tidy: Add a rule ensuring that // comments are followed by a space in Rust (#38698)

This shows up sometimes in code reviews, so it makes sense that tidy
enforces it. `rustfmt` supports this via comment normalization, but it
does many other things and is still an unstable feature (with bugs).

Testing: There are new tidy tests for this change.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/net/filemanager_thread.rs

@@ -34,7 +34,7 @@ use crate::fetch::methods::{CancellationListener, Data, RangeRequestBounds};
 use crate::protocols::get_range_request_bounds;
 use crate::resource_thread::CoreResourceThreadPool;
 
-pub const FILE_CHUNK_SIZE: usize = 32768; //32 KB
+pub const FILE_CHUNK_SIZE: usize = 32768; // 32 KB
 
 /// FileManagerStore's entry
 struct FileStoreEntry {

components/net/image_cache.rs

@@ -1018,7 +1018,7 @@ impl ImageCache for ImageCacheImpl {
                 let pending_load = store.pending_loads.get_by_key_mut(&id).unwrap();
                 pending_load.bytes.extend_from_slice(&data);
 
-                //jmr0 TODO: possibly move to another task?
+                // jmr0 TODO: possibly move to another task?
                 if pending_load.metadata.is_none() {
                     let mut reader = std::io::Cursor::new(pending_load.bytes.as_slice());
                     if let Ok(info) = imsz_from_reader(&mut reader) {

components/net/subresource_integrity.rs

@@ -123,7 +123,7 @@ fn apply_algorithm_to_response<S: ArrayLength<u8>, D: Digest<OutputSize = S>>(
 ) -> String {
     if let ResponseBody::Done(ref vec) = *body {
         hasher.update(vec);
-        let response_digest = hasher.finalize(); //Now hash
+        let response_digest = hasher.finalize(); // Now hash
         base64::engine::general_purpose::STANDARD.encode(&response_digest)
     } else {
         unreachable!("Tried to calculate digest of incomplete response body")

components/net/tests/fetch.rs

@@ -1299,7 +1299,7 @@ fn test_fetch_with_devtools() {
     let devhttprequests = expect_devtools_http_request(&devtools_port);
     let mut devhttpresponse = devtools_response_with_body(&devtools_port);
 
-    //Creating default headers for request
+    // Creating default headers for request
     let mut headers = HeaderMap::new();
 
     headers.insert(header::ACCEPT, HeaderValue::from_static("*/*"));

components/net/tests/http_loader.rs

@@ -352,7 +352,7 @@ fn test_request_and_response_data_with_network_messages() {
     let devhttprequests = expect_devtools_http_request(&devtools_port);
     let devhttpresponse = devtools_response_with_body(&devtools_port);
 
-    //Creating default headers for request
+    // Creating default headers for request
     let mut headers = HeaderMap::new();
 
     headers.insert(