diff --git a/src/librustc_errors/lib.rs b/src/librustc_errors/lib.rs
index 3e25f98ccd27c..64d5ca0c2a742 100644
--- a/src/librustc_errors/lib.rs
+++ b/src/librustc_errors/lib.rs
@@ -54,12 +54,28 @@ use syntax_pos::{BytePos,
                  Span,
                  NO_EXPANSION};
 
+/// Indicates the confidence in the correctness of a suggestion.
+///
+/// All suggestions are marked with an `Applicability`. Tools use the applicability of a suggestion
+/// to determine whether it should be automatically applied or if the user should be consulted
+/// before applying the suggestion.
 #[derive(Copy, Clone, Debug, PartialEq, Hash, RustcEncodable, RustcDecodable)]
 pub enum Applicability {
+    /// The suggestion is definitely what the user intended. This suggestion should be
+    /// automatically applied.
     MachineApplicable,
-    HasPlaceholders,
+
+    /// The suggestion may be what the user intended, but it is uncertain. The suggestion should
+    /// result in valid Rust code if it is applied.
     MaybeIncorrect,
-    Unspecified
+
+    /// The suggestion contains placeholders like `(...)` or `{ /* fields */ }`. The suggestion
+    /// cannot be applied automatically because it will not result in valid Rust code. The user
+    /// will need to fill in the placeholders.
+    HasPlaceholders,
+
+    /// The applicability of the suggestion is unknown.
+    Unspecified,
 }
 
 #[derive(Clone, Debug, PartialEq, Hash, RustcEncodable, RustcDecodable)]