Automatic trait implementation does not work - rust

Question

On the code below, you can see that I forced the implementation of A for everything that implements AsRef<[T]> and Index[T], so B should implement it, because it implements both of these. However this is not the case. Why?
use std::convert::{AsMut, AsRef};
use std::ops::{Index, IndexMut};
use std::slice::SliceIndex;
pub trait A<T, I: SliceIndex<[T], Output = T>>:
AsRef<[T]> + Index<I, Output = T>
{
fn len(&self) -> usize;
}
impl<
T: AsRef<[T]> + Index<I, Output = T>,
I: SliceIndex<[T], Output = T>,
> A<T, I> for T
{
fn len(&self) -> usize {
self.as_ref().len()
}
}
struct B<'a, T>{
data: &'a mut [T]
}
impl<'a, T> AsRef<[T]> for B<'a, T> {
fn as_ref(&self) -> &[T] {
self.data
}
}
impl<'a, T, I: SliceIndex<[T], Output = T>> Index<I> for B<'a, T> {
type Output = T;
fn index(
&self,
v: I,
) -> &Self::Output {
&self.as_ref()[v]
}
}
fn something<'a, T>(r: &'a mut[T]) -> Box<dyn A<T, usize>> {
let a = B{data: r};
Box::new(a)
}
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=aa2353139f3d097e2497ed700d678ed3
Error:
error[E0277]: the trait bound `B<'_, T>: A<T, usize>` is not satisfied
--> src/lib.rs:46:5
|
46 | Box::new(a)
| ^^^^^^^^^^^ the trait `A<T, usize>` is not implemented for `B<'_, T>`
|
= note: required for the cast to the object type `dyn A<T, usize, Output = T>`

Answer 1

You need to introduce an additional type parameter on the blanket impl for A:
impl<T, I, V> A<T, I> for V
where
V: AsRef<[T]> + Index<I, Output = T>,
I: SliceIndex<[T], Output = T>,
{
fn len(&self) -> usize {
self.as_ref().len()
}
}
After fixing that, you'll get a lifetime error since the trait object returned in the box infers a 'static lifetime, so you'll need to bind it to the input slice's 'a lifetime.
fn something<'a, T>(r: &'a mut [T]) -> Box<dyn A<T, usize> + 'a> {
let a = B { data: r };
Box::new(a)
}