Validate usage of inline attributes

[catamorphism]

syntax::attr::find_inline_attr has a FIXME saying "validate the usage of #[inline] and #[inline(always)]".

[pcwalton]

I don't believe this is backwards incompatible, renominating.

[pcwalton]

Also I don't know what this means.

[catamorphism]

I can't tell where the comment originally came from, but I'm assuming it means checking that #[inline] doesn't get used to annotate something that can't be inlined (because it's not a function).

[graydon]

just a bug, removing milestone/nomination.

[nikomatsakis]

Consensus is that this means a lint pass detecting #[inline] being applied
to bad code.

[graydon]

Visiting for triage, nontrivial amount of work so leaving as properly classified for now.

[steveklabnik]

Amusingly, today this says

    // FIXME (#2809)---validate the usage of #[inline] and #[inline]

No more always, but it's still there.

[nagisa]

#22490 validates well-formedness of the inline attribute, but it is still unknown whether there’s any ways to misuse #[inline] attribute in a way which would be invalid (e.g. not on a function).

If the comment in original report happens to have referred to validation of attribute well-formedness, this issue can then be closed. Do not forget to remove the FIXME comment too.