();
-      else $$ = new ExprConcatStrings($2);
+      else $$ = new ExprConcatStrings(true, $2);
   }
   | IND_STRING_OPEN ind_string_parts IND_STRING_CLOSE {
       $$ = stripIndentation(data->symbols, *$2);