Dyn trait here but not there

[Coding-Badly]

Hello!

#[derive(Clone, Copy, Debug)]
pub struct PortRange {
    first: u16,
    last: u16,
}

pub trait ToPortRange {
    fn to_port_range(&self) -> PortRange;
}

pub struct AllPorts {}

impl ToPortRange for AllPorts {
    fn to_port_range(&self) -> PortRange {
        PortRange {
            first: 1,
            last: 65353,
        }
    }
}

struct BrightLightUdpClientBuilder<R> {
    port_ranges: R,
}

impl<'a, R> BrightLightUdpClientBuilder<R>
where
    R: IntoIterator<Item = &'a dyn ToPortRange>,
{
    fn new(port_ranges: R) -> Self {
        Self {
            port_ranges,
        }
    }
    fn build(self) {
        for pr in self.port_ranges.into_iter() {
            println!("{:?}", pr.to_port_range());
        }
    }
}

fn main() -> Result<(), Box<dyn std::error::Error>> {
    println!();

    let port_ranges: Option<&dyn ToPortRange> = Some(&AllPorts{});
    let builder = BrightLightUdpClientBuilder::new(port_ranges);
    builder.build();

    // --- Why does this not work when the one above does?
    let builder = BrightLightUdpClientBuilder::new(Some(&AllPorts{}));
    //builder.build();
/*
error[E0271]: type mismatch resolving `<Option<&AllPorts> as IntoIterator>::Item == &dyn ToPortRange`
   --> src\main.rs:113:19
    |
113 |     let builder = BrightLightUdpClientBuilder::new(Some(&AllPorts{}));
    |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected trait object `dyn ToPortRange`, found struct `AllPorts`
    |
    = note: expected reference `&dyn ToPortRange`
               found reference `&AllPorts`
note: required by a bound in `BrightLightUdpClientBuilder::<R>::new`
   --> src\main.rs:88:21
    |
88  |     R: IntoIterator<Item = &'a dyn ToPortRange>,
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `BrightLightUdpClientBuilder::<R>::new`
89  | {
90  |     fn new(port_ranges: R) -> Self {
    |        --- required by a bound in this
*/

    println!();
    Ok(())
}

Playground.

I'd really like let builder = BrightLightUdpClientBuilder::new(Some(&AllPorts{})); to work (in addition to being able to pass a Vec). I just cannot find an incantation that appeases the compiler.

Is it possible to coerce the &AllPorts to be a &dyn ToPortRange?

[Cyborus]

You can cast it to a trait object with as:

let builder = BrightLightUdpClientBuilder::new(Some(&AllPorts{} as &dyn ToPortRange));

The compiler's just not making this coercion on its own

[quinedot]

You can tell the compiler you want it to infer some coercion via cast like so:

-    let builder = BrightLightUdpClientBuilder::new(Some(&AllPorts{}));
+    let builder = BrightLightUdpClientBuilder::new(Some(&AllPorts{} as _));

And then the compiler can figure it out for this example.

---

The top version works because you're initializing with an explicit type. The original bottom version does not work implicitly as such implicit coercions are not performed when matching traits.

[Coding-Badly]

Thank you both!