Change map-capacity-counting part of json_internal! macro to not repo…

…rt errors.

Since these errors are already reported by the part of the macro that actually parses the map,
it is unnecessary to report them twice, so the capacity-counting part can just return 0 if
it encounters an error.

src/macros.rs

@@ -254,7 +254,9 @@ macro_rules! json_internal {
         1 + json_internal!(@object_capacity () ($($rest)*) ($($rest)*))
     };
 
-    // Current entry followed by unexpected token. The actual parsing macro will print the error message.
+    // Current entry followed by unexpected token. The part that parses the values
+    // will trigger a reasonable error message; here, we just return 0
+    // so that there is not a duplicated error message.
     (@object_capacity entry $unexpected:tt $($rest:tt)*) => {
         0
     };
@@ -299,22 +301,30 @@ macro_rules! json_internal {
         json_internal!(@object_capacity entry)
     };
 
-    // Missing value for last entry. The actual parsing macro will print the error message.
+    // Missing value for last entry. The part that parses the values
+    // will trigger a reasonable error message; here, we just return 0
+    // so that there is not a duplicated error message.
     (@object_capacity ($($key:tt)+) (:) $copy:tt) => {
         0
     };
 
-    // Missing colon and value for last entry. The actual parsing macro will print the error message.
+    // Missing colon and value for last entry. The part that parses the values
+    // will trigger a reasonable error message; here, we just return 0
+    // so that there is not a duplicated error message.
     (@object_capacity ($($key:tt)+) () $copy:tt) => {
         0
     };
 
-    // Misplaced colon. The actual parsing macro will print the error message.
+    // Misplaced colon. The part that parses the values
+    // will trigger a reasonable error message; here, we just return 0
+    // so that there is not a duplicated error message.
     (@object_capacity () (: $($rest:tt)*) ($colon:tt $($copy:tt)*)) => {
         0
     };
 
-    // Found a comma inside a key. The actual parsing macro will print the error message.
+    // Found a comma inside a key. The part that parses the values
+    // will trigger a reasonable error message; here, we just return 0
+    // so that there is not a duplicated error message.
     (@object_capacity ($($key:tt)*) (, $($rest:tt)*) ($comma:tt $($copy:tt)*)) => {
         0
     };
@@ -326,8 +336,10 @@ macro_rules! json_internal {
     // };
 
     // Refuse to absorb colon token into key expression.
+    // The part that parses the values will trigger a reasonable error message;
+    // here, we just return 0 so that there is not a duplicated error message.
     (@object_capacity ($($key:tt)*) (: $($unexpected:tt)+) $copy:tt) => {
-        json_expect_expr_comma!($($unexpected)+)
+        0
     };
 
     // Munch a token into the current key.