Merge pull request #526 from yuzibo/drop-obsolete-section

Remove obsolete improperly reduced borrows example

Author: JohnTitor

src/lifetime-mismatch.md

@@ -71,53 +71,4 @@ blows up in our face!
 This program is clearly correct according to the reference semantics we actually
 care about, but the lifetime system is too coarse-grained to handle that.
 
-## Improperly reduced borrows
-
-The following code fails to compile, because Rust sees that a variable, `map`,
-is borrowed twice, and can not infer that the first borrow ceases to be needed
-before the second one occurs. This is caused by Rust conservatively falling back
-to using a whole scope for the first borrow. This will eventually get fixed.
-
-```rust,compile_fail
-# use std::collections::HashMap;
-# use std::hash::Hash;
-fn get_default<'m, K, V>(map: &'m mut HashMap<K, V>, key: K) -> &'m mut V
-where
-    K: Clone + Eq + Hash,
-    V: Default,
-{
-    match map.get_mut(&key) {
-        Some(value) => value,
-        None => {
-            map.insert(key.clone(), V::default());
-            map.get_mut(&key).unwrap()
-        }
-    }
-}
-```
-
-Because of the lifetime restrictions imposed, `&mut map`'s lifetime
-overlaps other mutable borrows, resulting in a compile error:
-
-```text
-error[E0499]: cannot borrow `*map` as mutable more than once at a time
-  --> src/main.rs:12:13
-   |
-4  |   fn get_default<'m, K, V>(map: &'m mut HashMap<K, V>, key: K) -> &'m mut V
-   |                  -- lifetime `'m` defined here
-...
-9  |       match map.get_mut(&key) {
-   |       -     --- first mutable borrow occurs here
-   |  _____|
-   | |
-10 | |         Some(value) => value,
-11 | |         None => {
-12 | |             map.insert(key.clone(), V::default());
-   | |             ^^^ second mutable borrow occurs here
-13 | |             map.get_mut(&key).unwrap()
-14 | |         }
-15 | |     }
-   | |_____- returning this value requires that `*map` is borrowed for `'m`
-```
-
 [ex2]: lifetimes.html#example-aliasing-a-mutable-reference