compilation warning#18

Rustc complains about multiple mutable borrows at the same time. I'm a bit worried about the "warning will become a hard error in the future" part. `rustc 1.36.0-nightly (e305df184 2019-04-24)` ``` warning[E0499]: cannot borrow `*self` as mutable more than once at a time --> /home/vagrant/.cargo/registry/src/github.com-1ecc6299db9ec823/qp-trie-0.7.3/src/node.rs:502:41 | 479 | &mut self, | - let's call the lifetime of this reference `'1` ... 485 | / match *self { 486 | | Node::Branch(ref mut branch) if branch.choice <= graft => { | | -------------- first mutable borrow occurs here 487 | | let index = branch.index(key.borrow()); 488 | | ... | 502 | | let node = mem::replace(self, Node::Branch(Branch::new(graft))); | | ^^^^ second mutable borrow occurs here ... | 517 | | } 518 | | } | |_________- returning this value requires that `self.0` is borrowed for `'1` | = warning: this error has been downgraded to a warning for backwards compatibility with previous releases = warning: this represents potential undefined behavior in your code and this warning will become a hard error in the future warning[E0499]: cannot borrow `*self` as mutable more than once at a time --> /home/vagrant/.cargo/registry/src/github.com-1ecc6299db9ec823/qp-trie-0.7.3/src/node.rs:505:45 | 479 | &mut self, | - let's call the lifetime of this reference `'1` ... 485 | / match *self { 486 | | Node::Branch(ref mut branch) if branch.choice <= graft => { | | -------------- first mutable borrow occurs here 487 | | let index = branch.index(key.borrow()); 488 | | ... | 505 | | let graft_branch = unsafe { self.unwrap_branch_mut() }; | | ^^^^ second mutable borrow occurs here ... | 517 | | } 518 | | } | |_________- returning this value requires that `self.0` is borrowed for `'1` | = warning: this error has been downgraded to a warning for backwards compatibility with previous releases = warning: this represents potential undefined behavior in your code and this warning will become a hard error in the future ```