Add SymbolTable::New()

Generates a new unnamed symbol.
Useful for creating temporaries in transforms.

Change-Id: I3aa037e4a6b2d400f24e01bbb3555af68da26748
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/41480
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>

src/symbol_table.cc

@@ -52,10 +52,15 @@ Symbol SymbolTable::Get(const std::string& name) const {
 
 std::string SymbolTable::NameFor(const Symbol symbol) const {
   auto it = symbol_to_name_.find(symbol);
-  if (it == symbol_to_name_.end())
-    return "";
+  if (it == symbol_to_name_.end()) {
+    return symbol.to_str();
+  }
 
   return it->second;
 }
 
+Symbol SymbolTable::New() {
+  return Symbol(next_symbol_++);
+}
+
 }  // namespace tint

src/symbol_table.h

@@ -58,6 +58,9 @@ class SymbolTable {
   /// @returns the symbol name or "" if not found
   std::string NameFor(const Symbol symbol) const;
 
+  /// @returns a new, unnamed symbol
+  Symbol New();
+
  private:
   // The value to be associated to the next registered symbol table entry.
   uint32_t next_symbol_ = 1;

src/symbol_table_test.cc

@@ -42,7 +42,7 @@ TEST_F(SymbolTableTest, ReturnsNameForSymbol) {
 
 TEST_F(SymbolTableTest, ReturnsBlankForMissingSymbol) {
   SymbolTable s;
-  EXPECT_EQ("", s.NameFor(Symbol(2)));
+  EXPECT_EQ("tint_symbol_2", s.NameFor(Symbol(2)));
 }
 
 TEST_F(SymbolTableTest, ReturnsInvalidForBlankString) {