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/gpui_macros.rs

@@ -1,11 +1,14 @@
+mod derive_app_context;
 mod derive_into_element;
 mod derive_path_static_str;
 mod derive_render;
+mod derive_visual_context;
 mod register_action;
 mod styles;
 mod test;
 
 use proc_macro::TokenStream;
+use syn::{DeriveInput, Ident};
 
 /// register_action! can be used to register an action with the GPUI runtime.
 /// You should typically use `gpui::actions!` or `gpui::impl_actions!` instead,
@@ -34,6 +37,57 @@ pub fn derive_path_static_str(input: TokenStream) -> TokenStream {
     derive_path_static_str::derive_path_static_str(input)
 }
 
+/// #[derive(AppContext)] is used to create a context out of anything that holds a `&mut App`
+/// Note that a `#[app]` attribute is required to identify the variable holding the &mut App.
+///
+/// Failure to add the attribute causes a compile error:
+///
+/// ```compile_fail
+/// # #[macro_use] extern crate gpui_macros;
+/// # #[macro_use] extern crate gpui;
+/// #[derive(AppContext)]
+/// struct MyContext<'a> {
+///     app: &'a mut gpui::App
+/// }
+/// ```
+#[proc_macro_derive(AppContext, attributes(app))]
+pub fn derive_app_context(input: TokenStream) -> TokenStream {
+    derive_app_context::derive_app_context(input)
+}
+
+/// #[derive(VisualContext)] is used to create a visual context out of anything that holds a `&mut Window` and
+/// implements `AppContext`
+/// Note that a `#[app]` and a `#[window]` attribute are required to identify the variables holding the &mut App,
+/// and &mut Window respectively.
+///
+/// Failure to add both attributes causes a compile error:
+///
+/// ```compile_fail
+/// # #[macro_use] extern crate gpui_macros;
+/// # #[macro_use] extern crate gpui;
+/// #[derive(VisualContext)]
+/// struct MyContext<'a, 'b> {
+///     #[app]
+///     app: &'a mut gpui::App,
+///     window: &'b mut gpui::Window
+/// }
+/// ```
+///
+/// ```compile_fail
+/// # #[macro_use] extern crate gpui_macros;
+/// # #[macro_use] extern crate gpui;
+/// #[derive(VisualContext)]
+/// struct MyContext<'a, 'b> {
+///     app: &'a mut gpui::App,
+///     #[window]
+///     window: &'b mut gpui::Window
+/// }
+/// ```
+#[proc_macro_derive(VisualContext, attributes(window, app))]
+pub fn derive_visual_context(input: TokenStream) -> TokenStream {
+    derive_visual_context::derive_visual_context(input)
+}
+
 /// Used by GPUI to generate the style helpers.
 #[proc_macro]
 #[doc(hidden)]
@@ -115,3 +169,15 @@ pub fn box_shadow_style_methods(input: TokenStream) -> TokenStream {
 pub fn test(args: TokenStream, function: TokenStream) -> TokenStream {
     test::test(args, function)
 }
+
+pub(crate) fn get_simple_attribute_field(ast: &DeriveInput, name: &'static str) -> Option<Ident> {
+    match &ast.data {
+        syn::Data::Struct(data_struct) => data_struct
+            .fields
+            .iter()
+            .find(|field| field.attrs.iter().any(|attr| attr.path.is_ident(name)))
+            .map(|field| field.ident.clone().unwrap()),
+        syn::Data::Enum(_) => None,
+        syn::Data::Union(_) => None,
+    }
+}