Fix typos in ARCHITECTURE.md and a number of crates

specifically: gen_lsp_server, ra_arena, ra_cli, ra_db, ra_hir

crates/ra_db/src/cancellation.rs

@@ -0,0 +1,44 @@
+//! 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 quickly 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 interfere with
+//! any background processing (this bit is handled by salsa, see the
+//! `BaseDatabase::check_canceled` method).
+
+/// An "error" signifing that the operation was canceled.
+#[derive(Clone, PartialEq, Eq, Hash)]
+pub struct Canceled {
+    _private: (),
+}
+
+pub type Cancelable<T> = Result<T, Canceled>;
+
+impl Canceled {
+    pub(crate) fn new() -> Canceled {
+        Canceled { _private: () }
+    }
+}
+
+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 {
+        write!(fmt, "Canceled")
+    }
+}
+
+impl std::error::Error for Canceled {}