'Nullability Update' upgrade guidance and info.

[Velfi]

This change has not yet been released. Track its status here.

The AWS Rust SDK and smithy-rs code generator have both been updated to respect smithy's @required trait. This means that many struct fields will no longer be wrapped in an unnecessary Option, with some caveats.

Codegen now respects nullability

• Required struct members that target simple types will no longer be wrapped in an Option.
• For AWS SDKs, required struct members that target structures or unions will continue to be treated as optional because these members can never transition from @required to optional using a @default trait.
• When generating generic smithy clients with default settings, required struct members that target structures or unions will no longer be wrapped in an Option. However, different "nullability check modes" may be specified during codegen to influence this. To do so, set the nullabilityCheckMode option in your smithy.json to either of the following values:
  • CLIENT (The default for generic smithy clients.)
  • CLIENT_CAREFUL (The default for generating AWS SDKs.)

e.g.

{
  "plugins": {
    "rust-client-codegen": {
      "codegen": {
        "nullabilityCheckMode": "CLIENT_CAREFUL"
      }
    }
  }
}

How to upgrade your usage of AWS SDKs if it was broken by this update.

Before this change, it was common for users to unwrap required struct fields. After this change, compiling projects with code like this will result in errors similar to these:

For builders that used to be fallible but aren't anymore:

error[E0599]: no method named `unwrap` found for type `SomeStruct` in the current scope
  --> my-crate/src/main.rs:XX:XX
   |
XX |         .some_struct(SomeStruct::builder().build().unwrap())
   |                                                   ^^^^^^^ method not found in `SomeStruct`

For struct fields that used to be Option-wrapped by aren't anymore:

error[E0599]: no method named `unwrap` found for reference `&str` in the current scope
  --> my-crate/src/main.rs:XX:XX
   |
XX |     let name = resp.name().unwrap();
   |                            ^^^^^^ method not found in `&str`

To fix these errors, simply remove the calls to .unwrap():

.some_struct(SomeStruct::builder().build())

 let name = resp.name();