Type inference complains about unknown size for dyn Any

[Wasabi375]

Inferring the resulting type of Box::downcast_unsafe complains about dyn Any not being sized. However if the type is specified directly, everything works as expected.

I tried this code:

pub struct BoxWrapper<T: Any> {
    data: Box<dyn Any>,

    _phantom: PhantomData<T>,
}

impl<T: Any> BoxWrapper<T> {

    pub fn downcast<TTarget: Any>(self) -> Result<BoxWrapper<TTarget>, Self> {
        if !self.data.is::<TTarget>() {
            return Err(self);
        }

        // let data: Box<TTarget> works fine
        let data: Box<_> = unsafe { self.data.downcast_unchecked() };

        Ok(BoxWrapper {
            data,
            _phantom: PhantomData,
        })
    }
}

This creates the following error message:

error[E0277]: the size for values of type `dyn Any` cannot be known at compilation time
    --> src/lib.rs:22:47
     |
22   |         let data: Box<_> = unsafe { self.data.downcast_unchecked() };
     |                                               ^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
     |
     = help: the trait `Sized` is not implemented for `dyn Any`
note: required by an implicit `Sized` bound in `Box::<(dyn Any + 'static), A>::downcast_unchecked`
    --> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/boxed.rs:1820:38
     |
1820 |     pub unsafe fn downcast_unchecked<T: Any>(self) -> Box<T, A> {
     |                                      ^ required by the implicit `Sized` requirement on this type parameter in `Box::<dyn Any, A>::downcast_unchecked`

However specifying the type directly (let data: Box<TTarget> = ... ) does not produce an error.

I would expect to either get no error message, or an error stating that the type can not be inferred.

Meta

rustc --version --verbose:

cargo 1.79.0-nightly (48eca1b16 2024-04-12)
release: 1.79.0-nightly
commit-hash: 48eca1b164695022295ce466b64b44e4e0228b08
commit-date: 2024-04-12
host: x86_64-unknown-linux-gnu
libgit2: 1.7.2 (sys:0.18.3 vendored)
libcurl: 8.6.0-DEV (sys:0.4.72+curl-8.6.0 vendored ssl:OpenSSL/1.1.1w)
ssl: OpenSSL 1.1.1w  11 Sep 2023
os: NixOS 23.11.0 [64-bit]

[marc0246]

The type you defined in your struct is Box<dyn Any>. This is the type that gets correctly inferred. It also correctly can't be downcast to because it's unsized. If you specify a concrete type to downcast to, what you are doing is downcasting and then unsizing again, which is really a no-op.

[marc0246]

@Wasabi375 Do you see why this is working as intended?

[Wasabi375]

Yeah, not sure how I got myself confused here. Thanks for the help anyways.