Adding upstream version 1.64.0+dfsg1.upstream/1.64.0+dfsg1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+% if let
+
+<small>There is a new edition of the book and this is an old link.</small>
+
+> The `if let` syntax lets you combine `if` and `let` into a less verbose way to handle values that match one pattern and ignore the rest.
+
+```rust
+let some_u8_value = Some(3u8);
+if let Some(3) = some_u8_value {
+    println!("three");
+}
+```
+
+---
+
+Here are the relevant sections in the new and old books:
+
+* [In the current edition: Ch 6.03 — Concise Control Flow with `if let`][2]
+* <small>[In the first edition: Ch 3.21 — if let][1]</small>
+
+[1]: https://doc.rust-lang.org/1.30.0/book/first-edition/if-let.html
+[2]: ch06-03-if-let.html