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Comments on How can I define a method on [&mut T] where T: MyTrait?
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How can I define a method on [&mut T] where T: MyTrait?
I'm trying to make a trait method callable on mutable slices of a type implementing that trait (see main). Rust doesn't like me for it.
use core::convert::AsMut;
trait A {
type B;
fn f(m: &mut [Self], b: Self::B);
}
impl<B, M: AsMut<[impl A<B=B>]>> M {
fn f(&mut self, b: B) {
A::f(self.as_mut(), b)
}
}
struct C (u64);
impl A for C {
type B = u8;
fn f(m: &mut [Self], b: u8) {
for c in m {
c.0 += b;
}
}
}
fn main() {
let v = vec![C(1), C(2), C(3), C(4)];
v.f(5);
assert_eq!(v, [C(6), C(7), C(8), C(9)]);
}
This gives me errors I don't understand, which is unusual for Rust:
Compiling playground v0.0.1 (/playground)
error[E0207]: the type parameter `B` is not constrained by the impl trait, self type, or predicates
--> src/main.rs:8:6
|
8 | impl<B, M: AsMut<[impl A<B=B>]>> M {
| ^ unconstrained type parameter
error[E0207]: the type parameter `impl A<B = B>` is not constrained by the impl trait, self type, or predicates
--> src/main.rs:8:19
|
8 | impl<B, M: AsMut<[impl A<B=B>]>> M {
| ^^^^^^^^^^^ unconstrained type parameter
error: aborting due to 2 previous errors
For more information about this error, try `rustc --explain E0207`.
error: could not compile `playground`.
Rewording my code slightly doesn't fix this. Is what I'm trying to do even possible in Rust? If so, how?
Post
This error doesn't do a good job at all of highlighting the important thing!
You can't define an inherent impl on a type parameter. You have to make it the impl of some trait. Here's an example: Try it online!
trait A: Sized {
type B;
fn f(m: &mut [Self], b: Self::B);
}
trait AsMutExt<Inner: A> {
fn f(&mut self, b: Inner::B);
}
impl<Inner: A, M: AsMut<[Inner]>> AsMutExt<Inner> for M {
fn f(&mut self, b: Inner::B) {
A::f(self.as_mut(), b)
}
}
#[derive(PartialEq, Debug)]
struct C (u64);
impl A for C {
type B = u8;
fn f(m: &mut [Self], b: u8) {
for c in m {
c.0 += b as u64;
}
}
}
fn main() {
let mut v = vec![C(1), C(2), C(3), C(4)];
v.f(5);
assert_eq!(v, [C(6), C(7), C(8), C(9)]);
}
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