Code compiles on playground but fails when passed via stdin to rustc

ekuber:

Would someone have the energy to write the MCP for merging this change?

I'll try to give it a go tomorrow. It'll be my first MCP. Thanks for picking this up! But ... Before I do ...

ekuber:

I believe that we might have to coordinate with cargo (to ensure the edition is being passed in always).

My worry would be autocfg. If anyone has probes with a leading #[deny(warnings)] then that could break with rather nasty consequences. (?) It'd be great if @cuviper could quickly weigh in as to whether that's a valid concern or not?