imp(CLI): remove null in pretty-printed json

Without all that clutter, it's so much more enjoyable to read the
output.

The implementation is based on a suggestion of @erickt, which is
converts into a json::Value (able to represent any json structure),
on which the filtering is applied.

If we should ever implement pretty-printing in json-tools, we might
still consider using these capabilities instead, as we would avoid
building potentially large datastructures, all we would need is
a sufficiently large destination buffer which is a single alloc and
a consecutive region in memory.

Closes #102
[skip ci]

src/mako/cli/lib/engine.mako

@@ -32,7 +32,7 @@
 %>\
 use cmn::{InvalidOptionsError, CLIError, JsonTokenStorage, arg_from_str, writer_from_opts, parse_kv_arg, 
           input_file_from_opts, input_mime_from_opts, FieldCursor, FieldError, CallType, UploadProtocol,
-          calltype_from_str};
+          calltype_from_str, remove_json_null_values};
 
 use std::default::Default;
 use std::str::FromStr;
@@ -318,7 +318,9 @@ if dry_run {
             if !download_mode {
             % endif
             % if mc.response_schema:
-            serde::json::to_writer_pretty(&mut ostream, &output_schema).unwrap();
+            let mut value = json::value::to_value(&output_schema);
+            remove_json_null_values(&mut value);
+            serde::json::to_writer_pretty(&mut ostream, &value).unwrap();
             % endif
             % if track_download_flag:
             } else {

src/rust/cli/cmn.rs

@@ -17,6 +17,29 @@ use std::default::Default;
 
 const FIELD_SEP: char = '.';
 
+// Based on @erickt user comment. Thanks for the idea !
+// Remove all keys whose values are null from given value (changed in place)
+pub fn remove_json_null_values(value: &mut json::value::Value) {
+    match *value {
+        json::value::Value::Object(ref mut map) => {
+            let mut for_removal = Vec::new();
+
+            for (key, mut value) in map.iter_mut() {
+                if value.is_null() {
+                    for_removal.push(key.clone());
+                } else {
+                    remove_json_null_values(&mut value);
+                }
+            }
+
+            for key in &for_removal {
+                map.remove(key);
+            }
+        }
+        _ => {}
+    }
+}
+
 fn did_you_mean<'a>(v: &str, possible_values: &[&'a str]) -> Option<&'a str> {
 
     let mut candidate: Option<(f64, &str)> = None;