Add error explanation for E0066.

This also updates the error message to remove mention of "managed
heap", which was removed in 8a91d33.

Author: Nick Hamann

src/librustc_typeck/check/mod.rs

@@ -3033,8 +3033,8 @@ fn check_expr_with_unifier<'a, 'tcx, F>(fcx: &FnCtxt<'a, 'tcx>,
           let mut checked = false;
           opt_place.as_ref().map(|place| match place.node {
               ast::ExprPath(None, ref path) => {
-                  // FIXME(pcwalton): For now we hardcode the two permissible
-                  // places: the exchange heap and the managed heap.
+                  // FIXME(pcwalton): For now we hardcode the only permissible
+                  // place: the exchange heap.
                   let definition = lookup_full_def(tcx, path.span, place.id);
                   let def_id = definition.def_id();
                   let referent_ty = fcx.expr_ty(&**subexpr);
@@ -3048,7 +3048,7 @@ fn check_expr_with_unifier<'a, 'tcx, F>(fcx: &FnCtxt<'a, 'tcx>,
 
           if !checked {
               span_err!(tcx.sess, expr.span, E0066,
-                  "only the managed heap and exchange heap are currently supported");
+                  "only the exchange heap is currently supported");
               fcx.write_ty(id, tcx.types.err);
           }
       }

src/librustc_typeck/diagnostics.rs

@@ -91,6 +91,16 @@ enum variant, one of the fields was not provided. Each field should be specified
 exactly once.
 "##,
 
+E0066: r##"
+Box placement expressions (like C++'s "placement new") do not support any
+place expression except the exchange heap (i.e. `std::boxed::HEAP`).
+Furthermore, the syntax is changing to use `in` instead of `box`. See [RFC
+470][rfc470] and [RFC 809][rfc809] for more details.
+
+[rfc470]: https://github.com/rust-lang/rfcs/pull/470
+[rfc809]: https://github.com/rust-lang/rfcs/blob/master/text/0809-box-and-in-for-stdlib.md
+"##,
+
 E0067: r##"
 The left-hand side of an assignment operator must be an lvalue expression. An
 lvalue expression represents a memory location and includes item paths (ie,
@@ -498,7 +508,6 @@ register_diagnostics! {
     E0059,
     E0060,
     E0061,
-    E0066,
     E0068,
     E0070,
     E0071,