Add comments about the checks for recursive variant definition, as requested by @nrc.

Author: quantheory

src/librustc/middle/check_static_recursion.rs

@@ -27,6 +27,10 @@ struct CheckCrateVisitor<'a, 'ast: 'a> {
     sess: &'a Session,
     def_map: &'a DefMap,
     ast_map: &'a ast_map::Map<'ast>,
+    // `discriminant_map` is a cache that associates the `NodeId`s of local
+    // variant definitions with the discriminant expression that applies to
+    // each one. If the variant uses the default values (starting from `0`),
+    // then `None` is stored.
     discriminant_map: RefCell<NodeMap<Option<&'ast ast::Expr>>>,
 }
 

src/test/compile-fail/issue-23302.rs

@@ -8,10 +8,14 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// Check that an enum with recursion in the discriminant throws
+// the appropriate error (rather than, say, blowing the stack).
 enum X {
     A = X::A as isize, //~ ERROR E0265
 }
 
+// Since `Y::B` here defaults to `Y::A+1`, this is also a
+// recursive definition.
 enum Y {
     A = Y::B as isize, //~ ERROR E0265
     B,