Rustfmt: merge_imports should be a little more conservative

[whatisaphone]

I turned on [merge_imports] and ran cargo fmt over a codebase of mine, with mixed results. Since that option is not yet stable, I thought you might be interested in some feedback.

Summary

Here is one change it made that I found useful:

-use behavior::Behavior;
-use behavior::NullBehavior;
+use behavior::{Behavior, NullBehavior};

From two lines down to one. Looking good 👍

Here is a change I found unnecessary:

-use std::io::{Read, Write};
-use std::thread;
+use std::{
+    io::{Read, Write},
+    thread,
+};

This one went from 2 lines to 4, and became a bit of a maze to read.

Possible changes

The options for merge_imports are true and false. The import porridge is always either too hot or too cold. It would be nice if there were a "just right" option. I can think of a couple possibilities for the specific implementation:

1. Only merge imported names that do not refer to a module:

   use std::io::{Read, Write};
   use std::mem;
   use std::thread;
   

2. Only merge at most a single level of names:

   use std::io::{Read, Write};
   use std::{mem, thread};
   

Whichever one is easier to implement or has fewest corner cases is probably the way to go.

---

In general, with the default options rustfmt has been very valuable to me and saved me a lot of time doing menial formatting. Thanks for working on it! 😄

[mulkieran]

Here is a change I found unnecessary:

-use std::io::{Read, Write};
-use std::thread;
+use std::{
+    io::{Read, Write},
+    thread,
+};

I actually liked this result. I think it should be possible to be prevented by a separating line between the two imports, so:

use std::io::{Read, Write};

use std::io::thread;

would remain unchanged due to the separating line. Maybe that always happens, anyway.