fix(macros): Allow macros to work even if Future is not in scope (#385)

`#[tool]` generates code that uses Future by name for async functions.
This means that consumers have to import Future. This commit fixes that
by changing the macro expansion to use std::future::Future instead.

The same applies to `#[prompt]` as well.

Author: swlynch99

crates/rmcp-macros/src/prompt.rs

@@ -108,10 +108,10 @@ pub fn prompt(attr: TokenStream, input: TokenStream) -> syn::Result<TokenStream>
             }
             match &fn_item.sig.output {
                 syn::ReturnType::Default => {
-                    quote! { -> futures::future::BoxFuture<#lt, ()> }
+                    quote! { -> ::std::pin::Pin<Box<dyn ::std::future::Future<Output = ()> + Send + #lt>> }
                 }
                 syn::ReturnType::Type(_, ty) => {
-                    quote! { -> futures::future::BoxFuture<#lt, #ty> }
+                    quote! { -> ::std::pin::Pin<Box<dyn ::std::future::Future<Output = #ty> + Send + #lt>> }
                 }
             }
         })?;

crates/rmcp-macros/src/tool.rs

@@ -234,7 +234,7 @@ pub fn tool(attr: TokenStream, input: TokenStream) -> syn::Result<TokenStream> {
     // modify the the input function
     if fn_item.sig.asyncness.is_some() {
         // 1. remove asyncness from sig
-        // 2. make return type: `std::pin::Pin<Box<dyn Future<Output = #ReturnType> + Send + '_>>`
+        // 2. make return type: `std::pin::Pin<Box<dyn std::future::Future<Output = #ReturnType> + Send + '_>>`
         // 3. make body: { Box::pin(async move { #body }) }
         let new_output = syn::parse2::<ReturnType>({
             let mut lt = quote! { 'static };
@@ -249,10 +249,10 @@ pub fn tool(attr: TokenStream, input: TokenStream) -> syn::Result<TokenStream> {
             }
             match &fn_item.sig.output {
                 syn::ReturnType::Default => {
-                    quote! { -> futures::future::BoxFuture<#lt, ()> }
+                    quote! { -> ::std::pin::Pin<Box<dyn ::std::future::Future<Output = ()> + Send + #lt>> }
                 }
                 syn::ReturnType::Type(_, ty) => {
-                    quote! { -> futures::future::BoxFuture<#lt, #ty> }
+                    quote! { -> ::std::pin::Pin<Box<dyn ::std::future::Future<Output = #ty> + Send + #lt>> }
                 }
             }
         })?;