embassy-executor: explicitly return impl Future in task inner task

embassy-executor-macros/src/macros/task.rs

@@ -112,25 +112,11 @@ pub fn run(args: TokenStream, item: TokenStream) -> TokenStream {
         }
     }
 
-    let task_ident = f.sig.ident.clone();
-    let task_inner_ident = format_ident!("__{}_task", task_ident);
-
-    let mut task_inner = f.clone();
-    let visibility = task_inner.vis.clone();
-    task_inner.vis = syn::Visibility::Inherited;
-    task_inner.sig.ident = task_inner_ident.clone();
-
-    // Forcefully mark the inner task as safe.
-    // SAFETY: We only ever call task_inner in functions
-    //         with the same safety preconditions as task_inner
-    task_inner.sig.unsafety = None;
-    let task_body = task_inner.body;
-    task_inner.body = quote! {
-        #[allow(unused_unsafe, reason = "Not all function bodies may require being in an unsafe block")]
-        unsafe {
-            #task_body
-        }
-    };
+    // Copy the generics + where clause to avoid more spurious errors.
+    let generics = &f.sig.generics;
+    let where_clause = &f.sig.generics.where_clause;
+    let unsafety = &f.sig.unsafety;
+    let visibility = &f.vis;
 
     // assemble the original input arguments,
     // including any attributes that may have
@@ -143,6 +129,51 @@ pub fn run(args: TokenStream, item: TokenStream) -> TokenStream {
         ));
     }
 
+    let task_ident = f.sig.ident.clone();
+    let task_inner_ident = format_ident!("__{}_task", task_ident);
+
+    let task_inner_future_output = match &f.sig.output {
+        ReturnType::Default => quote! {-> impl ::core::future::Future<Output = ()>},
+        // Special case the never type since we can't stuff it into a `impl Future<Output = !>`
+        ReturnType::Type(arrow, maybe_never) if matches!(**maybe_never, Type::Never(_)) => quote! {
+            #arrow #maybe_never
+        },
+        // Grab the arrow span, why not
+        ReturnType::Type(arrow, typ) if f.sig.asyncness.is_some() => quote! {
+            #arrow impl ::core::future::Future<Output = #typ>
+        },
+        // We assume that if `f` isn't async, it must return `-> impl Future<...>`
+        // This is checked using traits later
+        ReturnType::Type(arrow, typ) => quote! {
+            #arrow #typ
+        },
+    };
+
+    let task_inner_body = if errors.is_empty() {
+        quote! {
+            #f
+
+            // SAFETY: All the preconditions to `#task_ident` apply to
+            //         all contexts `#task_inner_ident` is called in
+            #unsafety {
+                #task_ident(#(#full_args,)*)
+            }
+        }
+    } else {
+        quote! {
+            async {::core::todo!()}
+        }
+    };
+
+    let task_inner = quote! {
+        #visibility fn #task_inner_ident #generics (#fargs)
+        #task_inner_future_output
+        #where_clause
+        {
+            #task_inner_body
+        }
+    };
+
     let spawn = if returns_impl_trait {
         quote!(spawn)
     } else {
@@ -185,7 +216,7 @@ pub fn run(args: TokenStream, item: TokenStream) -> TokenStream {
         unsafe { __task_pool_get(#task_inner_ident).#spawn(move || #task_inner_ident(#(#full_args,)*)) }
     };
 
-    let task_outer_attrs = task_inner.attrs.clone();
+    let task_outer_attrs = &f.attrs;
 
     if !errors.is_empty() {
         task_outer_body = quote! {
@@ -195,11 +226,6 @@ pub fn run(args: TokenStream, item: TokenStream) -> TokenStream {
         };
     }
 
-    // Copy the generics + where clause to avoid more spurious errors.
-    let generics = &f.sig.generics;
-    let where_clause = &f.sig.generics.where_clause;
-    let unsafety = &f.sig.unsafety;
-
     let result = quote! {
         // This is the user's task function, renamed.
         // We put it outside the #task_ident fn below, because otherwise
@@ -226,7 +252,7 @@ fn check_arg_ty(errors: &mut TokenStream, ty: &Type) {
 
     impl<'a, 'ast> Visit<'ast> for Visitor<'a> {
         fn visit_type_reference(&mut self, i: &'ast syn::TypeReference) {
-            // only check for elided lifetime here. If not elided, it's checked by `visit_lifetime`.
+            // Only check for elided lifetime here. If not elided, it's checked by `visit_lifetime`.
             if i.lifetime.is_none() {
                 error(
                     self.errors,

embassy-executor/tests/ui/nonstatic_struct_elided.stderr

@@ -8,3 +8,17 @@ help: indicate the anonymous lifetime
   |
 6 | async fn task(_x: Foo<'_>) {}
   |                      ++++
+
+error[E0700]: hidden type for `impl Sized` captures lifetime that does not appear in bounds
+ --> tests/ui/nonstatic_struct_elided.rs:5:1
+  |
+5 | #[embassy_executor::task]
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^ opaque type defined here
+6 | async fn task(_x: Foo) {}
+  |                   --- hidden type `impl Sized` captures the anonymous lifetime defined here
+  |
+  = note: this error originates in the attribute macro `embassy_executor::task` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: add a `use<...>` bound to explicitly capture `'_`
+  |
+5 | #[embassy_executor::task] + use<'_>
+  |                           +++++++++