sandboxes and macros

[Matthew Flatt]

If anyone is using a sandbox to run untrusted code that defines macros,
please see this information on a potential upcoming change to the
Racket macro expander:

https://github.com/racket/racket/pull/3915

Summary: Syntax arming and disarming were part of a design to allow
sandboxing untrusted code without unduly constraining the sandboxed
code. This proposal replaces that approach with a simpler one. The
trade-off is that some advanced macro-implementation tools, including
`syntax-local-value` and `local-expand`, cannot be referenced directly
within a sandbox (i.e., in a context where the code inspector is
different than the original one).


Matthew