问题描述
Rust的 std :: ops :: Index< T> ，接受密钥类型可以借用的任何类型（因此您可以使用& str HashMap< String，_> $ c>）。
This is just not true. HashMap, for example, accepts any type that the key type can borrow to (so you can index a HashMap<String, _> using a &str).
切片只允许您使用 usize 对其进行索引，所有容器类型都假装是或实际 ，在内存中是线性的。那是因为 usize 是用于索引它们的正确的类型。任何其他类型将无法访问容器的完整潜在范围，或者将允许不可能存在的索引。
Slices only allow you to index them using usize, as do all container types that either pretend to be, or actually are, linear in memory. That's because usize is the correct type with which to index them. Any other type would either not be able to access the full potential range of the container, or would allow for indices that cannot possibly exist.
人们过去曾询问过添加较小的类型以进行切片索引，但这将是向后不兼容的，因此不会很快发生。
People have asked in the past about adding smaller types to slice indexing, but this would be backward-incompatible, so it's not going to happen any time soon.
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