Add a contains method to VecDeque and LinkedList

[LukasKalbertodt]

Summary: add a contains method to VecDeque and LinkedList that checks if the collection contains a given item.

Rendered

[main--]

• also add BinaryHeap::contains, since it could be convenient for some use cases, too

It's appropriate for most collections, so I'd suggest adding it to Iterator (with the obvious default impl self.any(|e| e == item)). The common usecase foo.iter().contains(bar) is a little bit longer but more generic.

[LukasKalbertodt]

adding it to Iterator

While it definitely makes sense to do so, we already have an inconsistency here: there already is a contains (or contains_key) method for [T], *Set, *Map and (more or less) Vec. Only VecDeque and LinkedList are lacking it. Therefore this argument still applies:

• programmers that are used to call contains on a Vec are confused by the non-existence of the method for VecDeque or LinkedList

I also want to avoid creating many solutions for one programming task. I can already imagine confused students, asking about the difference of slice.contains(...) and slice.iter().contains(...). Or asking why you can write vec.contains(...), but need to write vec_deq.iter().contains(...).

I'm not overly happy with the current system due to inconsistencies, but I think adding contains to the remaining collections would make std more consistent. Also adding it to Iterator may be a good idea...

[ticki]

#1434

[alexcrichton]

🔔 This RFC is now entering its week-long final comment period 🔔

Note, though, that the libs team also figured that small RFCs like this which already fit existing conventions are also appropriate to just send a PR :)

[mattico]

With #1434 merged, if this gets merged we'll have .contains() for everything that it makes sense for except iterators (unless I've missed something). I think that's a good argument for implementing .contains() for iterators as well.

vec.contains(3); // works
hash_table.contains(3); // works
(0..100).contains(3); // works
"onomatopoeia".contains("e"); // works
vec.iter().map(|x| x*3).contains(3); // doesn't work?

Certainly there's opportunity for someone to unnecessarily .iter() something which natively implements .contains(), subtley reducing performance. I think it would be more likely that someone would be surprised that Iterator is missing this obvious (to me at least) method.

[alexcrichton]

The libs team discussed this RFC during triage today and the decision was to merge. Thanks again for the RFC @LukasKalbertodt!

[alexcrichton]

Tracking issue: rust-lang/rust#32630