Add as_xyz helper methods to Smithy unions / Rust enums

[rcoh]

Each union variant will define a as_xyz() -> Option<&Xyz> and is_xyz() -> bool method to enable easily extracting individual variants from the union without matching.

These should also be added to top level & combined errors.

• Union Shapes
• Error shapes

[barbu110]

Would like to jump on this. As a newbie with Rust and this project, this would be a good start to look through the code.

[barbu110]

Hey, what would be the preferred approach between the following two:

1. Implement a derive macro (e.g. MemberConvertible) in the Rust runtime, or
2. Generate the impl and functions from the code generator?

To detail more about option 1, I was thinking of something like generating the following Rust code for unions:

#[derive(MemberConvertible)]
pub enum SomeUnion {
  Foo(Foo),
  Bar(Bar),
}

where this MemberConvertible macro would generate an impl SomeUnion with the is_xyz and as_xyz for every variant of the enum.

What do you think?

[rcoh]

In general, I would favor generating impl functions in Kotlin. Since we're code generating everything anyway, there isn't a compelling reason to build a derive macro. Adding a derive macro is what a "regular" Rust project would probably do but it introduces a bunch of extra complexity

[barbu110]

Cool, then I will go with option 2. I'll submit a PR by tomorrow.

[rcoh]

[reopened to also track adding the same thing to enums]