Tool middleware in Rust

Annotate an implementation of a generated middleware trait with #[tool_middleware]. A transparent middleware presents and wraps the same tool:


use golem_rust::{tool::ToolInvokeError, tool_middleware};
 
struct EchoPolicy;
impl EchoPolicy { fn new() -> Self { Self } }
 
#[tool_middleware(name = "echo-policy", constructor = EchoPolicy::new)]
impl EchoMiddleware for EchoPolicy {
    async fn echo(
        &self,
        underlying: &EchoUnderlying,
        value: String,
    ) -> Result<String, ToolInvokeError<std::convert::Infallible>> {
        if value.is_empty() {
            Err(ToolInvokeError::ConstraintViolation("value is empty".into()))
        } else {
            underlying.echo(value).await
        }
    }
}

For an adapter, implement PresentedMiddleware<ExpectedUnderlying> and translate inputs, outputs, and custom errors explicitly. The supplied underlying handle is invocation-scoped: do not store it, return it, or use it after the middleware method finishes. Forward affine permission cards and streams exactly once.