gio: document the interaction of the Task result setter and getters

The three ways you can return a result from the task must be match in how you
set and get the fields.

Document this in the different functions and mark them as unsafe unconditionally
to indicate that there is no way within the type system to ensure that you are
using it safely.

Author: carlosmn

examples/gio_task/file_size/mod.rs

@@ -26,7 +26,9 @@ impl FileSize {
         callback: Q,
     ) {
         let closure = move |task: gio::LocalTask<i64>, source_object: Option<&glib::Object>| {
-            let value = task.propagate().unwrap();
+            // SAFETY: this is safe because we call propagate just once and the
+            // task sets the result as a value
+            let value = unsafe { task.propagate() }.unwrap();
             let source_object = source_object.unwrap().downcast_ref::<FileSize>().unwrap();
             callback(value, source_object);
         };
@@ -68,7 +70,8 @@ impl FileSize {
         callback: Q,
     ) {
         let closure = move |task: gio::Task<i64>, source_object: Option<&FileSize>| {
-            // SAFETY: this is safe because we call propagate just once
+            // SAFETY: this is safe because we call propagate just once and the
+            // task sets the result as a value
             let value = unsafe { task.propagate().unwrap() };
             let source_object = source_object.unwrap().downcast_ref::<FileSize>().unwrap();
             callback(value, source_object);

gio/src/task.rs

@@ -237,6 +237,13 @@ macro_rules! task_impl {
                 unsafe { from_glib(ffi::g_task_return_error_if_cancelled(self.to_glib_none().0)) }
             }
 
+            // rustdoc-stripper-ignore-next
+            /// Set the result of the task
+            ///
+            /// # Safety
+            ///
+            /// The value must be read with [`Task::propagate`],
+            /// `g_task_propagate_value` or `g_task_propagate_pointer`.
             #[doc(alias = "g_task_return_value")]
             #[doc(alias = "g_task_return_pointer")]
             #[doc(alias = "g_task_return_error")]
@@ -270,6 +277,13 @@ macro_rules! task_impl {
                 }
             }
 
+            // rustdoc-stripper-ignore-next
+            /// Set the result of the task as a boolean
+            ///
+            /// # Safety
+            ///
+            /// The value must be read with [`Task::propagate_boolean`],
+            /// or `g_task_propagate_boolean`.
             #[doc(alias = "g_task_return_boolean")]
             #[allow(unused_unsafe)]
             pub $($safety)? fn return_boolean_result(self, result: Result<bool, glib::Error>) {
@@ -279,6 +293,13 @@ macro_rules! task_impl {
                 }
             }
 
+            // rustdoc-stripper-ignore-next
+            /// Set the result of the task as an int
+            ///
+            /// # Safety
+            ///
+            /// The value must be read with [`Task::propagate_int`],
+            /// or `g_task_propagate_int`.
             #[doc(alias = "g_task_return_int")]
             #[allow(unused_unsafe)]
             pub $($safety)? fn return_int_result(self, result: Result<isize, glib::Error>) {
@@ -289,10 +310,18 @@ macro_rules! task_impl {
             }
 
 
+            // rustdoc-stripper-ignore-next
+            /// Gets the result of the task and transfers ownership of it
+            ///
+            /// # Safety
+            ///
+            /// This must only be called once, and only if the result was set
+            /// via [`Task::return_result`], `g_task_return_value` or
+            /// `g_task_return_pointer`.
             #[doc(alias = "g_task_propagate_value")]
             #[doc(alias = "g_task_propagate_pointer")]
             #[allow(unused_unsafe)]
-            pub $($safety)? fn propagate(self) -> Result<V, glib::Error> {
+            pub unsafe fn propagate(self) -> Result<V, glib::Error> {
                 let mut error = ptr::null_mut();
 
                 unsafe {
@@ -329,9 +358,16 @@ macro_rules! task_impl {
                 }
             }
 
+            // rustdoc-stripper-ignore-next
+            /// Gets the result of the task as a boolean, or the error
+            ///
+            /// # Safety
+            ///
+            /// This must only be called once, and only if the result was set
+            /// via [`Task::return_boolean_result`], or `g_task_return_boolean`.
             #[doc(alias = "g_task_propagate_boolean")]
             #[allow(unused_unsafe)]
-            pub $($safety)? fn propagate_boolean(self) -> Result<bool, glib::Error> {
+            pub unsafe fn propagate_boolean(self) -> Result<bool, glib::Error> {
                 let mut error = ptr::null_mut();
 
                 unsafe {
@@ -345,9 +381,16 @@ macro_rules! task_impl {
                 }
             }
 
+            // rustdoc-stripper-ignore-next
+            /// Gets the result of the task as an int, or the error
+            ///
+            /// # Safety
+            ///
+            /// This must only be called once, and only if the result was set
+            /// via [`Task::return_int_result`], or `g_task_return_int`.
             #[doc(alias = "g_task_propagate_int")]
             #[allow(unused_unsafe)]
-            pub $($safety)? fn propagate_int(self) -> Result<isize, glib::Error> {
+            pub unsafe fn propagate_int(self) -> Result<isize, glib::Error> {
                 let mut error = ptr::null_mut();
 
                 unsafe {