decouple parser and EvalState

there's no reason the parser itself should be doing semantic analysis
like bindVars. split this bit apart (retaining the previous name in
EvalState) and have the parser really do *only* parsing, decoupled from
EvalState.

(cherry picked from commit b596cc9e7960b9256bcd557334d81e9d555be5a2)
Change-Id: I481a7623afc783e9d28a6eb4627552cf8a780986

src/libexpr/parser.y

@@ -31,6 +31,18 @@
 #define YY_DECL int yylex \
     (YYSTYPE * yylval_param, YYLTYPE * yylloc_param, yyscan_t yyscanner, nix::ParserState * state)
 
+namespace nix {
+
+Expr * parseExprFromBuf(
+    char * text,
+    size_t length,
+    Pos::Origin origin,
+    const SourcePath & basePath,
+    SymbolTable & symbols,
+    PosTable & positions);
+
+}
+
 #endif
 
 }
@@ -51,7 +63,7 @@ void yyerror(YYLTYPE * loc, yyscan_t scanner, ParserState * state, const char *
 {
     throw ParseError({
         .msg = hintfmt(error),
-        .errPos = state->state.positions[state->at(*loc)]
+        .errPos = state->positions[state->at(*loc)]
     });
 }
 
@@ -140,7 +152,7 @@ expr_function
     { if (!$2->dynamicAttrs.empty())
         throw ParseError({
             .msg = hintfmt("dynamic attributes not allowed in let"),
-            .errPos = state->state.positions[CUR_POS]
+            .errPos = state->positions[CUR_POS]
         });
       $$ = new ExprLet($2, $4);
     }
@@ -230,7 +242,7 @@ expr_simple
       if (noURLLiterals)
           throw ParseError({
               .msg = hintfmt("URL literals are disabled"),
-              .errPos = state->state.positions[CUR_POS]
+              .errPos = state->positions[CUR_POS]
           });
       $$ = new ExprString(std::string($1));
   }
@@ -326,7 +338,7 @@ attrs
       } else
           throw ParseError({
               .msg = hintfmt("dynamic attributes not allowed in inherit"),
-              .errPos = state->state.positions[state->at(@2)]
+              .errPos = state->positions[state->at(@2)]
           });
     }
   | { $$ = new AttrPath; }
@@ -393,17 +405,18 @@ formal
 
 namespace nix {
 
-Expr * EvalState::parse(
+Expr * parseExprFromBuf(
     char * text,
     size_t length,
     Pos::Origin origin,
     const SourcePath & basePath,
-    std::shared_ptr<StaticEnv> & staticEnv)
+    SymbolTable & symbols,
+    PosTable & positions)
 {
     yyscan_t scanner;
     ParserState state {
-        .state = *this,
         .symbols = symbols,
+        .positions = positions,
         .basePath = basePath,
         .origin = {origin},
     };
@@ -414,8 +427,6 @@ Expr * EvalState::parse(
     yy_scan_buffer(text, length, scanner);
     yyparse(scanner, &state);
 
-    state.result->bindVars(*this, staticEnv);
-
     return state.result;
 }