Document if/while let chains

Author: est31

src/SUMMARY.md

@@ -42,6 +42,7 @@
   - [Language](rust-2024/language.md)
     - [RPIT lifetime capture rules](rust-2024/rpit-lifetime-capture.md)
     - [`if let` temporary scope](rust-2024/temporary-if-let-scope.md)
+    - [`let` chains in `if` and `while`](rust-2024/let-chains.md)
     - [Tail expression temporary scope](rust-2024/temporary-tail-expr-scope.md)
     - [Match ergonomics](rust-2024/match-ergonomics.md)
     - [Unsafe `extern` blocks](rust-2024/unsafe-extern.md)

src/rust-2024/let-chains.md

@@ -0,0 +1,37 @@
+# `let` chains in `if` and `while`
+
+## Summary
+
+- Allow chaining of `let` expressions in the condition operand of `if` and `while`.
+
+## Details
+
+Starting with the 2024 Edition, it is now allowed to have chaining of `let` expressions inside `if` and `while` condition operands,
+where chaining refers to `&&` chains. The `let` expressions still have to appear at the top level,
+so `if (let Some(hi) = foo || let Some(hi) = bar)` is not allowed.
+
+Before 2024, the `let` had to appear directly after the `if` or `while`, forming a `if let` or `while let` special variant.
+Now, `if` and `while` allow chains of one or more `let` expressions, possibly mixed with expressions that are `bool` typed.
+
+<!-- TODO: edition2024 -->
+```rust
+fn sum_first_two(nums: &[u8]) -> Option<u8> {
+    let mut iter = nums.iter();
+    if let Some(first) = iter.next()
+        && let Some(second) = iter.next()
+    {
+        first.checked_add(second)
+    } else {
+        None
+    }
+}
+```
+
+The feature is edition gated due to requiring [if let rescoping], which is a Edition 2024 change.
+
+## Migration
+
+The switch to Edition 2024 doesn't neccessitate any migrations due to this feature,
+as it creates a true extension of the set of allowed Rust programs.
+
+[if let rescoping]: temporary-if-let-scope.html