On wasm, provide intradoc-link for spawn() function in EventLoop docs (#3178)

src/platform/run_on_demand.rs

@@ -35,9 +35,9 @@ pub trait EventLoopExtRunOnDemand {
     /// See the [`set_control_flow()`] docs on how to change the event loop's behavior.
     ///
     /// # Caveats
-    /// - This extension isn't available on all platforms, since it's not always possible to
-    ///   return to the caller (specifically this is impossible on iOS and Web - though with
-    ///   the Web backend it is possible to use `spawn()` more than once instead).
+    /// - This extension isn't available on all platforms, since it's not always possible to return
+    ///   to the caller (specifically this is impossible on iOS and Web - though with the Web
+    ///   backend it is possible to use `EventLoopExtWebSys::spawn()`[^1] more than once instead).
     /// - No [`Window`] state can be carried between separate runs of the event loop.
     ///
     /// You are strongly encouraged to use [`EventLoop::run()`] for portability, unless you specifically need
@@ -57,8 +57,13 @@ pub trait EventLoopExtRunOnDemand {
     ///   on an event loop that is internal to the browser itself.
     /// - **iOS:** It's not possible to stop and start an `NSApplication` repeatedly on iOS.
     ///
-    /// [`exit()`]: EventLoopWindowTarget::exit
-    /// [`set_control_flow()`]: EventLoopWindowTarget::set_control_flow
+    #[cfg_attr(
+        not(wasm_platform),
+        doc = "[^1]: `spawn()` is only available on `wasm` platforms."
+    )]
+    ///
+    /// [`exit()`]: EventLoopWindowTarget::exit()
+    /// [`set_control_flow()`]: EventLoopWindowTarget::set_control_flow()
     fn run_on_demand<F>(&mut self, event_handler: F) -> Result<(), EventLoopError>
     where
         F: FnMut(Event<Self::UserEvent>, &EventLoopWindowTarget<Self::UserEvent>);

src/platform/web.rs

@@ -109,13 +109,28 @@ pub trait EventLoopExtWebSys {
 
     /// Initializes the winit event loop.
     ///
-    /// Unlike `run`, this returns immediately, and doesn't throw an exception in order to
-    /// satisfy its `!` return type.
+    /// Unlike
+    #[cfg_attr(
+        all(wasm_platform, target_feature = "exception-handling"),
+        doc = "`run()`"
+    )]
+    #[cfg_attr(
+        not(all(wasm_platform, target_feature = "exception-handling")),
+        doc = "[`run()`]"
+    )]
+    /// [^1], this returns immediately, and doesn't throw an exception in order to
+    /// satisfy its [`!`] return type.
     ///
     /// Once the event loop has been destroyed, it's possible to reinitialize another event loop
     /// by calling this function again. This can be useful if you want to recreate the event loop
     /// while the WebAssembly module is still loaded. For example, this can be used to recreate the
     /// event loop when switching between tabs on a single page application.
+    ///
+    #[cfg_attr(
+        not(all(wasm_platform, target_feature = "exception-handling")),
+        doc = "[`run()`]: EventLoop::run()"
+    )]
+    /// [^1]: `run()` is _not_ available on WASM when the target supports `exception-handling`.
     fn spawn<F>(self, event_handler: F)
     where
         F: 'static + FnMut(Event<Self::UserEvent>, &EventLoopWindowTarget<Self::UserEvent>);