Add a unary integer negation operator

This allows saying "-1" instead of "builtins.sub 0 1".

src/libexpr/parser.y

@@ -270,11 +270,12 @@ void yyerror(YYLTYPE * loc, yyscan_t scanner, ParseData * data, const char * err
 %left AND
 %nonassoc EQ NEQ
 %right UPDATE
-%left NEG
+%left NOT
 %left '+'
 %right CONCAT
 %nonassoc '?'
 %nonassoc '~'
+%nonassoc NEGATE
 
 %%
 
@@ -306,7 +307,8 @@ expr_if
   ;
 
 expr_op
-  : '!' expr_op %prec NEG { $$ = new ExprOpNot($2); }
+  : '!' expr_op %prec NOT { $$ = new ExprOpNot($2); }
+  | '-' expr_op %prec NEGATE { $$ = new ExprApp(new ExprApp(new ExprVar(data->symbols.create("__sub")), new ExprInt(0)), $2); }
   | expr_op EQ expr_op { $$ = new ExprOpEq($1, $3); }
   | expr_op NEQ expr_op { $$ = new ExprOpNEq($1, $3); }
   | expr_op AND expr_op { $$ = new ExprOpAnd($1, $3); }

tests/lang/eval-okay-arithmetic.exp

-1854
+1843

tests/lang/eval-okay-arithmetic.nix

@@ -16,6 +16,11 @@ let {
   range = range_ [];
   */
 
-  body = sum (range 1 50) + 123 + 456;
+  x = 12;
 
+  body = sum
+    [ (sum (range 1 50))
+      (123 + 456)
+      (0 + -10 + -(-11) + -x)
+    ];
 }