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pnkfelix describes how you might go about fixing the third example.This removes the lifetime constraint on the variable and resolves the problem.fn main() test.rs:: error: cannot borrow `map` as immutable because it is also borrowed as mutable test.rs:4 map.insert(2, *map.get(&1)); // XXX ^~~ test.rs:4:2: 4:5 note: second borrow of `map` occurs here test.rs:4 map.insert(2, *map.get(&1)); // XXX ^~~ Fortunately, this bug is pretty easy to work around, if a little annoying: move all of your sub-expressions to let-bindings before the ill-fated mutable borrow (see examples for a worked example).Note: the borrows that occur in these sub-expressions really do have to be temporary; otherwise, you have a legitimate “cannot borrow mutable twice” error on your hands.The key points were as follows: What I want to talk about in this post is a proposal (or proto-proposal) for bridging those two worlds in Rust. I’ll defer virtual methods for another post (or perhaps an RFC). In any case, better late than never, as some-great-relative-or-other always (no doubt) said.they can be viewed as a special case of specialization. Truth is, I really miss blogging regularly, so I’m going to make an effort to write up more “in progress” and half-baked ideas (yeah yeah, promises to blog more are a dime a dozen, I know).Unfortunately, it’s not possible to insert a new lexical scope, as was possible in the previous example.
is involved, you can usually arrange for the misbehaving borrow to be performed outside of the match statement, in a new, non-overlapping lexical scope.When you borrow a pointer, Rust assigns it a lexical scope that constitutes its lifetime.