`#[deny(missing_docs)]` ignores exported `macro_rules!`

[kaikalii]

If my crate exports a macro_rules! macro, the deny(missing_docs) attribute has no effect even though it probably should.

The following code should, at the very least, provide a warning

#[deny(missing_docs)]
#[macro_export]
macro_rules! foo {
    () => {}
}

Preventing this code from compiling would likely break some backwards compatability, but I don't see why it shouldn't at least give a warning.

[Centril]

cc @petrochenkov @rust-lang/lang I cannot decide whether this is expected behavior or not, but it makes sense that missing_docs should fire on exported macros.

[cramertj]

Preventing this code from compiling would likely break some backwards compatability

We don't guarantee backwards-compatibility for #![deny(warnings)], otherwise we would never be able to introduce a new warning. I agree that missing deny(missing_docs) should error on crate-exported macros with no documentation.

[scottmcm]

Lang team discussion agreed with the two comments above; the lint should warn for this.

[nnethercote]

This broke compilation of existing code in the rustc-perf benchmark suite, see rust-lang/rustc-perf#325

I see the comment "We don't guarantee backwards-compatibility for #![deny(warnings)]" above, but I thought it might be useful to note some practical consequences.