#[prompt]` macro doesn't respect `local` feature — unconditionally emits `+ Send` unlike `#[tool]

[WeekendSuperhero]

Summary

The #[tool] macro correctly omits + Send on generated handler futures when the local crate feature is enabled or #[tool(local)] is used. The #[prompt] macro does not — it unconditionally emits + Send on all generated futures, making it impossible to use !Send types in prompt handlers even with the local feature.

Context

The local feature was added in #740 to support !Send tool handlers (e.g., single-threaded platform APIs like Apple's EventKit on macOS). The #[tool] macro was updated to check for the feature:

rmcp-macros/src/tool.rs:339-341:

// 2. make return type: `std::pin::Pin<Box<dyn std::future::Future<Output = #ReturnType> + Send + '_>>`
//    (omit `+ Send` when the `local` crate feature is active or `#[tool(local)]` is used)
let omit_send = cfg!(feature = "local") || attribute.local;

The #[prompt] macro was not updated and unconditionally emits + Send:

rmcp-macros/src/prompt.rs:141:

quote! { -> ::std::pin::Pin<Box<dyn ::std::future::Future<Output = ()> + Send + #lt>> }

rmcp-macros/src/prompt.rs:144:

quote! { -> ::std::pin::Pin<Box<dyn ::std::future::Future<Output = #ty> + Send + #lt>> }

Reproduction

Enable the local feature and define a server struct holding a !Send type with both tool and prompt handlers:

use rmcp::*;
use std::rc::Rc; // !Send

struct MyServer {
    data: Rc<String>, // !Send
}

#[tool_router]
impl MyServer {
    #[tool(name = "greet")]
    async fn greet(&self) -> Result<CallToolResult, McpError> {
        // ✅ Compiles with `local` feature — future is not required to be Send
        Ok(CallToolResult::text(format!("Hello: {}", self.data)))
    }
}

#[prompt_router]
impl MyServer {
    #[prompt(name = "greeting")]
    async fn greeting(&self) -> Result<GetPromptResult, McpError> {
        // ❌ Fails to compile — future is required to be Send because
        // #[prompt] unconditionally emits `+ Send`
        Ok(GetPromptResult::new(vec![PromptMessage::new_text(
            PromptMessageRole::User,
            format!("Greeting: {}", self.data),
        )]))
    }
}

Speculative Fix

Apply the same omit_send logic from tool.rs to prompt.rs. In rmcp-macros/src/prompt.rs, around line 130-145 where the return type is generated:

// Add this (matching tool.rs:339-341)
let omit_send = cfg!(feature = "local");

// Then change lines 141 and 144 from:
quote! { -> ::std::pin::Pin<Box<dyn ::std::future::Future<Output = ()> + Send + #lt>> }
quote! { -> ::std::pin::Pin<Box<dyn ::std::future::Future<Output = #ty> + Send + #lt>> }

// To:
if omit_send {
    quote! { -> ::std::pin::Pin<Box<dyn ::std::future::Future<Output = ()> + #lt>> }
    quote! { -> ::std::pin::Pin<Box<dyn ::std::future::Future<Output = #ty> + #lt>> }
} else {
    quote! { -> ::std::pin::Pin<Box<dyn ::std::future::Future<Output = ()> + Send + #lt>> }
    quote! { -> ::std::pin::Pin<Box<dyn ::std::future::Future<Output = #ty> + Send + #lt>> }
}

Optionally, also support the per-handler #[prompt(local)] attribute for parity with #[tool(local)].

Use Case

eventkit-rs is an MCP server wrapping Apple's EventKit framework. The underlying Objective-C managers (EKEventStore, EKReminderStore) are !Send. Currently every tool and prompt call must allocate a fresh manager because the local feature can't be used — the #[prompt] handlers reference &self and the macro forces Send on the future. With this fix, managers could be stored directly on the server struct and reused across calls.