Adding trait bound causes loss of some method/capability

help
1

I defined some customized vector type:

pub enum LocalStorageVec<T, const N: usize> {
    Stack { buf: [T; N], len: usize },
    Heap(Vec<T>),
}

Then I want to implement the Index trait for it:

impl<T, const N: usize, I> std::ops::Index<I> for LocalStorageVec<T, N>
where
    I: std::slice::SliceIndex<[T]>,
    // [T]: std::ops::Index<I>, // Uncomment would cause compile error.
{
    type Output = I::Output;

    fn index(&self, index: I) -> &Self::Output {
        match self {
            LocalStorageVec::Stack { buf, len } => buf[0..*len].index(index),
            LocalStorageVec::Heap(data) => &data[index],
        }
    }
}

First, the trait bound [T]: std::ops::Index<I> seems useless, because it's implied in the std library:

impl<T, I> Index<I> for [T]
where
    I: SliceIndex<[T]>,
type Output = <I as SliceIndex<[T]>>::Output

So adding it should not have any effect at all? But actually, the compiler reports an error:

mismatched types
expected type parameter `I`
           found struct `std::ops::Range<usize>`

In addition, (I thought) trait bounds should bring some capability instead of causing the loss of some. Why is the opposite happening in this case?

2

Looks like another instance of the known bug - Trait impls from where clauses (`ParamEnv`) take precedence over freestanding trait impls · Issue #24066 · rust-lang/rust · GitHub.

2 Likes
3

This topic was automatically closed 90 days after the last reply. We invite you to open a new topic if you have further questions or comments.