Add documentation for let-else.

src/SUMMARY.md

@@ -59,6 +59,7 @@
         - [Guards](flow_control/match/guard.md)
         - [Binding](flow_control/match/binding.md)
     - [if let](flow_control/if_let.md)
+    - [let-else](flow_control/let_else.md)
     - [while let](flow_control/while_let.md)
 
 - [Functions](fn.md)

src/flow_control/let_else.md

@@ -0,0 +1,60 @@
+# let-else
+
+
+> 🛈 stable since: rust 1.65
+
+
+With `let`-`else`, a refutable pattern can match and bind variables
+in the surrounding scope like a normal `let`, or else diverge (e.g. `break`,
+`return`, `panic!`) when the pattern doesn't match.
+
+```rust
+use std::str::FromStr;
+
+fn get_count_item(s: &str) -> (u64, &str) {
+    let mut it = s.split(' ');
+    let (Some(count_str), Some(item)) = (it.next(), it.next()) else {
+        panic!("Can't segment count item pair: '{s}'");
+    };
+    let Ok(count) = u64::from_str(count_str) else {
+        panic!("Can't parse integer: '{count_str}'");
+    };
+    (count, item)
+}
+
+assert_eq!(get_count_item("3 chairs"), (3, "chairs"));
+```
+
+The scope of name bindings is the main thing that makes this different from
+`match` or `if let`-`else` expressions. You could previously approximate these
+patterns with an unfortunate bit of repetition and an outer `let`:
+
+```rust
+# use std::str::FromStr;
+# 
+# fn get_count_item(s: &str) -> (u64, &str) {
+#     let mut it = s.split(' ');
+    let (count_str, item) = match (it.next(), it.next()) {
+        (Some(count_str), Some(item)) => (count_str, item),
+        _ => panic!("Can't segment count item pair: '{s}'"),
+    };
+    let count = if let Ok(count) = u64::from_str(count_str) {
+        count
+    } else {
+        panic!("Can't parse integer: '{count_str}'");
+    };
+#     (count, item)
+# }
+# 
+# assert_eq!(get_count_item("3 chairs"), (3, "chairs"));
+```
+
+### See also:
+
+[option][option], [match][match], [if let][if_let] and the [let-else RFC][let_else_rfc].
+
+
+[match]: ./match.md
+[if_let]: ./if_let.md
+[let_else_rfc]: https://rust-lang.github.io/rfcs/3137-let-else.html
+[option]: ../std/option.md