Is it a convention to write ?Sized by itself as a trait bound?

Question

On page 297 of Programming Rust you can find the following

impl HashMap<K, V> where K: Eq + Hash
{
  fn get<Q: ?Sized>(&self, key: &Q) -> Option<&V>
      where K: Borrow<Q>,
            Q: Eq + Hash
}

I've seen this before where ?Sized is written by itself, and the rest of trait bounds are on a different line? Is this a convention? As I understand it, the above is effectively the same as the following?

impl HashMap<K, V> where K: Eq + Hash
{
  fn get<Q>(&self, key: &Q) -> Option<&V>
      where K: Borrow<Q>,
            Q: Eq + Hash + ?Sized
}

Why is ?Sized split off? You can see this in a similar example on page 295,

...
where T: AsRef<U>
      T: ?Sized, U: ?Sized
...

Sébastien Renauld · Accepted Answer · 2019-09-20T15:00:23.873

4

That is pure convention, and it is not set in stone, but it does have some merit.

The special syntax ?Sized lets the compiler remove this bound if it is not appropriate (when monomorphizing). As such, it does make sense, somewhat, to split it and put it in the generic definition rather than the where clause for ease of readability and to separate the fact that, unlike the others, it isn't a strict, rigid marker.

Some libraries out there go even further and list all the markers in the generic definition, and all traits in the where clause.

As stated in the comments and found by @PeterHall through the commit log, prior to rust 1.15, ?Sized was only available as a trait requirement in the type definition. This PR changed this to the behavior we have today.

edited Sep 20 '19 at 15:00

answered Sep 20 '19 at 11:10

Sébastien Renauld

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3

There are also historical reasons for this distinction, though. Early versions of the compiler did not allow for `?Sized` to be in the `where` clause. – E_net4 Sep 20 '19 at 12:11
@E_net4theMeta-RemoveR that is probably the "real" answer. – Peter Hall Sep 20 '19 at 14:49
@PeterHall I would turn it info an answer if I found the reference. All I know is word of mouth that I still haven't quite validated yet. – E_net4 Sep 20 '19 at 14:50
@E_net4theMeta-RemoveR https://github.com/rust-lang/rust/pull/37791 Stable in Rust 1.15 – Peter Hall Sep 20 '19 at 14:52
So you found it. :) – E_net4 Sep 20 '19 at 14:54
Edited the answer to include this detail – Sébastien Renauld Sep 20 '19 at 15:00

Is it a convention to write ?Sized by itself as a trait bound?

1 Answers1