Warn or Error on errors in constants?

[oli-obk]

Before the miri merger, you were allowed to write pub const FOO: usize = 0 - 1; (producing just a warning), but using it either in this crate or another produced an error.

Since the miri merger, these kinds of constants have become hard errors (just like within statics).

This is of course a breaking change. So we need to decide whether to move back to the warning, or break this now. The warnings have existed for a very long time, but were never moved to deny-by-default

cc @rust-lang/lang

related: #47054

[nikomatsakis]

@oli-obk do we have some idea of the fallout here?

[oli-obk]

Crater didn't find anything on the miri PR. But an old version of winapi is affected. Old versions of other things might be affected, too. Or more windows things. Both are untested on crater

[oli-obk]

This has been "fixed" on beta and nightly (there's no more error), but now there's also no more warning. I'm readding the warning by reintroducing the lint pass that visits all constants and type aliases

pub type Foo = [u8; 0 - 1]; // no warning on beta/nihgtly

[pietroalbini]

This still needs a backport to beta.