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

I think that, given the number and significance of edition differences that now exist, rustc ought to warn whenever it is invoked without an --edition, because almost nobody writing a rustc invocation today should be using the 2015 edition.

At one point I tried implementing such a warning; the complication I found was that very many of the “UI” tests (tests that compile code and look at the compiler output) were written without any edition set, and each one would gain a new warning unless modified to set the edition.