Introduce LocalFile trait

If you want to call `abs_path` or `load`, the file needs to be local. You call `as_local` which returns `Option<dyn LocalFile>` with those local-only methods. I think this makes it more explicit what works only locally vs everywhere.

crates/project/src/project.rs

@@ -938,7 +938,7 @@ impl Project {
         let buffer_abs_path;
         if let Some(file) = File::from_dyn(buffer.file()) {
             worktree = file.worktree.clone();
-            buffer_abs_path = file.abs_path(cx);
+            buffer_abs_path = file.as_local().map(|f| f.abs_path(cx));
         } else {
             return Task::ready(Err(anyhow!("buffer does not belong to any worktree")));
         };
@@ -2181,8 +2181,8 @@ mod tests {
         cx.update(|cx| {
             let target_buffer = definition.target_buffer.read(cx);
             assert_eq!(
-                target_buffer.file().unwrap().abs_path(cx),
-                Some(dir.path().join("a.rs"))
+                target_buffer.file().unwrap().as_local().unwrap().abs_path(cx),
+                dir.path().join("a.rs")
             );
             assert_eq!(definition.target_range.to_offset(target_buffer), 9..10);
             assert_eq!(

crates/project/src/worktree.rs

@@ -1366,6 +1366,14 @@ pub struct File {
 }
 
 impl language::File for File {
+    fn as_local(&self) -> Option<&dyn language::LocalFile> {
+        if self.is_local {
+            Some(self)
+        } else {
+            None
+        }
+    }
+
     fn mtime(&self) -> SystemTime {
         self.mtime
     }
@@ -1374,13 +1382,6 @@ impl language::File for File {
         &self.path
     }
 
-    fn abs_path(&self, cx: &AppContext) -> Option<PathBuf> {
-        self.worktree
-            .read(cx)
-            .as_local()
-            .map(|local_worktree| local_worktree.abs_path.join(&self.path))
-    }
-
     fn full_path(&self, cx: &AppContext) -> PathBuf {
         let mut full_path = PathBuf::new();
         full_path.push(self.worktree.read(cx).root_name());
@@ -1448,16 +1449,6 @@ impl language::File for File {
         })
     }
 
-    fn load_local(&self, cx: &AppContext) -> Option<Task<Result<String>>> {
-        let worktree = self.worktree.read(cx).as_local()?;
-        let abs_path = worktree.absolutize(&self.path);
-        let fs = worktree.fs.clone();
-        Some(
-            cx.background()
-                .spawn(async move { fs.load(&abs_path).await }),
-        )
-    }
-
     fn format_remote(
         &self,
         buffer_id: u64,
@@ -1510,6 +1501,25 @@ impl language::File for File {
     }
 }
 
+impl language::LocalFile for File {
+    fn abs_path(&self, cx: &AppContext) -> PathBuf {
+        self.worktree
+            .read(cx)
+            .as_local()
+            .unwrap()
+            .abs_path
+            .join(&self.path)
+    }
+
+    fn load(&self, cx: &AppContext) -> Task<Result<String>> {
+        let worktree = self.worktree.read(cx).as_local().unwrap();
+        let abs_path = worktree.absolutize(&self.path);
+        let fs = worktree.fs.clone();
+        cx.background()
+            .spawn(async move { fs.load(&abs_path).await })
+    }
+}
+
 impl File {
     pub fn from_dyn(file: Option<&dyn language::File>) -> Option<&Self> {
         file.and_then(|f| f.as_any().downcast_ref())