rust - Why is a borrow still held in the else block of an if let? -

why call self.f2() in following code trip borrow checker? isn't else block in different scope? quite conundrum!

use std::str::chars;  struct a;  impl {     fn f2(&mut self) {}      fn f1(&mut self) -> option<chars> {         none     }      fn f3(&mut self) {         if let some(x) = self.f1() {          } else {             self.f2()         }     } }  fn main() {     let mut = a; } 

playground

error[e0499]: cannot borrow `*self` mutable more once @ time   --> src/main.rs:16:13    | 13 |         if let some(x) = self.f1() {    |                          ---- first mutable borrow occurs here ... 16 |             self.f2()    |             ^^^^ second mutable borrow occurs here 17 |         }    |         - first borrow ends here 

doesn't scope of borrow self begin , end self.f1() call? once call f1() has returned f1() not using self anymore hence borrow checker should not have problem second borrow. note following code fails too...

// ... if let some(x) = self.f1() {     self.f2() } // ... 

playground

i think second borrow should fine here since f1 , f3 not using self @ same time f2.

it's annoying, can work around introducing inner scope , changing control flow bit:

fn f3(&mut self) {     {         if let some(x) = self.f1() {             // ...             return;         }     }     self.f2() } 

as pointed out in comments, works without braces. because if or if...let expression has implicit scope, , borrow lasts scope:

fn f3(&mut self) {     if let some(x) = self.f1() {         // ...         return;     }      self.f2() } 

here's log of irc chat between sandeep datta , mbrubeck:

mbrubeck: std:tr::chars contains borrowed reference string created it. full type name chars<'a>. f1(&mut self) -> option<chars> without elision f1(&'a mut self) -> option<chars<'a>> means self remains borrowed long return value f1 in scope.

sandeep datta: can use 'b self , 'a chars avoid problem?

mbrubeck: not if returning iterator on self. though if can make function &self -> chars (instead of &mut self -> chars) fix issue.