Allow using { $__loc__ } as shorthand for { __loc__: $__loc__ }

n.b.: { $__loc__: "foo" }  is still not allowed.

$__loc__ cannot be overwritten, and it is always an object.
Using { $x: "foo" } where $x is not a string is an error, so there is
not much of a point in supporting that.

src/parser.y

@@ -300,6 +300,11 @@ static block gen_update(block object, block val, int optype) {
                                                               optype)))));
 }
 
+static block gen_loc_object(location *loc, struct locfile *locations) {
+  return gen_const(JV_OBJECT(jv_string("file"), jv_copy(locations->fname),
+                             jv_string("line"), jv_number(locfile_get_line(locations, loc->start) + 1)));
+}
+
 %}
 
 %%
@@ -780,8 +785,7 @@ BINDING {
   jv_free($1);
 } |
 "$__loc__" {
-  $$ = gen_const(JV_OBJECT(jv_string("file"), jv_copy(locations->fname),
-                           jv_string("line"), jv_number(locfile_get_line(locations, @$.start) + 1)));
+  $$ = gen_loc_object(&@$, locations);
 } |
 IDENT {
   const char *s = jv_string_value($1);
@@ -981,6 +985,10 @@ IDENT ':' ExpD {
   $$ = gen_dictpair(gen_const(jv_copy($1)),
                     gen_index(gen_noop(), gen_const($1)));
   }
+| "$__loc__" {
+  $$ = gen_dictpair(gen_const(jv_string("__loc__")),
+                    gen_loc_object(&@$, locations));
+  }
 | Keyword {
   $$ = gen_dictpair(gen_const(jv_copy($1)),
                     gen_index(gen_noop(), gen_const($1)));

tests/jq.test

@@ -1792,3 +1792,9 @@ reduce .[] as $then (4 as $else | $else; . as $elif | . + $then * $elif)
 [ foreach .[] as $try (1 as $catch | $catch - 1; . + $try; .) ]
 [10,9,8,7]
 [10,19,27,34]
+
+
+# { $__loc__ } works
+{ a, $__loc__, c }
+{"a":[1,2,3],"b":"foo","c":{"hi":"hey"}}
+{"a":[1,2,3],"__loc__":{"file":"<top-level>","line":1},"c":{"hi":"hey"}}