rust - `if` condition remains borrowed in body -

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• why borrow still held in else block of if let? 4 answers

i came across behaviour in rust (1.12) cannot explain. have struct implements lazy loading refcell<option<i32>> , function access data:

struct foo {     data: refcell<option<i32>> }  impl foo {     fn get_data(&self) -> i32 {         if self.data.borrow().is_none() { // <--- (a)             let d = 1337;             self.data.borrow_mut() = some(d); // <--- (b)             d         } else {             self.data.borrow().unwrap()         }     } } 

this compiles yields runtime error: refcell complains borrow active when attempting borrow_mut on line (b). issue not occur, however, if change if statement following:

let is_none = self.data.borrow().is_none(); if is_none { 

question: why borrow in if condition on line (a) still active inside body of if statement? shouldn't call is_none() cause borrow end, since i'm holding on bool afterwards, , not borrowed value?

this due rust's lexical borrowing scope.

as have noticed, borrowing in condition borrows branch. discussed in this issue, , has since moved here.

this limitation fixed in rust 1.13 beta , released in next cycle.