remove backtraces from Cancelled

Hopefully we won't need them for debugging. If we do need them, it
should be easy to add back.

Cargo.lock

@@ -697,7 +697,6 @@ dependencies = [
 name = "ra_db"
 version = "0.1.0"
 dependencies = [
- "backtrace 0.3.13 (registry+https://github.com/rust-lang/crates.io-index)",
  "parking_lot 0.7.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "ra_editor 0.1.0",
  "ra_syntax 0.1.0",

crates/ra_db/Cargo.toml

@@ -5,7 +5,6 @@ version = "0.1.0"
 authors = ["Aleksey Kladov <aleksey.kladov@gmail.com>"]
 
 [dependencies]
-backtrace = "0.3.1"
 relative-path = "0.4.0"
 salsa = "0.9.0"
 rustc-hash = "1.0"

crates/ra_db/src/cancelation.rs

@@ -15,29 +15,17 @@
 //! any background processing (this bit is handled by salsa, see
 //! `BaseDatabase::check_canceled` method).
 
-use std::{
-    cmp,
-    hash::{Hash, Hasher},
-    sync::Arc,
-};
-
-use backtrace::Backtrace;
-use parking_lot::Mutex;
-
 /// An "error" signifing that the operation was canceled.
-#[derive(Clone)]
+#[derive(Clone, PartialEq, Eq, Hash)]
 pub struct Canceled {
-    backtrace: Arc<Mutex<Backtrace>>,
+    _private: (),
 }
 
 pub type Cancelable<T> = Result<T, Canceled>;
 
 impl Canceled {
     pub(crate) fn new() -> Canceled {
-        let bt = Backtrace::new_unresolved();
-        Canceled {
-            backtrace: Arc::new(Mutex::new(bt)),
-        }
+        Canceled { _private: () }
     }
 }
 
@@ -49,37 +37,8 @@ impl std::fmt::Display for Canceled {
 
 impl std::fmt::Debug for Canceled {
     fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        let mut bt = self.backtrace.lock();
-        let bt: &mut Backtrace = &mut *bt;
-        bt.resolve();
-        write!(fmt, "canceled at:\n{:?}", bt)
+        write!(fmt, "Canceled")
     }
 }
 
 impl std::error::Error for Canceled {}
-
-impl PartialEq for Canceled {
-    fn eq(&self, _: &Canceled) -> bool {
-        true
-    }
-}
-
-impl Eq for Canceled {}
-
-impl Hash for Canceled {
-    fn hash<H: Hasher>(&self, hasher: &mut H) {
-        ().hash(hasher)
-    }
-}
-
-impl cmp::Ord for Canceled {
-    fn cmp(&self, _: &Canceled) -> cmp::Ordering {
-        cmp::Ordering::Equal
-    }
-}
-
-impl cmp::PartialOrd for Canceled {
-    fn partial_cmp(&self, other: &Canceled) -> Option<cmp::Ordering> {
-        Some(self.cmp(other))
-    }
-}

crates/ra_lsp_server/src/main_loop.rs

@@ -432,7 +432,7 @@ impl<'a> PoolDispatcher<'a> {
                                     RawResponse::err(
                                         id,
                                         ErrorCode::ContentModified as i32,
-                                        format!("content modified: {:?}", e),
+                                        "content modified".to_string(),
                                     )
                                 } else {
                                     RawResponse::err(