Enhance GN string interpolation.

tools/gn/parser.cc

Index: tools/gn/parser.cc
diff --git a/tools/gn/parser.cc b/tools/gn/parser.cc
index c207ce7179ec72474ad20603d0e8ab929b7f0793..286409b5ed48a6d3b207a37c37ddfe306af313fb 100644
--- a/tools/gn/parser.cc
+++ b/tools/gn/parser.cc
@@ -56,10 +56,13 @@ const char kGrammar_Help[] =
     "  A string literal represents a string value consisting of the quoted\n"
     "  characters with possible escape sequences and variable expansions.\n"
     "\n"
-    "      string    = `\"` { char | escape | expansion } `\"` .\n"
-    "      escape    = `\\` ( \"$\" | `\"` | char ) .\n"
-    "      expansion = \"$\" ( identifier | \"{\" identifier \"}\" ) .\n"
-    "      char      = /* any character except \"$\", `\"`, or newline */ .\n"
+    "      string           = `\"` { char | escape | expansion } `\"` .\n"
+    "      escape           = `\\` ( \"$\" | `\"` | char ) .\n"
+    "      BracketExpansion = \"{\" ( identifier | ArrayAccess | ScopeAccess "
+                              ") \"}\" .\n"
+    "      expansion        = \"$\" ( identifier | BracketExpansion ) .\n"
+    "      char             = /* any character except \"$\", `\"`, or newline "
+                             "*/ .\n"
     "\n"
     "  After a backslash, certain sequences represent special characters:\n"
     "\n"
@@ -92,11 +95,12 @@ const char kGrammar_Help[] =
     "      Block         = \"{\" StatementList \"}\" .\n"
     "      StatementList = { Statement } .\n"
     "\n"
+    "      ArrayAccess = identifier \"[\" { identifier | integer } \"]\" .\n"
+    "      ScopeAccess = identifier \".\" identifier .\n"
     "      Expr        = UnaryExpr | Expr BinaryOp Expr .\n"
     "      UnaryExpr   = PrimaryExpr | UnaryOp UnaryExpr .\n"
     "      PrimaryExpr = identifier | integer | string | Call\n"
-    "                  | identifier \"[\" Expr \"]\"\n"
-    "                  | identifier \".\" identifier\n"
+    "                  | ArrayAccess | ScopeAccess\n"
     "                  | \"(\" Expr \")\"\n"
     "                  | \"[\" [ ExprList [ \",\" ] ] \"]\" .\n"
     "      ExprList    = Expr { \",\" Expr } .\n"