simplify parse error reporting

since nix doesn't use the bison `error` terminal anywhere any invocation
of yyerror will immediately cause a failure. since we're *already*
leaking tons of memory whatever little bit bison allocates internally
doesn't much matter any more, and we'll be replacing the parser soon anyway.

coincidentally this now also matches the error behavior of URIs when
they are disabled or ~/ paths in pure eval mode, duplicate attr
detection etc.

src/libexpr/parser.y

@@ -18,6 +18,7 @@
 
 #include <variant>
 
+#include "finally.hh"
 #include "util.hh"
 #include "users.hh"
 
@@ -60,7 +61,6 @@ namespace nix {
         Expr * result;
         SourcePath basePath;
         PosTable::Origin origin;
-        std::optional<ErrorInfo> error;
     };
 
 }
@@ -315,10 +315,10 @@ static inline PosIdx makeCurPos(const YYLTYPE & loc, ParseData * data)
 
 void yyerror(YYLTYPE * loc, yyscan_t scanner, ParseData * data, const char * error)
 {
-    data->error = {
+    throw ParseError({
         .msg = hintfmt(error),
         .errPos = data->state.positions[makeCurPos(*loc, data)]
-    };
+    });
 }
 
 
@@ -689,11 +689,10 @@ Expr * EvalState::parse(
     };
 
     yylex_init(&scanner);
-    yy_scan_buffer(text, length, scanner);
-    int res = yyparse(scanner, &data);
-    yylex_destroy(scanner);
+    Finally _destroy([&] { yylex_destroy(scanner); });
 
-    if (res) throw ParseError(data.error.value());
+    yy_scan_buffer(text, length, scanner);
+    yyparse(scanner, &data);
 
     data.result->bindVars(*this, staticEnv);