Newbie: str.as_bytes -> mem::transmute documentation vs reality

str.as_bytes uses mem::transmute. The mem::transmute documentation says that this copies the bytes.

But I find it hard to believe that as_bytes, which is used everywhere, include str.len, would be so involved. Further, a simple test shows that if you mutate the returned bytes, the original string is mutated.

So, either I'm misunderstanding mem::transmute's documentation, or I'm missing something very obvious.

A &str is represented in the following manner:

struct Str {
    data: *const u8,
    len: usize,

A &[u8] is represented in the following manner:

struct ByteSlice {
    data: *const u8,
    len: usize,

When as_bytes uses transmute, the bytes that are copied are the 16 bytes that make up that pointer and length. The data behind the pointer is not copied.


That makes some sense re mem::transmute.

But I'm still confused, if str has a len, why not use that directly? Maybe I'm looking at the wrong thing?

The language doesn't have any built-in syntax for accessing the length field of a &str. It's a special type provided by the compiler.


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