Cannot infer type for Option<T> when passed None

help
#1

I have a struct with generics like this:

struct Foo<T> {
    member: Option<T>,
}

impl<T> Foo<T> {
    pub fn new(member: Option<T>) -> Self {
        Foo { member }
    }
}

but when calling new with None, Rust cannot infer the type for T:

let y = Foo::new(None);

19 |     let y = Foo::new(None);
   |         -   ^^^^^^^^ cannot infer type for type parameter `T`
   |         |
   |         consider giving `y` the explicit type `Foo<T>`, where the type parameter `T` is specified

but what type should I specify for this None input? Here is my playground test.

#2

If you don't care what type you get, just pick any type. The unit type () is a reasonable default choice that shows you aren't using the type for anything:

let y: Foo<()> = Foo::new(None);

On the other hand, if you want to later do something with y that requires a specific type, then you can pick that type explicitly, or let it be inferred based on that later usage.

#3

Thanks, it will work for this case. I missed that in my real code there are trait bounds also for T. A simplified version is like this:

struct Foo<T> 
where T: Fn(usize) -> usize,
{
    member: Option<T>,
}

I would get error for the following:

    let y: Foo<()> = Foo::new(None);

error:

27 |     let y: Foo<()> = Foo::new(None);
   |            ^^^^^^^ expected an `Fn<(usize,)>` closure, found `()`
   |

I am asking because the actual trait bounds in my code is more complex and I was wondering if there is a simple way to infer or specify the type here.

#4

In this specific case you could probably do something like this:

let y: Foo<_> = Foo::new(None);
1 Like
#5

None with a type hint can be written as None::<()>.

1 Like
#6

None::<fn(usize) -> usize> should work.

The lowercase fn types are function pointers, and are the only nameable types that meet Fn trait bounds.

#7

It seems to me that the trait bounds in my code is too complex for adopting the suggested approaches, I ended up with a different design that don't need the Option<T> . Somehow I guess the complexity of generics / trait bounds forced me to find a simpler (and better I hope) design :wink:

Btw, the trait bounds for Option<F> in my code was something like:

    where
        F: Fn(Request<Body>) -> S + Send + Clone + 'static,
        S: Future<Output = Response<Body>>,
#8

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