In my Rust program, I’m getting back values of the form *mut Foo from a function written in C.

let x: *mut Foo = my_ffi_function();

What does this “*mut” notation even mean?

As far as I can tell, x is just a pointer to a mutable Foo struct, the same as &mut Foo.

This code compiles just fine:

    let mut s: String = String::new();
    let x: &mut String = &mut s;
    let y: *mut String = x;

What’s difference between the two forms?