gpui: Fix Global trait (#11187)

This PR restores the `Global` trait's status as a marker trait.

This was the original intent from #7095, when it was added, that had
been lost in #9777.

The purpose of the `Global` trait is to statically convey what types can
and can't be accessed as `Global` state, as well as provide a way of
restricting access to said globals.

For example, in the case of the `ThemeRegistry` we have a private
`GlobalThemeRegistry` that is marked as `Global`:
91b3c24ed3/crates/theme/src/registry.rs (L25-L34)

We're then able to permit reading the `ThemeRegistry` from the
`GlobalThemeRegistry` via a custom getter, while still restricting which
callers are able to mutate the global:
91b3c24ed3/crates/theme/src/registry.rs (L46-L61)

Release Notes:

- N/A

crates/assistant2/src/assistant2.rs

@@ -11,7 +11,7 @@ use feature_flags::FeatureFlagAppExt as _;
 use futures::{future::join_all, StreamExt};
 use gpui::{
     list, prelude::*, AnyElement, AppContext, AsyncWindowContext, EventEmitter, FocusHandle,
-    FocusableView, Global, ListAlignment, ListState, Render, Task, View, WeakView,
+    FocusableView, ListAlignment, ListState, Render, Task, View, WeakView,
 };
 use language::{language_settings::SoftWrap, LanguageRegistry};
 use open_ai::{FunctionContent, ToolCall, ToolCallContent};

crates/assistant2/src/completion_provider.rs

@@ -2,7 +2,7 @@ use anyhow::Result;
 use assistant_tooling::ToolFunctionDefinition;
 use client::{proto, Client};
 use futures::{future::BoxFuture, stream::BoxStream, FutureExt, StreamExt};
-use gpui::Global;
+use gpui::{AppContext, Global};
 use std::sync::Arc;
 
 pub use open_ai::RequestMessage as CompletionMessage;
@@ -11,6 +11,10 @@ pub use open_ai::RequestMessage as CompletionMessage;
 pub struct CompletionProvider(Arc<dyn CompletionProviderBackend>);
 
 impl CompletionProvider {
+    pub fn get(cx: &AppContext) -> &Self {
+        cx.global::<CompletionProvider>()
+    }
+
     pub fn new(backend: impl CompletionProviderBackend) -> Self {
         Self(Arc::new(backend))
     }

crates/file_icons/src/file_icons.rs

@@ -31,6 +31,10 @@ pub fn init(assets: impl AssetSource, cx: &mut AppContext) {
 }
 
 impl FileIcons {
+    pub fn get(cx: &AppContext) -> &Self {
+        cx.global::<FileIcons>()
+    }
+
     pub fn new(assets: impl AssetSource) -> Self {
         assets
             .load("icons/file_icons/file_types.json")

crates/gpui/src/gpui.rs

@@ -330,19 +330,12 @@ impl<T> Flatten<T> for Result<T> {
 
 /// A marker trait for types that can be stored in GPUI's global state.
 ///
+/// This trait exists to provide type-safe access to globals by restricting
+/// the scope from which they can be accessed. For instance, the actual type
+/// that implements [`Global`] can be private, with public accessor functions
+/// that enforce correct usage.
+///
 /// Implement this on types you want to store in the context as a global.
-pub trait Global: 'static + Sized {
-    /// Access the global of the implementing type. Panics if a global for that type has not been assigned.
-    fn get(cx: &AppContext) -> &Self {
-        cx.global()
-    }
-
-    /// Updates the global of the implementing type with a closure. Unlike `global_mut`, this method provides
-    /// your closure with mutable access to the `AppContext` and the global simultaneously.
-    fn update<C, R>(cx: &mut C, f: impl FnOnce(&mut Self, &mut C) -> R) -> R
-    where
-        C: BorrowAppContext,
-    {
-        cx.update_global(f)
-    }
+pub trait Global: 'static {
+    // This trait is intentionally left empty, by virtue of being a marker trait.
 }

crates/semantic_index/examples/index.rs

@@ -1,6 +1,6 @@
 use client::Client;
 use futures::channel::oneshot;
-use gpui::{App, Global};
+use gpui::App;
 use language::language_settings::AllLanguageSettings;
 use project::Project;
 use semantic_index::{OpenAiEmbeddingModel, OpenAiEmbeddingProvider, SemanticIndex};

crates/semantic_index/src/semantic_index.rs

@@ -921,7 +921,7 @@ fn db_key_for_path(path: &Arc<Path>) -> String {
 mod tests {
     use super::*;
     use futures::{future::BoxFuture, FutureExt};
-    use gpui::{Global, TestAppContext};
+    use gpui::TestAppContext;
     use language::language_settings::AllLanguageSettings;
     use project::Project;
     use settings::SettingsStore;

crates/settings/src/settings_store.rs

@@ -207,6 +207,13 @@ trait AnySettingValue: 'static + Send + Sync {
 struct DeserializedSetting(Box<dyn Any>);
 
 impl SettingsStore {
+    pub fn update<C, R>(cx: &mut C, f: impl FnOnce(&mut Self, &mut C) -> R) -> R
+    where
+        C: BorrowAppContext,
+    {
+        cx.update_global(f)
+    }
+
     /// Add a new type of setting to the store.
     pub fn register_setting<T: Settings>(&mut self, cx: &mut AppContext) {
         let setting_type_id = TypeId::of::<T>();

crates/tasks_ui/src/modal.rs

@@ -3,9 +3,9 @@ use std::sync::Arc;
 use crate::{active_item_selection_properties, schedule_resolved_task};
 use fuzzy::{StringMatch, StringMatchCandidate};
 use gpui::{
-    impl_actions, rems, AppContext, DismissEvent, EventEmitter, FocusableView, Global,
-    InteractiveElement, Model, ParentElement, Render, SharedString, Styled, Subscription, View,
-    ViewContext, VisualContext, WeakView,
+    impl_actions, rems, AppContext, DismissEvent, EventEmitter, FocusableView, InteractiveElement,
+    Model, ParentElement, Render, SharedString, Styled, Subscription, View, ViewContext,
+    VisualContext, WeakView,
 };
 use picker::{highlighted_match_with_paths::HighlightedText, Picker, PickerDelegate};
 use project::{Inventory, TaskSourceKind};