The error message wasn't stellar when this question was asked. The same code (Playground) shows a better error message today:
error[E0277]: the size for values of type `dyn Foo` cannot be known at compilation time
--> src/main.rs:26:3
|
26 | call_foo(&foo);
| ^^^^^^^^ doesn't have a size known at compile-time
|
= help: the trait `std::marker::Sized` is not implemented for `dyn Foo`
= note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
= note: required because of the requirements on the impl of `Foo` for `std::boxed::Box<dyn Foo>`
note: required by `call_foo`
--> src/main.rs:20:1
|
20 | fn call_foo<F>(foo: &F) where F: Foo {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The error message links to The Rust Programming Language Book, Chapter 19, section 4, "Dynamically sized types and the Sized
trait".
The problem arises because by default, a generic type parameter is assumed to be Sized
. However, a trait object (the dyn Foo
in Box<dyn Foo>
) does not have a known size. That's acceptable in this case, so we modify the blanket implementation to allow boxes things with an unknown size:
impl<F: ?Sized> Foo for Box<F>
where F: Foo
Here is the fixed code, updated to the Rust 2018 edition:
trait Foo {
fn foo(&self);
}
impl Foo for () {
fn foo(&self) {
println!("hello world");
}
}
impl<F: ?Sized> Foo for Box<F>
where F: Foo
{
fn foo(&self) {
(**self).foo()
}
}
fn call_foo<F>(foo: &F)
where F: Foo
{
foo.foo()
}
fn main() {
let foo: Box<dyn Foo> = Box::new(());
call_foo(&foo);
}
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…