refactor(sacp): add tool_fn!() macro for MCP tool registration

Adds a convenience macro that expands to the to_future_hack closure
`|t, args, cx| Box::pin(t(args, cx))`. This is needed until
return-type-notation is stabilized in Rust.

Updated all usages in docs and tests to use the new macro.

Author: nikomatsakis

src/sacp-conductor/tests/mcp_integration/proxy.rs

@@ -34,7 +34,7 @@ impl Component for ProxyComponent {
                         result: format!("Echo: {}", params.message),
                     })
                 },
-                |f, args, cx| Box::pin(f(args, cx)),
+                sacp::tool_fn!(),
             )
             .build();
 

src/sacp-conductor/tests/mcp_server_handler_chain.rs

@@ -82,7 +82,7 @@ impl Component for ProxyWithMcpAndHandler {
                         result: format!("Echo: {}", params.message),
                     })
                 },
-                |f, args, cx| Box::pin(f(args, cx)),
+                sacp::tool_fn!(),
             )
             .build();
 

src/sacp-conductor/tests/test_session_id_in_mcp_tools.rs

@@ -40,7 +40,7 @@ fn create_echo_proxy() -> Result<sacp::DynComponent, sacp::Error> {
                     acp_url: context.acp_url(),
                 })
             },
-            |f, args, cx| Box::pin(f(args, cx)),
+            sacp::tool_fn!(),
         )
         .build();
 

src/sacp/src/lib.rs

@@ -228,3 +228,16 @@ pub use sacp_derive::{JrNotification, JrRequest, JrResponsePayload};
 
 mod session;
 pub use session::*;
+
+/// This macro is used for the value of the `to_future_hack` parameter of [`mcp_server::McpServerBuilder::tool_fn`].
+///
+/// It expands to `|t, args, cx| Box::pin(t(args, cx))`.
+///
+/// This is needed until [return-type-notation](https://github.com/rust-lang/rust/issues/109417)
+/// is stabilized.
+#[macro_export]
+macro_rules! tool_fn {
+    () => {
+        |t, args, cx| Box::pin(t(args, cx))
+    };
+}

src/sacp/src/mcp_server/builder.rs

@@ -34,7 +34,7 @@ use crate::{
 ///         "greet",
 ///         "Greet someone by name",
 ///         async |input: GreetInput, _cx| Ok(format!("Hello, {}!", input.name)),
-///         |t, args, cx| Box::pin(t(args, cx)),
+///         sacp::tool_fn!(),
 ///     )
 ///     .build();
 /// ```
@@ -87,7 +87,7 @@ where
     /// * `description`: The description of the tool.
     /// * `func`: The function that implements the tool. Use an async closure like `async |args, cx| { .. }`.
     /// * `to_future_hack`: A function that converts the tool function into a future.
-    ///   You should always write `|t, args, cx| Box::pin(t(args, cx))`.
+    ///   You should always use the [`sacp::tool_fn!()`](crate::tool_fn) macro here.
     ///   This is needed to sidestep current Rust language limitations.
     ///
     /// # Examples
@@ -98,7 +98,7 @@ where
     ///         "greet",
     ///         "Greet someone by name",
     ///         async |input: GreetInput, _cx| Ok(format!("Hello, {}!", input.name)),
-    ///         |t, args, cx| Box::pin(t(args, cx)),
+    ///         sacp::tool_fn!(),
     ///     )
     /// ```
     pub fn tool_fn<P, R, F, H>(