From b5fbbf9b734a4a6702c210aba50687a066f23406 Mon Sep 17 00:00:00 2001
From: Matt Keeter <matt.j.keeter@gmail.com>
Date: Sun, 17 Mar 2024 14:20:49 -0400
Subject: [PATCH] Update spec for `Option<T>` encoding (#702)

* Update spec for Option<T> encoding

* Fix header rank
---
 docs/spec.md | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/docs/spec.md b/docs/spec.md
index 02d3095b..d7456a2d 100644
--- a/docs/spec.md
+++ b/docs/spec.md
@@ -86,6 +86,23 @@ assert_eq!(encoded.as_slice(), &[
 ]);
 ```
 
+### Options
+`Option<T>` is always serialized using a single byte for the discriminant, even in `Fixint` encoding (which normally uses a `u32` for discriminant).
+
+```rust
+let data: Option<u32> = Some(123);
+let encoded = bincode::encode_to_vec(data, bincode::config::legacy()).unwrap();
+assert_eq!(encoded.as_slice(), &[
+    1, 123, 0, 0, 0  // the Some(..) tag is the leading 1
+]);
+
+let data: Option<u32> = None;
+let encoded = bincode::encode_to_vec(data, bincode::config::legacy()).unwrap();
+assert_eq!(encoded.as_slice(), &[
+    0 // the None tag is simply 0
+]);
+```
+
 # Collections
 
 Collections are encoded with their length value first, following by each entry of the collection. The length value is based on your `IntEncoding`.