Pre-parse token list.

This CL updates the parser and lexer to pre-parse the token list and
then the parser works off the list of tokens. This allows the parser to
work with references to the tokens and not have to worry about them
going stale.

For any splittable token a placeholder token is injected after the
token. If the token ends up getting split the placeholder is overwritten
with the new type.

Change-Id: I5a8ccca15d8c14b5027df7dd2734be6753e46fa9
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/97070
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>

src/tint/reader/wgsl/lexer.cc

@@ -37,6 +37,8 @@ static_assert(sizeof(decltype(tint::Source::FileContent::data[0])) == sizeof(uin
               "tint::reader::wgsl requires the size of a std::string element "
               "to be a single byte");
 
+static constexpr size_t kDefaultListSize = 512;
+
 bool read_blankspace(std::string_view str, size_t i, bool* is_blankspace, size_t* blankspace_size) {
     // See https://www.w3.org/TR/WGSL/#blankspace
 
@@ -88,6 +90,27 @@ Lexer::Lexer(const Source::File* file) : file_(file), location_{1, 1} {}
 
 Lexer::~Lexer() = default;
 
+std::vector<Token> Lexer::Lex() {
+    std::vector<Token> tokens;
+    tokens.reserve(kDefaultListSize);
+    while (true) {
+        tokens.emplace_back(next());
+
+        // If the token can be split, we insert a placeholder element into
+        // the stream to hold the split character.
+        if (tokens.back().IsSplittable()) {
+            auto src = tokens.back().source();
+            src.range.begin.column++;
+            tokens.emplace_back(Token(Token::Type::kPlaceholder, src));
+        }
+
+        if (tokens.back().IsEof() || tokens.back().IsError()) {
+            break;
+        }
+    }
+    return tokens;
+}
+
 const std::string_view Lexer::line() const {
     if (file_->content.lines.size() == 0) {
         static const char* empty_string = "";

src/tint/reader/wgsl/lexer.h

@@ -16,6 +16,7 @@
 #define SRC_TINT_READER_WGSL_LEXER_H_
 
 #include <string>
+#include <vector>
 
 #include "src/tint/reader/wgsl/token.h"
 
@@ -29,11 +30,14 @@ class Lexer {
     explicit Lexer(const Source::File* file);
     ~Lexer();
 
+    /// @return the token list.
+    std::vector<Token> Lex();
+
+  private:
     /// Returns the next token in the input stream.
     /// @return Token
     Token next();
 
-  private:
     /// Advances past blankspace and comments, if present at the current position.
     /// @returns error token, EOF, or uninitialized
     Token skip_blankspace_and_comments();

src/tint/reader/wgsl/lexer_test.cc

@@ -46,25 +46,34 @@ using LexerTest = testing::Test;
 TEST_F(LexerTest, Empty) {
     Source::File file("", "");
     Lexer l(&file);
-    auto t = l.next();
+
-    EXPECT_TRUE(t.IsEof());
+    auto list = l.Lex();
+    ASSERT_EQ(1u, list.size());
+    EXPECT_TRUE(list[0].IsEof());
 }
 
 TEST_F(LexerTest, Skips_Blankspace_Basic) {
     Source::File file("", "\t\r\n\t    ident\t\n\t  \r ");
     Lexer l(&file);
 
-    auto t = l.next();
+    auto list = l.Lex();
+    ASSERT_EQ(2u, list.size());
+
+    {
+        auto& t = list[0];
         EXPECT_TRUE(t.IsIdentifier());
         EXPECT_EQ(t.source().range.begin.line, 2u);
         EXPECT_EQ(t.source().range.begin.column, 6u);
         EXPECT_EQ(t.source().range.end.line, 2u);
         EXPECT_EQ(t.source().range.end.column, 11u);
         EXPECT_EQ(t.to_str(), "ident");
+    }
 
-    t = l.next();
+    {
+        auto& t = list[1];
         EXPECT_TRUE(t.IsEof());
     }
+}
 
 TEST_F(LexerTest, Skips_Blankspace_Exotic) {
     Source::File file("",                              //
@@ -73,17 +82,24 @@ TEST_F(LexerTest, Skips_Blankspace_Exotic) {
                       kVTab kFF kNL kLS kPS kL2R kR2L);
     Lexer l(&file);
 
-    auto t = l.next();
+    auto list = l.Lex();
+    ASSERT_EQ(2u, list.size());
+
+    {
+        auto& t = list[0];
         EXPECT_TRUE(t.IsIdentifier());
         EXPECT_EQ(t.source().range.begin.line, 6u);
         EXPECT_EQ(t.source().range.begin.column, 7u);
         EXPECT_EQ(t.source().range.end.line, 6u);
         EXPECT_EQ(t.source().range.end.column, 12u);
         EXPECT_EQ(t.to_str(), "ident");
+    }
 
-    t = l.next();
+    {
+        auto& t = list[1];
         EXPECT_TRUE(t.IsEof());
     }
+}
 
 TEST_F(LexerTest, Skips_Comments_Line) {
     Source::File file("", R"(//starts with comment
@@ -92,25 +108,34 @@ ident1 //ends with comment
  ident2)");
     Lexer l(&file);
 
-    auto t = l.next();
+    auto list = l.Lex();
+    ASSERT_EQ(3u, list.size());
+
+    {
+        auto& t = list[0];
         EXPECT_TRUE(t.IsIdentifier());
         EXPECT_EQ(t.source().range.begin.line, 2u);
         EXPECT_EQ(t.source().range.begin.column, 1u);
         EXPECT_EQ(t.source().range.end.line, 2u);
         EXPECT_EQ(t.source().range.end.column, 7u);
         EXPECT_EQ(t.to_str(), "ident1");
+    }
 
-    t = l.next();
+    {
+        auto& t = list[1];
         EXPECT_TRUE(t.IsIdentifier());
         EXPECT_EQ(t.source().range.begin.line, 4u);
         EXPECT_EQ(t.source().range.begin.column, 2u);
         EXPECT_EQ(t.source().range.end.line, 4u);
         EXPECT_EQ(t.source().range.end.column, 8u);
         EXPECT_EQ(t.to_str(), "ident2");
+    }
 
-    t = l.next();
+    {
+        auto& t = list[2];
         EXPECT_TRUE(t.IsEof());
     }
+}
 
 TEST_F(LexerTest, Skips_Comments_Unicode) {
     Source::File file("", R"(// starts with 🙂🙂🙂
@@ -119,25 +144,34 @@ ident1 //ends with 🙂🙂🙂
  ident2)");
     Lexer l(&file);
 
-    auto t = l.next();
+    auto list = l.Lex();
+    ASSERT_EQ(3u, list.size());
+
+    {
+        auto& t = list[0];
         EXPECT_TRUE(t.IsIdentifier());
         EXPECT_EQ(t.source().range.begin.line, 2u);
         EXPECT_EQ(t.source().range.begin.column, 1u);
         EXPECT_EQ(t.source().range.end.line, 2u);
         EXPECT_EQ(t.source().range.end.column, 7u);
         EXPECT_EQ(t.to_str(), "ident1");
+    }
 
-    t = l.next();
+    {
+        auto& t = list[1];
         EXPECT_TRUE(t.IsIdentifier());
         EXPECT_EQ(t.source().range.begin.line, 4u);
         EXPECT_EQ(t.source().range.begin.column, 2u);
         EXPECT_EQ(t.source().range.end.line, 4u);
         EXPECT_EQ(t.source().range.end.column, 8u);
         EXPECT_EQ(t.to_str(), "ident2");
+    }
 
-    t = l.next();
+    {
+        auto& t = list[2];
         EXPECT_TRUE(t.IsEof());
     }
+}
 
 using LineCommentTerminatorTest = testing::TestWithParam<const char*>;
 TEST_P(LineCommentTerminatorTest, Terminators) {
@@ -150,21 +184,29 @@ TEST_P(LineCommentTerminatorTest, Terminators) {
     Source::File file("", src);
     Lexer l(&file);
 
-    auto t = l.next();
+    auto is_same_line = [](std::string_view v) {
+        return v == kSpace || v == kHTab || v == kL2R || v == kR2L;
+    };
+
+    auto list = l.Lex();
+    ASSERT_EQ(is_same_line(c) ? 2u : 3u, list.size());
+
+    size_t idx = 0;
+
+    {
+        auto& t = list[idx++];
         EXPECT_TRUE(t.Is(Token::Type::kConst));
         EXPECT_EQ(t.source().range.begin.line, 1u);
         EXPECT_EQ(t.source().range.begin.column, 1u);
         EXPECT_EQ(t.source().range.end.line, 1u);
         EXPECT_EQ(t.source().range.end.column, 6u);
-
+    }
-    auto is_same_line = [](std::string_view v) {
-        return v == kSpace || v == kHTab || v == kL2R || v == kR2L;
-    };
 
     if (!is_same_line(c)) {
         size_t line = is_same_line(c) ? 1u : 2u;
         size_t col = is_same_line(c) ? 25u : 1u;
-        t = l.next();
+
+        auto& t = list[idx++];
         EXPECT_TRUE(t.IsIdentifier());
         EXPECT_EQ(t.source().range.begin.line, line);
         EXPECT_EQ(t.source().range.begin.column, col);
@@ -173,9 +215,11 @@ TEST_P(LineCommentTerminatorTest, Terminators) {
         EXPECT_EQ(t.to_str(), "ident");
     }
 
-    t = l.next();
+    {
+        auto& t = list[idx];
         EXPECT_TRUE(t.IsEof());
     }
+}
 INSTANTIATE_TEST_SUITE_P(LexerTest,
                          LineCommentTerminatorTest,
                          testing::Values(
@@ -198,17 +242,24 @@ TEST_F(LexerTest, Skips_Comments_Block) {
 text */ident)");
     Lexer l(&file);
 
-    auto t = l.next();
+    auto list = l.Lex();
+    ASSERT_EQ(2u, list.size());
+
+    {
+        auto& t = list[0];
         EXPECT_TRUE(t.IsIdentifier());
         EXPECT_EQ(t.source().range.begin.line, 2u);
         EXPECT_EQ(t.source().range.begin.column, 8u);
         EXPECT_EQ(t.source().range.end.line, 2u);
         EXPECT_EQ(t.source().range.end.column, 13u);
         EXPECT_EQ(t.to_str(), "ident");
+    }
 
-    t = l.next();
+    {
+        auto& t = list[1];
         EXPECT_TRUE(t.IsEof());
     }
+}
 
 TEST_F(LexerTest, Skips_Comments_Block_Nested) {
     Source::File file("", R"(/* comment
@@ -216,17 +267,24 @@ text // nested line comments are ignored /* more text
 /////**/ */*/ident)");
     Lexer l(&file);
 
-    auto t = l.next();
+    auto list = l.Lex();
+    ASSERT_EQ(2u, list.size());
+
+    {
+        auto& t = list[0];
         EXPECT_TRUE(t.IsIdentifier());
         EXPECT_EQ(t.source().range.begin.line, 3u);
         EXPECT_EQ(t.source().range.begin.column, 14u);
         EXPECT_EQ(t.source().range.end.line, 3u);
         EXPECT_EQ(t.source().range.end.column, 19u);
         EXPECT_EQ(t.to_str(), "ident");
+    }
 
-    t = l.next();
+    {
+        auto& t = list[1];
         EXPECT_TRUE(t.IsEof());
     }
+}
 
 TEST_F(LexerTest, Skips_Comments_Block_Unterminated) {
     // I had to break up the /* because otherwise the clang readability check
@@ -237,7 +295,10 @@ TEST_F(LexerTest, Skips_Comments_Block_Unterminated) {
 abcd)");
     Lexer l(&file);
 
-    auto t = l.next();
+    auto list = l.Lex();
+    ASSERT_EQ(1u, list.size());
+
+    auto& t = list[0];
     ASSERT_TRUE(t.Is(Token::Type::kError));
     EXPECT_EQ(t.to_str(), "unterminated block comment");
     EXPECT_EQ(t.source().range.begin.line, 2u);
@@ -250,7 +311,10 @@ TEST_F(LexerTest, Null_InBlankspace_IsError) {
     Source::File file("", std::string{' ', 0, ' '});
     Lexer l(&file);
 
-    auto t = l.next();
+    auto list = l.Lex();
+    ASSERT_EQ(1u, list.size());
+
+    auto& t = list[0];
     EXPECT_TRUE(t.IsError());
     EXPECT_EQ(t.source().range.begin.line, 1u);
     EXPECT_EQ(t.source().range.begin.column, 2u);
@@ -263,7 +327,10 @@ TEST_F(LexerTest, Null_InLineComment_IsError) {
     Source::File file("", std::string{'/', '/', ' ', 0, ' '});
     Lexer l(&file);
 
-    auto t = l.next();
+    auto list = l.Lex();
+    ASSERT_EQ(1u, list.size());
+
+    auto& t = list[0];
     EXPECT_TRUE(t.IsError());
     EXPECT_EQ(t.source().range.begin.line, 1u);
     EXPECT_EQ(t.source().range.begin.column, 4u);
@@ -276,7 +343,10 @@ TEST_F(LexerTest, Null_InBlockComment_IsError) {
     Source::File file("", std::string{'/', '*', ' ', 0, '*', '/'});
     Lexer l(&file);
 
-    auto t = l.next();
+    auto list = l.Lex();
+    ASSERT_EQ(1u, list.size());
+
+    auto& t = list[0];
     EXPECT_TRUE(t.IsError());
     EXPECT_EQ(t.source().range.begin.line, 1u);
     EXPECT_EQ(t.source().range.begin.column, 4u);
@@ -292,10 +362,17 @@ TEST_F(LexerTest, Null_InIdentifier_IsError) {
     Source::File file("", std::string{'a', 0, 'c'});
     Lexer l(&file);
 
-    auto t = l.next();
+    auto list = l.Lex();
+    ASSERT_EQ(2u, list.size());
+
+    {
+        auto& t = list[0];
         EXPECT_TRUE(t.IsIdentifier());
         EXPECT_EQ(t.to_str(), "a");
-    t = l.next();
+    }
+
+    {
+        auto& t = list[1];
         EXPECT_TRUE(t.IsError());
         EXPECT_EQ(t.source().range.begin.line, 1u);
         EXPECT_EQ(t.source().range.begin.column, 2u);
@@ -303,6 +380,7 @@ TEST_F(LexerTest, Null_InIdentifier_IsError) {
         EXPECT_EQ(t.source().range.end.column, 2u);
         EXPECT_EQ(t.to_str(), "null character found");
     }
+}
 
 struct FloatData {
     const char* input;
@@ -318,7 +396,11 @@ TEST_P(FloatTest, Parse) {
     Source::File file("", params.input);
     Lexer l(&file);
 
-    auto t = l.next();
+    auto list = l.Lex();
+    ASSERT_EQ(2u, list.size());
+
+    {
+        auto& t = list[0];
         if (std::string(params.input).back() == 'f') {
             EXPECT_TRUE(t.Is(Token::Type::kFloatLiteral_F));
         } else if (std::string(params.input).back() == 'h') {
@@ -331,10 +413,13 @@ TEST_P(FloatTest, Parse) {
         EXPECT_EQ(t.source().range.begin.column, 1u);
         EXPECT_EQ(t.source().range.end.line, 1u);
         EXPECT_EQ(t.source().range.end.column, 1u + strlen(params.input));
+    }
 
-    t = l.next();
+    {
+        auto& t = list[1];
         EXPECT_TRUE(t.IsEof());
     }
+}
 INSTANTIATE_TEST_SUITE_P(LexerTest,
                          FloatTest,
                          testing::Values(
@@ -437,7 +522,10 @@ TEST_P(FloatTest_Invalid, Handles) {
     Source::File file("", GetParam());
     Lexer l(&file);
 
-    auto t = l.next();
+    auto list = l.Lex();
+    ASSERT_FALSE(list.empty());
+
+    auto& t = list[0];
     EXPECT_FALSE(t.Is(Token::Type::kFloatLiteral) || t.Is(Token::Type::kFloatLiteral_F) ||
                  t.Is(Token::Type::kFloatLiteral_H));
 }
@@ -478,7 +566,11 @@ TEST_P(AsciiIdentifierTest, Parse) {
     Source::File file("", GetParam());
     Lexer l(&file);
 
-    auto t = l.next();
+    auto list = l.Lex();
+    ASSERT_EQ(2u, list.size());
+
+    {
+        auto& t = list[0];
         EXPECT_TRUE(t.IsIdentifier());
         EXPECT_EQ(t.source().range.begin.line, 1u);
         EXPECT_EQ(t.source().range.begin.column, 1u);
@@ -486,6 +578,12 @@ TEST_P(AsciiIdentifierTest, Parse) {
         EXPECT_EQ(t.source().range.end.column, 1u + strlen(GetParam()));
         EXPECT_EQ(t.to_str(), GetParam());
     }
+
+    {
+        auto& t = list[1];
+        EXPECT_TRUE(t.IsEof());
+    }
+}
 INSTANTIATE_TEST_SUITE_P(LexerTest,
                          AsciiIdentifierTest,
                          testing::Values("a",
@@ -510,7 +608,11 @@ TEST_P(ValidUnicodeIdentifierTest, Parse) {
     Source::File file("", GetParam().utf8);
     Lexer l(&file);
 
-    auto t = l.next();
+    auto list = l.Lex();
+    ASSERT_EQ(2u, list.size());
+
+    {
+        auto& t = list[0];
         EXPECT_TRUE(t.IsIdentifier());
         EXPECT_EQ(t.source().range.begin.line, 1u);
         EXPECT_EQ(t.source().range.begin.column, 1u);
@@ -518,6 +620,12 @@ TEST_P(ValidUnicodeIdentifierTest, Parse) {
         EXPECT_EQ(t.source().range.end.column, 1u + GetParam().count);
         EXPECT_EQ(t.to_str(), GetParam().utf8);
     }
+
+    {
+        auto& t = list[1];
+        EXPECT_TRUE(t.IsEof());
+    }
+}
 INSTANTIATE_TEST_SUITE_P(
     LexerTest,
     ValidUnicodeIdentifierTest,
@@ -554,7 +662,10 @@ TEST_P(InvalidUnicodeIdentifierTest, Parse) {
     Source::File file("", GetParam());
     Lexer l(&file);
 
-    auto t = l.next();
+    auto list = l.Lex();
+    ASSERT_FALSE(list.empty());
+
+    auto& t = list[0];
     EXPECT_TRUE(t.IsError());
     EXPECT_EQ(t.source().range.begin.line, 1u);
     EXPECT_EQ(t.source().range.begin.column, 1u);
@@ -602,7 +713,10 @@ TEST_F(LexerTest, IdentifierTest_SingleUnderscoreDoesNotMatch) {
     Source::File file("", "_");
     Lexer l(&file);
 
-    auto t = l.next();
+    auto list = l.Lex();
+    ASSERT_FALSE(list.empty());
+
+    auto& t = list[0];
     EXPECT_FALSE(t.IsIdentifier());
 }
 
@@ -610,7 +724,10 @@ TEST_F(LexerTest, IdentifierTest_DoesNotStartWithDoubleUnderscore) {
     Source::File file("", "__test");
     Lexer l(&file);
 
-    auto t = l.next();
+    auto list = l.Lex();
+    ASSERT_FALSE(list.empty());
+
+    auto& t = list[0];
     EXPECT_FALSE(t.IsIdentifier());
 }
 
@@ -618,7 +735,10 @@ TEST_F(LexerTest, IdentifierTest_DoesNotStartWithNumber) {
     Source::File file("", "01test");
     Lexer l(&file);
 
-    auto t = l.next();
+    auto list = l.Lex();
+    EXPECT_FALSE(list.empty());
+
+    auto& t = list[0];
     EXPECT_FALSE(t.IsIdentifier());
 }
 
@@ -641,7 +761,12 @@ TEST_P(ParseIntegerTest, Parse) {
     auto params = std::get<1>(GetParam());
     Source::File file("", params.input);
 
-    auto t = Lexer(&file).next();
+    Lexer l(&file);
+
+    auto list = l.Lex();
+    ASSERT_FALSE(list.empty());
+
+    auto& t = list[0];
     switch (suffix) {
         case 'i':
             EXPECT_TRUE(t.Is(Token::Type::kIntLiteral_I));
@@ -770,7 +895,12 @@ TEST_P(ParseIntegerTest_CannotBeRepresented, Parse) {
     auto type = std::get<0>(GetParam());
     auto source = std::get<1>(GetParam());
     Source::File file("", source);
-    auto t = Lexer(&file).next();
+
+    Lexer l(&file);
+    auto list = l.Lex();
+    ASSERT_FALSE(list.empty());
+
+    auto& t = list[0];
     EXPECT_TRUE(t.Is(Token::Type::kError));
     auto expect = "value cannot be represented as '" + std::string(type) + "'";
     EXPECT_EQ(t.to_str(), expect);
@@ -800,7 +930,12 @@ INSTANTIATE_TEST_SUITE_P(u32,
 using ParseIntegerTest_LeadingZeros = testing::TestWithParam<const char*>;
 TEST_P(ParseIntegerTest_LeadingZeros, Parse) {
     Source::File file("", GetParam());
-    auto t = Lexer(&file).next();
+
+    Lexer l(&file);
+    auto list = l.Lex();
+    ASSERT_FALSE(list.empty());
+
+    auto& t = list[0];
     EXPECT_TRUE(t.Is(Token::Type::kError));
     EXPECT_EQ(t.to_str(), "integer literal cannot have leading 0s");
 }
@@ -815,7 +950,12 @@ INSTANTIATE_TEST_SUITE_P(LeadingZero,
 using ParseIntegerTest_NoSignificantDigits = testing::TestWithParam<const char*>;
 TEST_P(ParseIntegerTest_NoSignificantDigits, Parse) {
     Source::File file("", GetParam());
-    auto t = Lexer(&file).next();
+
+    Lexer l(&file);
+    auto list = l.Lex();
+    ASSERT_FALSE(list.empty());
+
+    auto& t = list[0];
     EXPECT_TRUE(t.Is(Token::Type::kError));
     EXPECT_EQ(t.to_str(), "integer or float hex literal has no significant digits");
 }
@@ -849,16 +989,23 @@ TEST_P(PunctuationTest, Parses) {
     Source::File file("", params.input);
     Lexer l(&file);
 
-    auto t = l.next();
+    auto list = l.Lex();
+    ASSERT_GE(list.size(), 2u);
+
+    {
+        auto& t = list[0];
         EXPECT_TRUE(t.Is(params.type));
         EXPECT_EQ(t.source().range.begin.line, 1u);
         EXPECT_EQ(t.source().range.begin.column, 1u);
         EXPECT_EQ(t.source().range.end.line, 1u);
         EXPECT_EQ(t.source().range.end.column, 1u + strlen(params.input));
+    }
 
-    t = l.next();
+    {
+        auto& t = list[1];
         EXPECT_EQ(t.source().range.begin.column, 1 + std::string(params.input).size());
     }
+}
 INSTANTIATE_TEST_SUITE_P(LexerTest,
                          PunctuationTest,
                          testing::Values(TokenData{"&", Token::Type::kAnd},
@@ -876,8 +1023,6 @@ INSTANTIATE_TEST_SUITE_P(LexerTest,
                                          TokenData{"=", Token::Type::kEqual},
                                          TokenData{"==", Token::Type::kEqualEqual},
                                          TokenData{">", Token::Type::kGreaterThan},
-                                         TokenData{">=", Token::Type::kGreaterThanEqual},
-                                         TokenData{">>", Token::Type::kShiftRight},
                                          TokenData{"<", Token::Type::kLessThan},
                                          TokenData{"<=", Token::Type::kLessThanEqual},
                                          TokenData{"<<", Token::Type::kShiftLeft},
@@ -906,21 +1051,60 @@ INSTANTIATE_TEST_SUITE_P(LexerTest,
                                          TokenData{"|=", Token::Type::kOrEqual},
                                          TokenData{"^=", Token::Type::kXorEqual}));
 
+using SplittablePunctuationTest = testing::TestWithParam<TokenData>;
+TEST_P(SplittablePunctuationTest, Parses) {
+    auto params = GetParam();
+    Source::File file("", params.input);
+    Lexer l(&file);
+
+    auto list = l.Lex();
+    ASSERT_GE(list.size(), 3u);
+
+    {
+        auto& t = list[0];
+        EXPECT_TRUE(t.Is(params.type));
+        EXPECT_EQ(t.source().range.begin.line, 1u);
+        EXPECT_EQ(t.source().range.begin.column, 1u);
+        EXPECT_EQ(t.source().range.end.line, 1u);
+        EXPECT_EQ(t.source().range.end.column, 1u + strlen(params.input));
+    }
+
+    {
+        auto& t = list[1];
+        EXPECT_TRUE(t.Is(Token::Type::kPlaceholder));
+        EXPECT_EQ(t.source().range.begin.line, 1u);
+        EXPECT_EQ(t.source().range.begin.column, 2u);
+        EXPECT_EQ(t.source().range.end.line, 1u);
+        EXPECT_EQ(t.source().range.end.column, 1u + strlen(params.input));
+    }
+
+    {
+        auto& t = list[2];
+        EXPECT_EQ(t.source().range.begin.column, 1 + std::string(params.input).size());
+    }
+}
+INSTANTIATE_TEST_SUITE_P(LexerTest,
+                         SplittablePunctuationTest,
+                         testing::Values(TokenData{">=", Token::Type::kGreaterThanEqual},
+                                         TokenData{">>", Token::Type::kShiftRight}));
+
 using KeywordTest = testing::TestWithParam<TokenData>;
 TEST_P(KeywordTest, Parses) {
     auto params = GetParam();
     Source::File file("", params.input);
     Lexer l(&file);
 
-    auto t = l.next();
+    auto list = l.Lex();
+    ASSERT_GE(list.size(), 2u);
+
+    auto& t = list[0];
     EXPECT_TRUE(t.Is(params.type)) << params.input;
     EXPECT_EQ(t.source().range.begin.line, 1u);
     EXPECT_EQ(t.source().range.begin.column, 1u);
     EXPECT_EQ(t.source().range.end.line, 1u);
     EXPECT_EQ(t.source().range.end.column, 1u + strlen(params.input));
 
-    t = l.next();
+    EXPECT_EQ(list[1].source().range.begin.column, 1 + std::string(params.input).size());
-    EXPECT_EQ(t.source().range.begin.column, 1 + std::string(params.input).size());
 }
 INSTANTIATE_TEST_SUITE_P(
     LexerTest,

src/tint/reader/wgsl/parser_impl.cc

@@ -123,7 +123,7 @@ const char kStageAttribute[] = "stage";
 const char kWorkgroupSizeAttribute[] = "workgroup_size";
 
 // https://gpuweb.github.io/gpuweb/wgsl.html#reserved-keywords
-bool is_reserved(Token t) {
+bool is_reserved(const Token& t) {
     return t == "asm" || t == "bf16" || t == "do" || t == "enum" || t == "f64" || t == "handle" ||
            t == "i8" || t == "i16" || t == "i64" || t == "mat" || t == "premerge" ||
            t == "regardless" || t == "typedef" || t == "u8" || t == "u16" || t == "u64" ||
@@ -181,7 +181,7 @@ class ParserImpl::MultiTokenSource {
     /// Implicit conversion to Source that returns the combined source from start
     /// to the current last token's source.
     operator Source() const {
-        Source end = parser_->last_token().source().End();
+        Source end = parser_->last_source().End();
         if (end < start_) {
             end = start_;
         }
@@ -241,7 +241,7 @@ ParserImpl::VarDeclInfo::VarDeclInfo(Source source_in,
 
 ParserImpl::VarDeclInfo::~VarDeclInfo() = default;
 
-ParserImpl::ParserImpl(Source::File const* file) : lexer_(std::make_unique<Lexer>(file)) {}
+ParserImpl::ParserImpl(Source::File const* file) : file_(file) {}
 
 ParserImpl::~ParserImpl() = default;
 
@@ -274,33 +274,67 @@ void ParserImpl::deprecated(const Source& source, const std::string& msg) {
                                        "use of deprecated language feature: " + msg, source);
 }
 
-Token ParserImpl::next() {
+const Token& ParserImpl::next() {
-    if (!token_queue_.empty()) {
+    if (!tokens_[next_token_idx_].IsEof() && !tokens_[next_token_idx_].IsError()) {
-        auto t = token_queue_.front();
+        // Skip over any placeholder elements
-        token_queue_.pop_front();
+        while (true) {
-        last_token_ = t;
+            if (!tokens_[next_token_idx_].IsPlaceholder()) {
-        return last_token_;
+                break;
             }
-    last_token_ = lexer_->next();
+            next_token_idx_++;
-    return last_token_;
+        }
+    }
+    last_source_ = tokens_[next_token_idx_].source();
+    return tokens_[next_token_idx_++];
 }
 
-Token ParserImpl::peek(size_t idx) {
+const Token& ParserImpl::peek(size_t idx) {
-    while (token_queue_.size() < (idx + 1)) {
+    if (next_token_idx_ + idx >= tokens_.size()) {
-        token_queue_.push_back(lexer_->next());
+        return tokens_[tokens_.size() - 1];
     }
-    return token_queue_[idx];
+
+    // Skip over any placeholder elements
+    while (true) {
+        if (!tokens_[next_token_idx_ + idx].IsPlaceholder()) {
+            break;
+        }
+        idx++;
+    }
+
+    return tokens_[next_token_idx_ + idx];
 }
 
 bool ParserImpl::peek_is(Token::Type tok, size_t idx) {
     return peek(idx).Is(tok);
 }
 
-Token ParserImpl::last_token() const {
+void ParserImpl::split_token(Token::Type lhs, Token::Type rhs) {
-    return last_token_;
+    if (next_token_idx_ == 0) {
+        TINT_ICE(Reader, builder_.Diagnostics())
+            << "attempt to update placeholder at beginning of tokens";
+    }
+    if (next_token_idx_ >= tokens_.size()) {
+        TINT_ICE(Reader, builder_.Diagnostics())
+            << "attempt to update placeholder past end of tokens";
+    }
+    if (!tokens_[next_token_idx_].IsPlaceholder()) {
+        TINT_ICE(Reader, builder_.Diagnostics()) << "attempt to update non-placeholder token";
+    }
+    tokens_[next_token_idx_ - 1].SetType(lhs);
+    tokens_[next_token_idx_].SetType(rhs);
+}
+
+Source ParserImpl::last_source() const {
+    return last_source_;
+}
+
+void ParserImpl::InitializeLex() {
+    Lexer l{file_};
+    tokens_ = l.Lex();
 }
 
 bool ParserImpl::Parse() {
+    InitializeLex();
     translation_unit();
     return !has_error();
 }
@@ -310,7 +344,7 @@ bool ParserImpl::Parse() {
 void ParserImpl::translation_unit() {
     bool after_global_decl = false;
     while (continue_parsing()) {
-        auto p = peek();
+        auto& p = peek();
         if (p.IsEof()) {
             break;
         }
@@ -353,7 +387,7 @@ Maybe<bool> ParserImpl::enable_directive() {
 
         // Match the extension name.
         Expect<std::string> name = {""};
-        auto t = peek();
+        auto& t = peek();
         if (t.IsIdentifier()) {
             synchronized_ = true;
             next();
@@ -508,7 +542,7 @@ Maybe<bool> ParserImpl::global_decl() {
     }
 
     // We have a statement outside of a function?
-    auto t = peek();
+    auto& t = peek();
     auto stat = without_error([&] { return statement(); });
     if (stat.matched) {
         // Attempt to jump to the next '}' - the function might have just been
@@ -882,7 +916,7 @@ Maybe<const ast::Type*> ParserImpl::depth_texture() {
 //  | 'rgba32sint'
 //  | 'rgba32float'
 Expect<ast::TexelFormat> ParserImpl::expect_texel_format(std::string_view use) {
-    auto t = next();
+    auto& t = next();
     if (t == "rgba8unorm") {
         return ast::TexelFormat::kRgba8Unorm;
     }
@@ -951,7 +985,7 @@ Expect<ParserImpl::TypedIdentifier> ParserImpl::expect_variable_ident_decl(std::
         return Failure::kErrored;
     }
 
-    auto t = peek();
+    auto& t = peek();
     auto type = type_decl();
     if (type.errored) {
         return Failure::kErrored;
@@ -1021,7 +1055,7 @@ Maybe<const ast::Alias*> ParserImpl::type_alias() {
         return Failure::kNoMatch;
     }
 
-    auto t = next();
+    auto& t = next();
     const char* use = "type alias";
 
     auto name = expect_ident(use);
@@ -1069,7 +1103,7 @@ Maybe<const ast::Alias*> ParserImpl::type_alias() {
 //   | MAT4x4 LESS_THAN type_decl GREATER_THAN
 //   | texture_samplers
 Maybe<const ast::Type*> ParserImpl::type_decl() {
-    auto t = peek();
+    auto& t = peek();
     Source source;
     if (match(Token::Type::kIdentifier, &source)) {
         return builder_.create<ast::TypeName>(source, builder_.Symbols().Register(t.to_str()));
@@ -1139,7 +1173,7 @@ Expect<const ast::Type*> ParserImpl::expect_type(std::string_view use) {
     return type.value;
 }
 
-Expect<const ast::Type*> ParserImpl::expect_type_decl_pointer(Token t) {
+Expect<const ast::Type*> ParserImpl::expect_type_decl_pointer(const Token& t) {
     const char* use = "ptr declaration";
 
     auto storage_class = ast::StorageClass::kNone;
@@ -1180,7 +1214,7 @@ Expect<const ast::Type*> ParserImpl::expect_type_decl_pointer(Token t) {
                                access);
 }
 
-Expect<const ast::Type*> ParserImpl::expect_type_decl_atomic(Token t) {
+Expect<const ast::Type*> ParserImpl::expect_type_decl_atomic(const Token& t) {
     const char* use = "atomic declaration";
 
     auto subtype = expect_lt_gt_block(use, [&] { return expect_type(use); });
@@ -1191,7 +1225,7 @@ Expect<const ast::Type*> ParserImpl::expect_type_decl_atomic(Token t) {
     return builder_.ty.atomic(make_source_range_from(t.source()), subtype.value);
 }
 
-Expect<const ast::Type*> ParserImpl::expect_type_decl_vector(Token t) {
+Expect<const ast::Type*> ParserImpl::expect_type_decl_vector(const Token& t) {
     uint32_t count = 2;
     if (t.Is(Token::Type::kVec3)) {
         count = 3;
@@ -1212,7 +1246,7 @@ Expect<const ast::Type*> ParserImpl::expect_type_decl_vector(Token t) {
     return builder_.ty.vec(make_source_range_from(t.source()), subtype, count);
 }
 
-Expect<const ast::Type*> ParserImpl::expect_type_decl_array(Token t) {
+Expect<const ast::Type*> ParserImpl::expect_type_decl_array(const Token& t) {
     const char* use = "array declaration";
 
     const ast::Expression* size = nullptr;
@@ -1244,7 +1278,7 @@ Expect<const ast::Type*> ParserImpl::expect_type_decl_array(Token t) {
     return builder_.ty.array(make_source_range_from(t.source()), subtype.value, size);
 }
 
-Expect<const ast::Type*> ParserImpl::expect_type_decl_matrix(Token t) {
+Expect<const ast::Type*> ParserImpl::expect_type_decl_matrix(const Token& t) {
     uint32_t rows = 2;
     uint32_t columns = 2;
     if (t.IsMat3xN()) {
@@ -1305,7 +1339,7 @@ Expect<ast::StorageClass> ParserImpl::expect_storage_class(std::string_view use)
 // struct_decl
 //   : STRUCT IDENT struct_body_decl
 Maybe<const ast::Struct*> ParserImpl::struct_decl() {
-    auto t = peek();
+    auto& t = peek();
     auto source = t.source();
 
     if (!match(Token::Type::kStruct)) {
@@ -1334,7 +1368,7 @@ Expect<ast::StructMemberList> ParserImpl::expect_struct_body_decl() {
         bool errored = false;
         while (continue_parsing()) {
             // Check for the end of the list.
-            auto t = peek();
+            auto& t = peek();
             if (!t.IsIdentifier() && !t.Is(Token::Type::kAttr)) {
                 break;
             }
@@ -1481,7 +1515,7 @@ Expect<ast::ParameterList> ParserImpl::expect_param_list() {
     ast::ParameterList ret;
     while (continue_parsing()) {
         // Check for the end of the list.
-        auto t = peek();
+        auto& t = peek();
         if (!t.IsIdentifier() && !t.Is(Token::Type::kAttr)) {
             break;
         }
@@ -1522,7 +1556,7 @@ Expect<ast::Parameter*> ParserImpl::expect_param() {
 //   | FRAGMENT
 //   | COMPUTE
 Expect<ast::PipelineStage> ParserImpl::expect_pipeline_stage() {
-    auto t = peek();
+    auto& t = peek();
     if (t == kVertexStage) {
         next();  // Consume the peek
         return {ast::PipelineStage::kVertex, t.source()};
@@ -2010,7 +2044,7 @@ Maybe<const ast::CaseStatement*> ParserImpl::switch_body() {
         return Failure::kNoMatch;
     }
 
-    auto t = next();
+    auto& t = next();
     auto source = t.source();
 
     ast::CaseSelectorList selector_list;
@@ -2263,8 +2297,8 @@ Maybe<const ast::WhileStatement*> ParserImpl::while_stmt() {
 // func_call_stmt
 //    : IDENT argument_expression_list
 Maybe<const ast::CallStatement*> ParserImpl::func_call_stmt() {
-    auto t = peek();
+    auto& t = peek();
-    auto t2 = peek(1);
+    auto& t2 = peek(1);
     if (!t.IsIdentifier() || !t2.Is(Token::Type::kParenLeft)) {
         return Failure::kNoMatch;
     }
@@ -2325,7 +2359,7 @@ Maybe<const ast::BlockStatement*> ParserImpl::continuing_stmt() {
 //   | paren_expression
 //   | BITCAST LESS_THAN type_decl GREATER_THAN paren_expression
 Maybe<const ast::Expression*> ParserImpl::primary_expression() {
-    auto t = peek();
+    auto& t = peek();
     auto source = t.source();
 
     auto lit = const_literal();
@@ -2491,7 +2525,7 @@ Expect<ast::ExpressionList> ParserImpl::expect_argument_expression_list(std::str
 //   | STAR unary_expression
 //   | AND unary_expression
 Maybe<const ast::Expression*> ParserImpl::unary_expression() {
-    auto t = peek();
+    auto& t = peek();
 
     if (match(Token::Type::kPlusPlus) || match(Token::Type::kMinusMinus)) {
         add_error(t.source(),
@@ -2556,7 +2590,7 @@ Expect<const ast::Expression*> ParserImpl::expect_multiplicative_expr(const ast:
             return lhs;
         }
 
-        auto t = next();
+        auto& t = next();
         auto source = t.source();
         auto name = t.to_name();
 
@@ -2603,7 +2637,7 @@ Expect<const ast::Expression*> ParserImpl::expect_additive_expr(const ast::Expre
             return lhs;
         }
 
-        auto t = next();
+        auto& t = next();
         auto source = t.source();
 
         auto rhs = multiplicative_expression();
@@ -2651,7 +2685,7 @@ Expect<const ast::Expression*> ParserImpl::expect_shift_expr(const ast::Expressi
             return lhs;
         }
 
-        auto t = next();
+        auto& t = next();
         auto source = t.source();
         auto rhs = additive_expression();
         if (rhs.errored) {
@@ -2702,7 +2736,7 @@ Expect<const ast::Expression*> ParserImpl::expect_relational_expr(const ast::Exp
             return lhs;
         }
 
-        auto t = next();
+        auto& t = next();
         auto source = t.source();
         auto name = t.to_name();
 
@@ -2749,7 +2783,7 @@ Expect<const ast::Expression*> ParserImpl::expect_equality_expr(const ast::Expre
             return lhs;
         }
 
-        auto t = next();
+        auto& t = next();
         auto source = t.source();
         auto name = t.to_name();
 
@@ -2790,7 +2824,7 @@ Expect<const ast::Expression*> ParserImpl::expect_and_expr(const ast::Expression
             return lhs;
         }
 
-        auto t = next();
+        auto& t = next();
         auto source = t.source();
 
         auto rhs = equality_expression();
@@ -2903,7 +2937,7 @@ Expect<const ast::Expression*> ParserImpl::expect_logical_and_expr(const ast::Ex
             return lhs;
         }
 
-        auto t = next();
+        auto& t = next();
         auto source = t.source();
 
         auto rhs = inclusive_or_expression();
@@ -3013,7 +3047,7 @@ Maybe<ast::BinaryOp> ParserImpl::compound_assignment_operator() {
 // decrement_stmt
 // | lhs_expression MINUS_MINUS
 Maybe<const ast::Statement*> ParserImpl::assignment_stmt() {
-    auto t = peek();
+    auto& t = peek();
     auto source = t.source();
 
     // tint:295 - Test for `ident COLON` - this is invalid grammar, and without
@@ -3076,7 +3110,7 @@ Maybe<const ast::Statement*> ParserImpl::assignment_stmt() {
 //   | TRUE
 //   | FALSE
 Maybe<const ast::LiteralExpression*> ParserImpl::const_literal() {
-    auto t = peek();
+    auto& t = peek();
     if (match(Token::Type::kIntLiteral)) {
         return create<ast::IntLiteralExpression>(t.source(), t.to_i64(),
                                                  ast::IntLiteralExpression::Suffix::kNone);
@@ -3141,7 +3175,7 @@ Maybe<ast::AttributeList> ParserImpl::attribute_list() {
 }
 
 Expect<const ast::Attribute*> ParserImpl::expect_attribute() {
-    auto t = peek();
+    auto& t = peek();
     auto attr = attribute();
     if (attr.errored) {
         return Failure::kErrored;
@@ -3154,7 +3188,7 @@ Expect<const ast::Attribute*> ParserImpl::expect_attribute() {
 
 Maybe<const ast::Attribute*> ParserImpl::attribute() {
     using Result = Maybe<const ast::Attribute*>;
-    auto t = next();
+    auto& t = next();
 
     if (!t.IsIdentifier()) {
         return Failure::kNoMatch;
@@ -3204,7 +3238,7 @@ Maybe<const ast::Attribute*> ParserImpl::attribute() {
             ast::InterpolationType type;
             ast::InterpolationSampling sampling = ast::InterpolationSampling::kNone;
 
-            auto type_tok = next();
+            auto& type_tok = next();
             if (type_tok == "perspective") {
                 type = ast::InterpolationType::kPerspective;
             } else if (type_tok == "linear") {
@@ -3217,7 +3251,7 @@ Maybe<const ast::Attribute*> ParserImpl::attribute() {
 
             if (match(Token::Type::kComma)) {
                 if (!peek_is(Token::Type::kParenRight)) {
-                    auto sampling_tok = next();
+                    auto& sampling_tok = next();
                     if (sampling_tok == "center") {
                         sampling = ast::InterpolationSampling::kCenter;
                     } else if (sampling_tok == "centroid") {
@@ -3227,7 +3261,6 @@ Maybe<const ast::Attribute*> ParserImpl::attribute() {
                     } else {
                         return add_error(sampling_tok, "invalid interpolation sampling");
                     }
-
                     match(Token::Type::kComma);
                 }
             }
@@ -3384,7 +3417,7 @@ bool ParserImpl::expect_attributes_consumed(ast::AttributeList& in) {
 }
 
 bool ParserImpl::match(Token::Type tok, Source* source /*= nullptr*/) {
-    auto t = peek();
+    auto& t = peek();
 
     if (source != nullptr) {
         *source = t.source();
@@ -3398,7 +3431,7 @@ bool ParserImpl::match(Token::Type tok, Source* source /*= nullptr*/) {
 }
 
 bool ParserImpl::expect(std::string_view use, Token::Type tok) {
-    auto t = peek();
+    auto& t = peek();
     if (t.Is(tok)) {
         next();
         synchronized_ = true;
@@ -3414,9 +3447,9 @@ bool ParserImpl::expect(std::string_view use, Token::Type tok) {
         auto source = t.source();
         source.range.begin.column++;
         if (t.Is(Token::Type::kShiftRight)) {
-            token_queue_.push_front(Token(Token::Type::kGreaterThan, source));
+            split_token(Token::Type::kGreaterThan, Token::Type::kGreaterThan);
         } else if (t.Is(Token::Type::kGreaterThanEqual)) {
-            token_queue_.push_front(Token(Token::Type::kEqual, source));
+            split_token(Token::Type::kGreaterThan, Token::Type::kEqual);
         }
 
         synchronized_ = true;
@@ -3439,7 +3472,7 @@ bool ParserImpl::expect(std::string_view use, Token::Type tok) {
 }
 
 Expect<int32_t> ParserImpl::expect_sint(std::string_view use) {
-    auto t = peek();
+    auto& t = peek();
     if (!t.Is(Token::Type::kIntLiteral) && !t.Is(Token::Type::kIntLiteral_I)) {
         return add_error(t.source(), "expected signed integer literal", use);
     }
@@ -3482,7 +3515,7 @@ Expect<uint32_t> ParserImpl::expect_nonzero_positive_sint(std::string_view use)
 }
 
 Expect<std::string> ParserImpl::expect_ident(std::string_view use) {
-    auto t = peek();
+    auto& t = peek();
     if (t.IsIdentifier()) {
         synchronized_ = true;
         next();
@@ -3578,7 +3611,7 @@ bool ParserImpl::sync_to(Token::Type tok, bool consume) {
     BlockCounters counters;
 
     for (size_t i = 0; i < kMaxResynchronizeLookahead; i++) {
-        auto t = peek(i);
+        auto& t = peek(i);
         if (counters.consume(t) > 0) {
             continue;  // Nested block
         }

src/tint/reader/wgsl/parser_impl.h

@@ -15,7 +15,6 @@
 #ifndef SRC_TINT_READER_WGSL_PARSER_IMPL_H_
 #define SRC_TINT_READER_WGSL_PARSER_IMPL_H_
 
-#include <deque>
 #include <memory>
 #include <string>
 #include <string_view>
@@ -300,6 +299,10 @@ class ParserImpl {
     explicit ParserImpl(Source::File const* file);
     ~ParserImpl();
 
+    /// Reads tokens from the source file. This will be called automatically
+    /// by |parse|.
+    void InitializeLex();
+
     /// Run the parser
     /// @returns true if the parse was successful, false otherwise.
     bool Parse();
@@ -330,19 +333,19 @@ class ParserImpl {
     ProgramBuilder& builder() { return builder_; }
 
     /// @returns the next token
-    Token next();
+    const Token& next();
     /// Peeks ahead and returns the token at `idx` ahead of the current position
     /// @param idx the index of the token to return
     /// @returns the token `idx` positions ahead without advancing
-    Token peek(size_t idx = 0);
+    const Token& peek(size_t idx = 0);
     /// Peeks ahead and returns true if the token at `idx` ahead of the current
     /// position is |tok|
     /// @param idx the index of the token to return
     /// @param tok the token to look for
     /// @returns true if the token `idx` positions ahead is |tok|
     bool peek_is(Token::Type tok, size_t idx = 0);
-    /// @returns the last token that was returned by `next()`
+    /// @returns the last source location that was returned by `next()`
-    Token last_token() const;
+    Source last_source() const;
     /// Appends an error at `t` with the message `msg`
     /// @param t the token to associate the error with
     /// @param msg the error message
@@ -812,11 +815,11 @@ class ParserImpl {
     /// Used to ensure that all attributes are consumed.
     bool expect_attributes_consumed(ast::AttributeList& list);
 
-    Expect<const ast::Type*> expect_type_decl_pointer(Token t);
+    Expect<const ast::Type*> expect_type_decl_pointer(const Token& t);
-    Expect<const ast::Type*> expect_type_decl_atomic(Token t);
+    Expect<const ast::Type*> expect_type_decl_atomic(const Token& t);
-    Expect<const ast::Type*> expect_type_decl_vector(Token t);
+    Expect<const ast::Type*> expect_type_decl_vector(const Token& t);
-    Expect<const ast::Type*> expect_type_decl_array(Token t);
+    Expect<const ast::Type*> expect_type_decl_array(const Token& t);
-    Expect<const ast::Type*> expect_type_decl_matrix(Token t);
+    Expect<const ast::Type*> expect_type_decl_matrix(const Token& t);
 
     Expect<const ast::Type*> expect_type(std::string_view use);
 
@@ -824,6 +827,8 @@ class ParserImpl {
     Maybe<const ast::Statement*> for_header_initializer();
     Maybe<const ast::Statement*> for_header_continuing();
 
+    void split_token(Token::Type lhs, Token::Type rhs);
+
     class MultiTokenSource;
     MultiTokenSource make_source_range();
     MultiTokenSource make_source_range_from(const Source& start);
@@ -837,9 +842,10 @@ class ParserImpl {
         return builder_.create<T>(std::forward<ARGS>(args)...);
     }
 
-    std::unique_ptr<Lexer> lexer_;
+    Source::File const* const file_;
-    std::deque<Token> token_queue_;
+    std::vector<Token> tokens_;
-    Token last_token_;
+    size_t next_token_idx_ = 0;
+    Source last_source_;
     bool synchronized_ = true;
     uint32_t parse_depth_ = 0;
     std::vector<Token::Type> sync_tokens_;

src/tint/reader/wgsl/parser_impl_pipeline_stage_test.cc

@@ -40,7 +40,7 @@ TEST_P(PipelineStageTest, Parses) {
     EXPECT_EQ(stage.source.range.end.line, 1u);
     EXPECT_EQ(stage.source.range.end.column, 1u + params.input.size());
 
-    auto t = p->next();
+    auto& t = p->next();
     EXPECT_TRUE(t.IsEof());
 }
 INSTANTIATE_TEST_SUITE_P(

src/tint/reader/wgsl/parser_impl_storage_class_test.cc

@@ -37,7 +37,7 @@ TEST_P(StorageClassTest, Parses) {
     EXPECT_FALSE(p->has_error());
     EXPECT_EQ(sc.value, params.result);
 
-    auto t = p->next();
+    auto& t = p->next();
     EXPECT_TRUE(t.IsEof());
 }
 INSTANTIATE_TEST_SUITE_P(

src/tint/reader/wgsl/parser_impl_test_helper.h

@@ -38,6 +38,7 @@ class ParserImplTest : public testing::Test, public ProgramBuilder {
     std::unique_ptr<ParserImpl> parser(const std::string& str) {
         auto file = std::make_unique<Source::File>("test.wgsl", str);
         auto impl = std::make_unique<ParserImpl>(file.get());
+        impl->InitializeLex();
         files_.emplace_back(std::move(file));
         return impl;
     }
@@ -60,6 +61,7 @@ class ParserImplTestWithParam : public testing::TestWithParam<T>, public Program
     std::unique_ptr<ParserImpl> parser(const std::string& str) {
         auto file = std::make_unique<Source::File>("test.wgsl", str);
         auto impl = std::make_unique<ParserImpl>(file.get());
+        impl->InitializeLex();
         files_.emplace_back(std::move(file));
         return impl;
     }

src/tint/reader/wgsl/parser_impl_variable_decl_test.cc

@@ -68,7 +68,7 @@ TEST_F(ParserImplTest, VariableDecl_MissingVar) {
     EXPECT_FALSE(v.errored);
     EXPECT_FALSE(p->has_error());
 
-    auto t = p->next();
+    auto& t = p->next();
     ASSERT_TRUE(t.IsIdentifier());
 }
 

src/tint/reader/wgsl/parser_impl_variable_qualifier_test.cc

@@ -40,7 +40,7 @@ TEST_P(VariableQualifierTest, ParsesStorageClass) {
     EXPECT_EQ(sc->storage_class, params.storage_class);
     EXPECT_EQ(sc->access, params.access);
 
-    auto t = p->next();
+    auto& t = p->next();
     EXPECT_TRUE(t.IsEof());
 }
 INSTANTIATE_TEST_SUITE_P(
@@ -83,7 +83,7 @@ TEST_F(ParserImplTest, VariableQualifier_MissingLessThan) {
     EXPECT_FALSE(sc.errored);
     EXPECT_FALSE(sc.matched);
 
-    auto t = p->next();
+    auto& t = p->next();
     ASSERT_TRUE(t.Is(Token::Type::kIdentifier));
 }
 
@@ -94,7 +94,7 @@ TEST_F(ParserImplTest, VariableQualifier_MissingLessThan_AfterSC) {
     EXPECT_FALSE(sc.errored);
     EXPECT_FALSE(sc.matched);
 
-    auto t = p->next();
+    auto& t = p->next();
     ASSERT_TRUE(t.Is(Token::Type::kIdentifier));
 }
 

src/tint/reader/wgsl/token.cc

@@ -37,6 +37,8 @@ std::string_view Token::TypeToName(Type type) {
             return "'i'-suffixed integer literal";
         case Token::Type::kIntLiteral_U:
             return "'u'-suffixed integer literal";
+        case Token::Type::kPlaceholder:
+            return "placeholder";
         case Token::Type::kUninitialized:
             return "uninitialized";
 
@@ -285,13 +287,9 @@ Token::Token(Type type, const Source& source) : type_(type), source_(source) {}
 
 Token::Token(Token&&) = default;
 
-Token::Token(const Token&) = default;
-
 Token::~Token() = default;
 
-Token& Token::operator=(const Token& rhs) = default;
+bool Token::operator==(std::string_view ident) const {
-
-bool Token::operator==(std::string_view ident) {
     if (type_ != Type::kIdentifier) {
         return false;
     }

src/tint/reader/wgsl/token.h

@@ -32,6 +32,8 @@ class Token {
         kError = -2,
         /// Uninitialized token
         kUninitialized = 0,
+        /// Placeholder token which maybe fillled in later
+        kPlaceholder = 1,
         /// End of input string reached
         kEOF,
 
@@ -310,19 +312,16 @@ class Token {
     Token(Type type, const Source& source, double val);
     /// Move constructor
     Token(Token&&);
-    /// Copy constructor
-    Token(const Token&);
     ~Token();
 
-    /// Assignment operator
-    /// @param b the token to copy
-    /// @return Token
-    Token& operator=(const Token& b);
-
     /// Equality operator with an identifier
     /// @param ident the identifier string
     /// @return true if this token is an identifier and is equal to ident.
-    bool operator==(std::string_view ident);
+    bool operator==(std::string_view ident) const;
+
+    /// Sets the token to the given type
+    /// @param type the type to set
+    void SetType(Token::Type type) { type_ = type; }
 
     /// Returns true if the token is of the given type
     /// @param t the type to check against.
@@ -331,6 +330,8 @@ class Token {
 
     /// @returns true if the token is uninitialized
     bool IsUninitialized() const { return type_ == Type::kUninitialized; }
+    /// @returns true if the token is a placeholder
+    bool IsPlaceholder() const { return type_ == Type::kPlaceholder; }
     /// @returns true if the token is EOF
     bool IsEof() const { return type_ == Type::kEOF; }
     /// @returns true if the token is Error
@@ -372,6 +373,11 @@ class Token {
         return type_ == Type::kVec2 || type_ == Type::kVec3 || type_ == Type::kVec4;
     }
 
+    /// @returns true if the token can be split during parse into component tokens
+    bool IsSplittable() const {
+        return Is(Token::Type::kShiftRight) || Is(Token::Type::kGreaterThanEqual);
+    }
+
     /// @returns the source information for this token
     Source source() const { return source_; }