Fix window double borrows (#23739)

Fix bugs caused by the window context PR, where the window could be on
the stack and is then requested from the App.
This PR also adds derive macros for `AppContext` and `VisualContext` so
that it's easy to define further contexts in API code, such as
`editor::BlockContext`.

Release Notes:

- N/A

crates/gpui_macros/src/derive_visual_context.rs

@@ -0,0 +1,71 @@
+use proc_macro::TokenStream;
+use quote::quote;
+use syn::{parse_macro_input, DeriveInput};
+
+use super::get_simple_attribute_field;
+
+pub fn derive_visual_context(input: TokenStream) -> TokenStream {
+    let ast = parse_macro_input!(input as DeriveInput);
+
+    let Some(window_variable) = get_simple_attribute_field(&ast, "window") else {
+        return quote! {
+            compile_error!("Derive must have a #[window] attribute to detect the &mut Window field");
+        }
+        .into();
+    };
+
+    let Some(app_variable) = get_simple_attribute_field(&ast, "app") else {
+        return quote! {
+            compile_error!("Derive must have a #[app] attribute to detect the &mut App field");
+        }
+        .into();
+    };
+
+    let type_name = &ast.ident;
+    let (impl_generics, type_generics, where_clause) = ast.generics.split_for_impl();
+
+    let gen = quote! {
+        impl #impl_generics gpui::VisualContext for #type_name #type_generics
+        #where_clause
+        {
+            fn window_handle(&self) -> gpui::AnyWindowHandle {
+                self.#window_variable.window_handle()
+            }
+
+            fn update_window_entity<T: 'static, R>(
+                &mut self,
+                model: &gpui::Entity<T>,
+                update: impl FnOnce(&mut T, &mut gpui::Window, &mut gpui::Context<T>) -> R,
+            ) -> Self::Result<R> {
+                gpui::AppContext::update_entity(self.#app_variable, model, |entity, cx| update(entity, self.#window_variable, cx))
+            }
+
+            fn new_window_entity<T: 'static>(
+                &mut self,
+                build_model: impl FnOnce(&mut gpui::Window, &mut gpui::Context<'_, T>) -> T,
+            ) -> Self::Result<gpui::Entity<T>> {
+                gpui::AppContext::new(self.#app_variable, |cx| build_model(self.#window_variable, cx))
+            }
+
+            fn replace_root_view<V>(
+                &mut self,
+                build_view: impl FnOnce(&mut gpui::Window, &mut gpui::Context<V>) -> V,
+            ) -> Self::Result<gpui::Entity<V>>
+            where
+                V: 'static + gpui::Render,
+            {
+                self.#window_variable.replace_root(self.#app_variable, build_view)
+            }
+
+            fn focus<V>(&mut self, model: &gpui::Entity<V>) -> Self::Result<()>
+            where
+                V: gpui::Focusable,
+            {
+                let focus_handle = gpui::Focusable::focus_handle(model, self.#app_variable);
+                self.#window_variable.focus(&focus_handle)
+            }
+        }
+    };
+
+    gen.into()
+}