gpui: Improve Global ergonomics (#11923)

This PR adds some ergonomic improvements when working with GPUI
`Global`s.

Two new traits have been added—`ReadGlobal` and `UpdateGlobal`—that
provide associated functions on any type that implements `Global` for
accessing and updating the global without needing to call the methods on
the `cx` directly (which generally involves qualifying the type).

I looked into adding `ObserveGlobal` as well, but this seems a bit
trickier to implement as the signatures of `cx.observe_global` vary
slightly between the different contexts.

Release Notes:

- N/A

crates/assistant/src/assistant.rs

@@ -12,7 +12,7 @@ use assistant_settings::{AnthropicModel, AssistantSettings, OpenAiModel, ZedDotD
 use client::{proto, Client};
 use command_palette_hooks::CommandPaletteFilter;
 pub(crate) use completion_provider::*;
-use gpui::{actions, AppContext, BorrowAppContext, Global, SharedString};
+use gpui::{actions, AppContext, Global, SharedString, UpdateGlobal};
 pub(crate) use saved_conversation::*;
 use serde::{Deserialize, Serialize};
 use settings::{Settings, SettingsStore};
@@ -233,13 +233,13 @@ pub fn init(client: Arc<Client>, cx: &mut AppContext) {
     CommandPaletteFilter::update_global(cx, |filter, _cx| {
         filter.hide_namespace(Assistant::NAMESPACE);
     });
-    cx.update_global(|assistant: &mut Assistant, cx: &mut AppContext| {
+    Assistant::update_global(cx, |assistant, cx| {
         let settings = AssistantSettings::get_global(cx);
 
         assistant.set_enabled(settings.enabled, cx);
     });
     cx.observe_global::<SettingsStore>(|cx| {
-        cx.update_global(|assistant: &mut Assistant, cx: &mut AppContext| {
+        Assistant::update_global(cx, |assistant, cx| {
             let settings = AssistantSettings::get_global(cx);
 
             assistant.set_enabled(settings.enabled, cx);

crates/assistant/src/assistant_panel.rs

@@ -28,8 +28,8 @@ use gpui::{
     AsyncWindowContext, AvailableSpace, ClipboardItem, Context, Entity, EventEmitter, FocusHandle,
     FocusableView, FontStyle, FontWeight, HighlightStyle, InteractiveElement, IntoElement, Model,
     ModelContext, ParentElement, Pixels, Render, SharedString, StatefulInteractiveElement, Styled,
-    Subscription, Task, TextStyle, UniformListScrollHandle, View, ViewContext, VisualContext,
-    WeakModel, WeakView, WhiteSpace, WindowContext,
+    Subscription, Task, TextStyle, UniformListScrollHandle, UpdateGlobal, View, ViewContext,
+    VisualContext, WeakModel, WeakView, WhiteSpace, WindowContext,
 };
 use language::{language_settings::SoftWrap, Buffer, LanguageRegistry, Point, ToOffset as _};
 use multi_buffer::MultiBufferRow;
@@ -240,7 +240,7 @@ impl AssistantPanel {
             || prev_settings_version != CompletionProvider::global(cx).settings_version()
         {
             self.authentication_prompt =
-                Some(cx.update_global::<CompletionProvider, _>(|provider, cx| {
+                Some(CompletionProvider::update_global(cx, |provider, cx| {
                     provider.authentication_prompt(cx)
                 }));
         }
@@ -1088,7 +1088,7 @@ impl AssistantPanel {
     }
 
     fn authenticate(&mut self, cx: &mut ViewContext<Self>) -> Task<Result<()>> {
-        cx.update_global::<CompletionProvider, _>(|provider, cx| provider.authenticate(cx))
+        CompletionProvider::update_global(cx, |provider, cx| provider.authenticate(cx))
     }
 
     fn render_signed_in(&mut self, cx: &mut ViewContext<Self>) -> impl IntoElement {

crates/assistant/src/assistant_settings.rs

@@ -418,7 +418,7 @@ fn merge<T: Copy>(target: &mut T, value: Option<T>) {
 
 #[cfg(test)]
 mod tests {
-    use gpui::{AppContext, BorrowAppContext};
+    use gpui::{AppContext, UpdateGlobal};
     use settings::SettingsStore;
 
     use super::*;
@@ -440,7 +440,7 @@ mod tests {
         );
 
         // Ensure backward-compatibility.
-        cx.update_global::<SettingsStore, _>(|store, cx| {
+        SettingsStore::update_global(cx, |store, cx| {
             store
                 .set_user_settings(
                     r#"{
@@ -460,7 +460,7 @@ mod tests {
                 low_speed_timeout_in_seconds: None,
             }
         );
-        cx.update_global::<SettingsStore, _>(|store, cx| {
+        SettingsStore::update_global(cx, |store, cx| {
             store
                 .set_user_settings(
                     r#"{
@@ -482,7 +482,7 @@ mod tests {
         );
 
         // The new version supports setting a custom model when using zed.dev.
-        cx.update_global::<SettingsStore, _>(|store, cx| {
+        SettingsStore::update_global(cx, |store, cx| {
             store
                 .set_user_settings(
                     r#"{