Rust String AsRef<T> does not auto borrow for [u8]

Question

Rust String implements both AsRef<str> and AsRef<[u8]>

But why does Rust correctly allow String to be borrowed as &str but not as &[u8] even when we provide explicit type annotation hint?

let s = String::from("hello");
let item: &str = &s; // Works
let item2: &[u8] = &s; // Fails

Ofcourse explicit invocation works, but curious why above doesn't work

let item2: &[u8] = s.as_ref();

Answer 1

When a &String coerces to &str, that happens via the Deref trait (not AsRef, and not Borrow). The Deref trait is not generic — it only allows one type to be chosen to “dereference to”. In the case of String, that's str.

The AsRef and Borrow traits are not special to the compiler. They are available for generic code to use explicitly, not implicitly.