Fix TextGenerator::UniqueIdentifier() failing with empty string arg

Real problem was that we relied on SymbolTable::New() to be called with
the default arg of "tint_symbol", which isn't ergonomic when forwarding
to this function from others, like TextGenerator::UniqueIdentifier().
Instead, make New() take emptry string by default, and make it use
"tint_symbol" if input arg is empty string.

Also made it so that SymbolTable::Register() must not take an empty
string, so we now assert, rather than return an invalid symbol.

Change-Id: I386ece318c86d1d399f1dd1557a95fecac01f7ec
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/56461
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>

src/ast/identifier_expression_test.cc

@@ -54,7 +54,7 @@ TEST_F(IdentifierExpressionTest, Assert_DifferentProgramID_Symbol) {
       {
         ProgramBuilder b1;
         ProgramBuilder b2;
-        b1.Expr(b2.Sym(""));
+        b1.Expr(b2.Sym("b2"));
       },
       "internal compiler error");
 }

src/symbol_table.cc

@@ -32,8 +32,7 @@ SymbolTable& SymbolTable::operator=(const SymbolTable& other) = default;
 SymbolTable& SymbolTable::operator=(SymbolTable&&) = default;
 
 Symbol SymbolTable::Register(const std::string& name) {
-  if (name == "")
-    return Symbol();
+  TINT_ASSERT(Symbol, !name.empty());
 
   auto it = name_to_symbol_.find(name);
   if (it != name_to_symbol_.end())
@@ -63,7 +62,10 @@ std::string SymbolTable::NameFor(const Symbol symbol) const {
   return it->second;
 }
 
-Symbol SymbolTable::New(std::string prefix /* = "tint_symbol" */) {
+Symbol SymbolTable::New(std::string prefix /* = "" */) {
+  if (prefix.empty()) {
+    prefix = "tint_symbol";
+  }
   auto it = name_to_symbol_.find(prefix);
   if (it == name_to_symbol_.end()) {
     return Register(prefix);

src/symbol_table.h

@@ -66,7 +66,7 @@ class SymbolTable {
   /// @returns a new, unnamed symbol with the given name. If the name is already
   /// taken then this will be suffixed with an underscore and a unique numerical
   /// value
-  Symbol New(std::string name = "tint_symbol");
+  Symbol New(std::string name = "");
 
   /// Foreach calls the callback function `F` for each symbol in the table.
   /// @param callback must be a function or function-like object with the

src/symbol_table_test.cc

@@ -14,7 +14,7 @@
 
 #include "src/symbol_table.h"
 
-#include "gtest/gtest.h"
+#include "gtest/gtest-spi.h"
 
 namespace tint {
 namespace {
@@ -49,10 +49,14 @@ TEST_F(SymbolTableTest, ReturnsBlankForMissingSymbol) {
   EXPECT_EQ("$2", s.NameFor(Symbol(2, program_id)));
 }
 
-TEST_F(SymbolTableTest, ReturnsInvalidForBlankString) {
+TEST_F(SymbolTableTest, AssertsForBlankString) {
+  EXPECT_FATAL_FAILURE(
+      {
         auto program_id = ProgramID::New();
         SymbolTable s{program_id};
-  EXPECT_FALSE(s.Register("").IsValid());
+        s.Register("");
+      },
+      "internal compiler error");
 }
 
 }  // namespace

src/writer/text_generator.h

@@ -62,7 +62,7 @@ class TextGenerator {
 
   /// @return a new, unique identifier with the given prefix.
   /// @param prefix optional prefix to apply to the generated identifier. If
-  /// empty "tint" will be used.
+  /// empty "tint_symbol" will be used.
   std::string UniqueIdentifier(const std::string& prefix = "");
 
  protected: