Replacing a trait bound with a trait alias says “the size for values cannot be known at compilation time” [duplicate]

问题

After reading about trait aliases, I tried to switch this code:

pub fn authorize<LoadClient>(get_client: LoadClient) -> Result<String, ()> 
where
   LoadClient: FnOnce(String) -> Result<Client, LoadingError>,
{
    unimplemented!()
}

#![feature(trait_alias)]

trait LoadClient = FnOnce(String) -> Result<Client, ClientLoadingError>;

pub fn authorize(get_client: LoadClient) -> Result<String, ()> {
    unimplemented!()
}

This gives me an error:

warning: trait objects without an explicit `dyn` are deprecated
   --> src/oauth2/mod.rs:396:21
    |
396 |         get_client: LoadClient,
    |                     ^^^^^^^^^^ help: use `dyn`: `dyn LoadClient`
    |
    = note: `#[warn(bare_trait_objects)]` on by default

error[E0277]: the size for values of type `(dyn std::ops::FnOnce(std::string::String) -> std::result::Result<oauth2::Client, oauth2::ClientLoadingError> + 'static)` cannot be known at compilation time
   --> src/oauth2/mod.rs:396:9
    |
396 |         get_client: LoadClient,
    |         ^^^^^^^^^^ doesn't have a size known at compile-time
    |

Is it possible to use a trait alias in this way? This function is used elsewhere so it would be nice to have a shared definition rather than redefine it at each place it's used.

This is related to Can match on Result here be replaced with map_err and "?"

回答1:

You still need to use it as generic parameter:

#![feature(trait_alias)]

pub trait LoadClient = FnOnce(String) -> Result<Client, ClientLoadingError>;

pub fn authorize<T>(get_client: T) -> Result<String, ()>
where
    T: LoadClient,
{
    unimplemented!()
}

You also need to make the trait alias public since it is part of a public interface.

回答2:

It seems this was just a naive mistake related to using traits as arguments, rather than anything specific to trait aliases. I need to use impl LoadClient as described in the rust book when using traits as function parameters.

With the signature

pub fn authorize(get_client: impl LoadClient) -> Result<String, ()>

the code compiles.

来源：https://stackoverflow.com/questions/59720444/replacing-a-trait-bound-with-a-trait-alias-says-the-size-for-values-cannot-be-k

标签

rust