Improve the ergonomics of creating local buffers (#10347)

This PR renames `language::Buffer::new` to `language::Buffer::local` and
simplifies its interface. Instead of taking a replica id (which should
always be 0 for the local case) and a `BufferId`, which was awkward and
verbose to construct, it simply takes text and a `cx`.

It uses the `cx` to derive a `BufferId` from the `EntityId` associated
with the `cx`, which should always be positive based on the following
analysis...

We convert the entity id to a u64 using this method on `EntityId`, which
is defined by macros in the `slotmap` crate:

```rust
    pub fn as_ffi(self) -> u64 {
        (u64::from(self.version.get()) << 32) | u64::from(self.idx)
    }
```

If you look at the type of `version` in `KeyData`, it is non-zero:

```rust
#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub struct KeyData {
    idx: u32,
    version: NonZeroU32,
}
```

This commit also adds `Context::reserve_model` and
`Context::insert_model` to determine a model's entity ID before it is
created, which we need in order to assign a `BufferId` in the background
when loading a buffer asynchronously.

Release Notes:

- N/A

---------

Co-authored-by: Piotr Osiewicz <24362066+osiewicz@users.noreply.github.com>

crates/project/src/project.rs

@@ -178,7 +178,6 @@ pub struct Project {
     collaborators: HashMap<proto::PeerId, Collaborator>,
     client_subscriptions: Vec<client::Subscription>,
     _subscriptions: Vec<gpui::Subscription>,
-    next_buffer_id: BufferId,
     loading_buffers: HashMap<BufferId, Vec<oneshot::Sender<Result<Model<Buffer>, anyhow::Error>>>>,
     incomplete_remote_buffers: HashMap<BufferId, Model<Buffer>>,
     shared_buffers: HashMap<proto::PeerId, HashSet<BufferId>>,
@@ -675,7 +674,6 @@ impl Project {
                 flush_language_server_update: None,
                 pending_language_server_update: None,
                 collaborators: Default::default(),
-                next_buffer_id: BufferId::new(1).unwrap(),
                 opened_buffers: Default::default(),
                 shared_buffers: Default::default(),
                 loading_buffers_by_path: Default::default(),
@@ -792,7 +790,6 @@ impl Project {
                 pending_language_server_update: None,
                 flush_language_server_update: None,
                 loading_buffers_by_path: Default::default(),
-                next_buffer_id: BufferId::new(1).unwrap(),
                 loading_buffers: Default::default(),
                 shared_buffers: Default::default(),
                 incomplete_remote_buffers: Default::default(),
@@ -1849,9 +1846,8 @@ impl Project {
         if self.is_remote() {
             return Err(anyhow!("creating buffers as a guest is not supported yet"));
         }
-        let id = self.next_buffer_id.next();
         let buffer = cx.new_model(|cx| {
-            Buffer::new(self.replica_id(), id, text)
+            Buffer::local(text, cx)
                 .with_language(language.unwrap_or_else(|| language::PLAIN_TEXT.clone()), cx)
         });
         self.register_buffer(&buffer, cx)?;
@@ -1950,10 +1946,9 @@ impl Project {
         worktree: Model<Worktree>,
         cx: &mut ModelContext<Self>,
     ) -> Task<Result<Model<Buffer>>> {
-        let buffer_id = self.next_buffer_id.next();
         let load_buffer = worktree.update(cx, |worktree, cx| {
             let worktree = worktree.as_local_mut().unwrap();
-            worktree.load_buffer(buffer_id, &path, cx)
+            worktree.load_buffer(&path, cx)
         });
         fn is_not_found_error(error: &anyhow::Error) -> bool {
             error
@@ -1967,7 +1962,7 @@ impl Project {
                 Err(error) if is_not_found_error(&error) => {
                     worktree.update(&mut cx, |worktree, cx| {
                         let worktree = worktree.as_local_mut().unwrap();
-                        worktree.new_buffer(buffer_id, path, cx)
+                        worktree.new_buffer(path, cx)
                     })
                 }
                 Err(e) => Err(e),