1 files changed, 85 insertions, 0 deletions

diff --git a/crates/ra_db/src/cancelation.rs b/crates/ra_db/src/cancelation.rs
new file mode 100644
index 000000000..73444b015
--- /dev/null
+++ b/crates/ra_db/src/cancelation.rs
@@ -0,0 +1,85 @@
+//! Utility types to support cancellation.
+//!
+//! In a typical IDE use-case, requests and modification happen concurrently, as
+//! in the following scenario:
+//!
+//!   * user types a character,
+//!   * a syntax highlighting process is started
+//!   * user types next character, while syntax highlighting *is still in
+//!     progress*.
+//!
+//! In this situation, we want to react to modification as quckly as possible.
+//! At the same time, in-progress results are not very interesting, because they
+//! are invalidated by the edit anyway. So, we first cancel all in-flight
+//! requests, and then apply modification knowing that it won't intrfere with
+//! 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)]
+pub struct Canceled {
+    backtrace: Arc<Mutex<Backtrace>>,
+}
+
+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)),
+        }
+    }
+}
+
+impl std::fmt::Display for Canceled {
+    fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        fmt.write_str("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)
+    }
+}
+
+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))
+    }
+}