add comment

Author: lcnr

compiler/rustc_middle/src/traits/select.rs

@@ -105,6 +105,12 @@ pub type EvaluationCache<'tcx> = Cache<
 /// parameter environment.
 #[derive(PartialEq, Eq, Debug, Clone, TypeFoldable, TypeVisitable)]
 pub enum SelectionCandidate<'tcx> {
+    /// A builtin implementation for some specific traits, used in cases
+    /// where we cannot rely an ordinary library implementations.
+    ///
+    /// The most notable examples are `sized`, `Copy` and `Clone`. This is also
+    /// used for the `DiscriminantKind` and `Pointee` trait, both of which have
+    /// an associated type.
     BuiltinCandidate {
         /// `false` if there are no *further* obligations.
         has_nested: bool,