What are the possible operators for traits in a where clause in Rust?

Question

I'm learning Rust and got to the chapter on trait bounds. In that chapter, they give an example with the + (plus) operator to enumerate all required traits in a where clause.

What other operators are allowed on traits in Rust? I searched but I wasn't able to find any documentation about this. Does Rust support 'or' notation, brackets, negation?

score 4 · Accepted Answer · answered Oct 16 '17 at 21:42

4

'or' notation

No. Such a thing doesn't make sense to me — what would the code do if something could be A OR B?

brackets

No, unless you count the parenthesis in Fn(...) and friends, the use of angle brackets as generics (Vec<T>) or the use of square brackets in slices ([u8]), but these are parts of types.

Again, such a request doesn't make sense to me, why would you need any kind of grouping?

negation

No. I believe this is due to the fact that handling negative logic makes things a lot more complicated.

other operators

There's the ? (question mark) bound.

Source: I've written a parser of Rust code. While it's possible I've missed something, it parses all the ~37500 Rust files I've thrown at it, so it's a pretty good fit.

answered Oct 16 '17 at 21:42

Shepmaster

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Than you. I think, if Rust support optional traits, one can create 'or' operator as ?Trait1 + ?Trait2. Or not? – George Shuklin Oct 17 '17 at 11:44
@GeorgeShuklin Rust does **not** support optional traits. The `?` (question mark) only applies to the specific trait `Sized`. For ergonomics, every generic type has an implicit `T: Sized` bound *unless* you remove that default bound using `?Sized`. – Shepmaster Oct 17 '17 at 12:02
It's not available, but it does make sense. Say you have a function like `fn print_related_stuff(a: A, b: B){ print(a.to_string(), b.to_string()) }`. A where clause like `where A:Stringable, B:Stringable, (A == B or (A: ISomething and B: ISomethingElse))` makes a lot of sense to me if you want the compiler to not allow just any `Stringable` types in the function, but rather to only allow types that are related in any one of several possible relationships – user986730 Aug 15 '20 at 22:15
@user986730 that specific example would be achieved by using only one generic type, which will ensure that the two values have the same type. – Shepmaster Aug 17 '20 at 12:47
I am still not that experienced with rust, so not very sure. Still I think I needed that `or` thing today. Wanted to have a trait that with supertrait BitOr. T could be any integer (u8, u32, i32 etc). It could be done by specifying trait bounds on the trait itself but then trait could be implemented multiple times and also funcs receiving my trait as arg will have unecessary generic bounds. As the trait implemented `BitOr` with any integer, bit or could be done on type implementing my trait with an integer expression (whose type i32, i64 etc would be inferred automatically). Contd.. – Mihir Luthra Nov 17 '20 at 15:19
`BitOr` would be nice but it says this is only possible in functions. I am not sure if it's even possible but will there by something like `BitOr` in super traits possible in future versions of rust? – Mihir Luthra Nov 17 '20 at 15:21
@Mihir I've read your comment a few times, but I'm not sure that I understand your goal. What I'd suggest is to create a [MRE] that demonstrates what you'd _like_ to do and then post a top-level question instead of a comment, That way, we can answer the question or identify any possible duplicates, but with a lot less guessing. – Shepmaster Nov 17 '20 at 18:24
@Shepmaster, I switched to a different approach. But here is an example of what I was trying to do: [example](https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=823333393886039b4ec7fe3af7274a76) – Mihir Luthra Nov 18 '20 at 00:06

What are the possible operators for traits in a where clause in Rust?

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