Grammar: Don't allow inner attrs in certain block expressions

[fmease]

rustc doesn't syntactically permit inner attributes (1) in the consequent & alternate blocks of if-expressions, (2) in the alternate block of let-statements (let/else) and (3) in blocks matched by macro metavariables of type block.

Intuitively that makes sense since inner attributes generally target the "owner" of the surrounding block (e.g., the overarching function item or while-expression), neither "the block itself" nor the sequence of statements in said block (so fn f(_: Undef) { #![cfg(false)] } = #[cfg(false)] fn f(_: Undef) {} and while undef { #![cfg(false)] } = #[cfg(false)] while undef {}) which would be very surprising for if-exprs and let/else stmts I feel like and straight up ill-defined in the MBE case.

Concretely, it would mean that if undef { /*…*/ } else if undef { /*…*/ } else { #![cfg(false)] } would be equivalent to #[cfg(false)] if …; similarly for let self::undef = /*…*/ else { #![cfg(false)] }; / #[cfg(false)] let …;.

I'm not sure if that's the "official" reasoning, I've yet to find that out. As far as I can tell, inner attributes have been illegal in then/else blocks since version 1.0 at least¹.

Fixes #2212.

Footnotes

1. However, back in version 1.0 inner attributes weren't permitted in most expressions with blocks like in while expressions or "normal" block exprs either (but they were already allowed in the block of function items) which were all lifted at some point except for if-expressions, of course. ↩

[fmease]

Naming inspired by TypeNoBounds.

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