How would you expect

// This is a long comment that's going to be wrapped.
// Short

to be formatted?

// This is a long comment
// that's going to be
// wrapped.
// Short

or 

// This is a long comment
// that's going to be
// wrapped. Short

We also don't push short comments onto a single line, e.g.

// This
// is
// a
// long

into

// This is a long

Right... I thought about those two cases. I actually wanted to see what rustfmt does but that's unstable and I was lazy to install an unsafe version of it to try it.

My guess is that this:

// This is a long comment that's going to be wrapped.
// Short

is formatted like this:

// This is a long comment
// that's going to be
// wrapped.
// Short

The main reason is that otherwise if you really want a newline after "wrapped." then the formatter would undo it (your second comment "We also don't push short comments onto a single line, e.g.")

This is especially tricky if code in comments is being formatted. If you write this:

// If we have a loop like this:
// 
// for i in 0..10 {
//   println("hello!");
// }

it would be kind of bad if that println always moves up because there's space.

I know it ends up a bit annoying that a wrapped comment might not look good, but my thought of this feature is that it makes sure you don't exceed the maximum, and the comment might end up not looking very nice and you'll fix it, always in a way that doesn't exceed the maximum.

One other thing: I think the feature in rustfmt is unstable because it has many bugs. I don't know if those bugs are because it's not clear how comments should be wrapped (this discussion).