spurious dead code warning with trait-based API definition

[davepacheco]

With this code (a version of the code from #1128 where I've moved the trait definition to the top level):

#[dropshot::api_description]
trait MyApi {
    type Context;
}
fn main() {
    let description = my_api_mod::stub_api_description().unwrap();
    let spec = description.openapi("Counter Server", "1.0.0");
    println!(
        "{}",
        serde_json::to_string_pretty(&spec.json().unwrap()).unwrap()
    );
}

When I run it, it works, but I get this when compiling it:

dap@zathras dropshot-operation-id $ cargo version
cargo 1.81.0 (2dbb1af80 2024-08-20)
dap@zathras dropshot-operation-id $ cargo check --example=api-trait-broken
    Checking dropshot v0.12.1-dev (/Users/dap/oxide/dropshot-operation-id/dropshot)
warning: trait `MyApi` is never used
 --> dropshot/examples/api-trait-broken.rs:2:7
  |
2 | trait MyApi {
  |       ^^^^^
  |
  = note: `#[warn(dead_code)]` on by default

warning: `dropshot` (example "api-trait-broken") generated 1 warning
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.56s

I get that technically the trait itself is unused but I think from a Dropshot user's perspective this warning is spurious and we should silence it if we can.

[sunshowers]

Oh interesting -- yeah we should silence this.