Overhaul inline assistant (#12846)

This pull request introduces a new diff mechanism that helps users
understand exactly which lines were changed by the LLM.

Release Notes:

- N/A

Cargo.toml

@@ -340,6 +340,7 @@ serde_repr = "0.1"
 sha2 = "0.10"
 shellexpand = "2.1.0"
 shlex = "1.3.0"
+similar = "1.3"
 smallvec = { version = "1.6", features = ["union"] }
 smol = "1.2"
 strum = { version = "0.25.0", features = ["derive"] }

crates/assistant/Cargo.toml

@@ -47,6 +47,7 @@ semantic_index.workspace = true
 serde.workspace = true
 serde_json.workspace = true
 settings.workspace = true
+similar.workspace = true
 smol.workspace = true
 strsim = "0.11"
 strum.workspace = true

crates/assistant/src/assistant_panel.rs

@@ -1222,6 +1222,10 @@ impl Context {
         }
     }
 
+    pub(crate) fn token_count(&self) -> Option<usize> {
+        self.token_count
+    }
+
     pub(crate) fn count_remaining_tokens(&mut self, cx: &mut ModelContext<Self>) {
         let request = self.to_completion_request(cx);
         self.pending_token_count = cx.spawn(|this, mut cx| {