fix(AIP-149): add backwards compatibility section (#1366)

Author: noahdietz

aip/general/0149.md

@@ -50,6 +50,17 @@ field behavior as defined in [AIP-203][]. For example, a field labeled with
 `google.api.field_behavior = REQUIRED` if the field must be set. If you only
 want to document the server perceived behavior of a field, read [AIP-203][].
 
+### Backwards compatibility
+
+It is a backwards incompatible change to add or remove the `optional` qualifier
+to an existing field. This is because the compiled src API is changed (in some
+languages). For example, in Golang, adding `optional` changes the field type of
+primitives to be the pointer variant of their original type, e.g. a field
+formerly of type `string` becomes `*string`, etc. Accordingly, this change
+requires that both clients and servers update their usage of the changed field
+in unison, which is risky and error prone. Additional information is
+[documented by Protobuf][proto docs].
+
 ## Rationale
 
 ### field behavior and `optional`
@@ -66,6 +77,8 @@ unrelated in practice and can reasonably be used together.
 
 ## Changelog
 
+- **2024-06-05**: Add backwards compatibility considerations
 - **2023-06-20**: Differentiate from field behavior documentation
 
 [AIP-203]: ./0203.md
+[proto docs]: https://protobuf.dev/programming-guides/field_presence/#considerations-for-change-compatibility

aip/general/0180.md

@@ -242,9 +242,11 @@ version.
 - For understanding stability levels and expectations, see [AIP-181][].
 - For compatibility with client library resource name parsing, see [AIP-4231][]
 - For compatibility with client library method signatures, see [AIP-4232][]
+- For compatibility around field presence changes, see [AIP-149][].
 
 ## Changelog
 
+- **2024-06-05**: Added reference to field presence compatibility.
 - **2023-07-26**: Added reference to field behavior compatibility.
 - **2023-07-26**: Added note on APIs which have limited clients.
 - **2022-08-11**: Added "Moving components between files" section.
@@ -254,6 +256,7 @@ version.
 
 <!-- prettier-ignore-start -->
 [aip-122]: ./0122.md
+[aip-149]: ./0149.md
 [aip-151]: ./0151.md
 [aip-158]: ./0158.md
 [aip-181]: ./0181.md