executor: add Pender, rework Cargo features.

This introduces a `Pender` struct with enum cases for thread-mode, interrupt-mode and
custom callback executors. This avoids calls through function pointers when using only
the thread or interrupt executors. Faster, and friendlier to `cargo-call-stack`.

`embassy-executor` now has `arch-xxx` Cargo features to select the arch and to enable
the builtin executors (thread and interrupt).

embassy-executor/src/arch/wasm.rs

@@ -1,74 +1,88 @@
-use core::marker::PhantomData;
+#[cfg(feature = "executor-interrupt")]
+compile_error!("`executor-interrupt` is not supported with `arch-wasm`.");
 
-use js_sys::Promise;
-use wasm_bindgen::prelude::*;
+#[cfg(feature = "executor-thread")]
+pub use thread::*;
+#[cfg(feature = "executor-thread")]
+mod thread {
 
-use super::raw::util::UninitCell;
-use super::raw::{self};
-use super::Spawner;
+    use core::marker::PhantomData;
 
-/// WASM executor, wasm_bindgen to schedule tasks on the JS event loop.
-pub struct Executor {
-    inner: raw::Executor,
-    ctx: &'static WasmContext,
-    not_send: PhantomData<*mut ()>,
-}
+    #[cfg(feature = "nightly")]
+    pub use embassy_macros::main_wasm as main;
+    use js_sys::Promise;
+    use wasm_bindgen::prelude::*;
 
-pub(crate) struct WasmContext {
-    promise: Promise,
-    closure: UninitCell<Closure<dyn FnMut(JsValue)>>,
-}
+    use crate::raw::util::UninitCell;
+    use crate::raw::{Pender, PenderInner};
+    use crate::{raw, Spawner};
 
-impl WasmContext {
-    pub fn new() -> Self {
-        Self {
-            promise: Promise::resolve(&JsValue::undefined()),
-            closure: UninitCell::uninit(),
-        }
+    /// WASM executor, wasm_bindgen to schedule tasks on the JS event loop.
+    pub struct Executor {
+        inner: raw::Executor,
+        ctx: &'static WasmContext,
+        not_send: PhantomData<*mut ()>,
     }
-}
 
-impl Executor {
-    /// Create a new Executor.
-    pub fn new() -> Self {
-        let ctx = &*Box::leak(Box::new(WasmContext::new()));
-        let inner = raw::Executor::new(
-            |p| unsafe {
-                let ctx = &*(p as *const () as *const WasmContext);
-                let _ = ctx.promise.then(ctx.closure.as_mut());
-            },
-            ctx as *const _ as _,
-        );
-        Self {
-            inner,
-            not_send: PhantomData,
-            ctx,
+    pub(crate) struct WasmContext {
+        promise: Promise,
+        closure: UninitCell<Closure<dyn FnMut(JsValue)>>,
+    }
+
+    #[derive(Copy, Clone)]
+    pub(crate) struct ThreadPender(&'static WasmContext);
+
+    impl ThreadPender {
+        #[allow(unused)]
+        pub(crate) fn pend(self) {
+            let _ = self.0.promise.then(unsafe { self.0.closure.as_mut() });
         }
     }
 
-    /// Run the executor.
-    ///
-    /// The `init` closure is called with a [`Spawner`] that spawns tasks on
-    /// this executor. Use it to spawn the initial task(s). After `init` returns,
-    /// the executor starts running the tasks.
-    ///
-    /// To spawn more tasks later, you may keep copies of the [`Spawner`] (it is `Copy`),
-    /// for example by passing it as an argument to the initial tasks.
-    ///
-    /// This function requires `&'static mut self`. This means you have to store the
-    /// Executor instance in a place where it'll live forever and grants you mutable
-    /// access. There's a few ways to do this:
-    ///
-    /// - a [StaticCell](https://docs.rs/static_cell/latest/static_cell/) (safe)
-    /// - a `static mut` (unsafe)
-    /// - a local variable in a function you know never returns (like `fn main() -> !`), upgrading its lifetime with `transmute`. (unsafe)
-    pub fn start(&'static mut self, init: impl FnOnce(Spawner)) {
-        unsafe {
-            let executor = &self.inner;
-            self.ctx.closure.write(Closure::new(move |_| {
-                executor.poll();
-            }));
-            init(self.inner.spawner());
+    impl WasmContext {
+        pub fn new() -> Self {
+            Self {
+                promise: Promise::resolve(&JsValue::undefined()),
+                closure: UninitCell::uninit(),
+            }
+        }
+    }
+
+    impl Executor {
+        /// Create a new Executor.
+        pub fn new() -> Self {
+            let ctx = &*Box::leak(Box::new(WasmContext::new()));
+            Self {
+                inner: raw::Executor::new(Pender(PenderInner::Thread(ThreadPender(ctx)))),
+                not_send: PhantomData,
+                ctx,
+            }
+        }
+
+        /// Run the executor.
+        ///
+        /// The `init` closure is called with a [`Spawner`] that spawns tasks on
+        /// this executor. Use it to spawn the initial task(s). After `init` returns,
+        /// the executor starts running the tasks.
+        ///
+        /// To spawn more tasks later, you may keep copies of the [`Spawner`] (it is `Copy`),
+        /// for example by passing it as an argument to the initial tasks.
+        ///
+        /// This function requires `&'static mut self`. This means you have to store the
+        /// Executor instance in a place where it'll live forever and grants you mutable
+        /// access. There's a few ways to do this:
+        ///
+        /// - a [StaticCell](https://docs.rs/static_cell/latest/static_cell/) (safe)
+        /// - a `static mut` (unsafe)
+        /// - a local variable in a function you know never returns (like `fn main() -> !`), upgrading its lifetime with `transmute`. (unsafe)
+        pub fn start(&'static mut self, init: impl FnOnce(Spawner)) {
+            unsafe {
+                let executor = &self.inner;
+                self.ctx.closure.write(Closure::new(move |_| {
+                    executor.poll();
+                }));
+                init(self.inner.spawner());
+            }
         }
     }
 }