libexpr: Switch parser.y to %skeleton lalr1.cc

Since the parser is now LALR we can easily switch
over to the less ugly sketelon than the default C one.
This would allow us to switch from %union to %define api.value.type variant
in the future to avoid the need for triviall POD types.

src/libexpr/include/nix/expr/parser-state.hh

@@ -24,7 +24,6 @@ struct StringToken
     }
 };
 
-// This type must be trivially copyable; see YYLTYPE_IS_TRIVIAL in parser.y.
 struct ParserLocation
 {
     int beginOffset;

src/libexpr/lexer-helpers.cc

@@ -1,11 +1,11 @@
 #include "lexer-helpers.hh"
 
-void nix::lexer::internal::initLoc(YYLTYPE * loc)
+void nix::lexer::internal::initLoc(Parser::location_type * loc)
 {
     loc->beginOffset = loc->endOffset = 0;
 }
 
-void nix::lexer::internal::adjustLoc(yyscan_t yyscanner, YYLTYPE * loc, const char * s, size_t len)
+void nix::lexer::internal::adjustLoc(yyscan_t yyscanner, Parser::location_type * loc, const char * s, size_t len)
 {
     loc->stash();
 

src/libexpr/lexer-helpers.hh

@@ -2,16 +2,12 @@
 
 #include <cstddef>
 
-// including the generated headers twice leads to errors
+#include "parser-scanner-decls.hh"
-#ifndef BISON_HEADER
-#  include "lexer-tab.hh"
-#  include "parser-tab.hh"
-#endif
 
 namespace nix::lexer::internal {
 
-void initLoc(YYLTYPE * loc);
+void initLoc(Parser::location_type * loc);
 
-void adjustLoc(yyscan_t yyscanner, YYLTYPE * loc, const char * s, size_t len);
+void adjustLoc(yyscan_t yyscanner, Parser::location_type * loc, const char * s, size_t len);
 
 } // namespace nix::lexer::internal

src/libexpr/lexer.l

@@ -82,6 +82,10 @@ static void requireExperimentalFeature(const ExperimentalFeature & feature, cons
 
 }
 
+using enum nix::Parser::token::token_kind_type;
+using YYSTYPE = nix::Parser::value_type;
+using YYLTYPE = nix::Parser::location_type;
+
 // yacc generates code that uses unannotated fallthrough.
 #pragma GCC diagnostic ignored "-Wimplicit-fallthrough"
 

src/libexpr/parser-scanner-decls.hh

@@ -0,0 +1,17 @@
+#pragma once
+
+#ifndef BISON_HEADER
+#  include "parser-tab.hh"
+using YYSTYPE = nix::parser::BisonParser::value_type;
+using YYLTYPE = nix::parser::BisonParser::location_type;
+#  include "lexer-tab.hh" // IWYU pragma: export
+#endif
+
+namespace nix {
+
+class Parser : public parser::BisonParser
+{
+    using BisonParser::BisonParser;
+};
+
+} // namespace nix

src/libexpr/parser.y

@@ -1,5 +1,7 @@
+%skeleton "lalr1.cc"
 %define api.location.type { ::nix::ParserLocation }
-%define api.pure
+%define api.namespace { ::nix::parser }
+%define api.parser.class { BisonParser }
 %locations
 %define parse.error verbose
 %defines
@@ -26,19 +28,12 @@
 #include "nix/expr/eval-settings.hh"
 #include "nix/expr/parser-state.hh"
 
-// Bison seems to have difficulty growing the parser stack when using C++ with
+#define YY_DECL                                    \
-// a custom location type. This undocumented macro tells Bison that our
+    int yylex(                                     \
-// location type is "trivially copyable" in C++-ese, so it is safe to use the
+        nix::Parser::value_type * yylval_param,    \
-// same memcpy macro it uses to grow the stack that it uses with its own
+        nix::Parser::location_type * yylloc_param, \
-// default location type. Without this, we get "error: memory exhausted" when
+        yyscan_t yyscanner,                        \
-// parsing some large Nix files. Our other options are to increase the initial
+        nix::ParserState * state)
-// stack size (200 by default) to be as large as we ever want to support (so
-// that growing the stack is unnecessary), or redefine the stack-relocation
-// macro ourselves (which is also undocumented).
-#define YYLTYPE_IS_TRIVIAL 1
-
-#define YY_DECL int yylex \
-    (YYSTYPE * yylval_param, YYLTYPE * yylloc_param, yyscan_t yyscanner, nix::ParserState * state)
 
 // For efficiency, we only track offsets; not line,column coordinates
 # define YYLLOC_DEFAULT(Current, Rhs, N)                                \
@@ -78,24 +73,30 @@ Expr * parseExprFromBuf(
 
 %{
 
-#include "parser-tab.hh"
+/* The parser is very performance sensitive and loses out on a lot
-#include "lexer-tab.hh"
+   of performance even with basic stdlib assertions. Since those don't
+   affect ABI we can disable those just for this file. */
+#if defined(_GLIBCXX_ASSERTIONS) && !defined(_GLIBCXX_DEBUG)
+#undef _GLIBCXX_ASSERTIONS
+#endif
+
+#include "parser-scanner-decls.hh"
 
 YY_DECL;
 
 using namespace nix;
 
-#define CUR_POS state->at(yyloc)
+#define CUR_POS state->at(yylhs.location)
 
-
+void parser::BisonParser::error(const location_type &loc_, const std::string &error)
-void yyerror(YYLTYPE * loc, yyscan_t scanner, ParserState * state, const char * error)
 {
+    auto loc = loc_;
     if (std::string_view(error).starts_with("syntax error, unexpected end of file")) {
-        loc->beginOffset = loc->endOffset;
+        loc.beginOffset = loc.endOffset;
     }
     throw ParseError({
         .msg = HintFmt(error),
-        .pos = state->positions[state->at(*loc)]
+        .pos = state->positions[state->at(loc)]
     });
 }
 
@@ -182,7 +183,7 @@ start: expr {
   state->result = $1;
 
   // This parser does not use yynerrs; suppress the warning.
-  (void) yynerrs;
+  (void) yynerrs_;
 };
 
 expr: expr_function;
@@ -563,7 +564,8 @@ Expr * parseExprFromBuf(
     Finally _destroy([&] { yylex_destroy(scanner); });
 
     yy_scan_buffer(text, length, scanner);
-    yyparse(scanner, &state);
+    Parser parser(scanner, &state);
+    parser.parse();
 
     return state.result;
 }