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+#### Note: this error code is no longer emitted by the compiler.
+
+Box placement expressions (like C++'s "placement new") do not yet 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]
+and [RFC 809] for more details.
+
+[RFC 470]: https://github.com/rust-lang/rfcs/pull/470
+[RFC 809]: https://github.com/rust-lang/rfcs/pull/809