引用自<OReilly.Programming Rust> | Chapter 5 | Sharing Versus Mutation

You can reborrow from a mutable reference:
let mut v = (136, 139);
let m = &mut v;
let m0 = &mut m.0; // ok: reborrowing mutable from mutable
*m0 = 137;
let r1 = &m.1; // ok: reborrowing shared from mutable,
// and doesn't overlap with m0
v.1; // error: access through other paths still forbidden

有个疑问，既然Sharable 和 Mutable 不能同时存在，为何这里时合法的呢？

let r1 = &m.1; // ok: reborrowing shared from mutable,