Narrow down the list of targets `#[link_section]` and other link attributes can be applied to

[JonathanBrouwer]

The #147418 PR restores the old behaviour of link_section, in that you can apply it to all method-like things, including trait methods without a default implementation.

We should take a look at the implementation to see which targets actually have an effect, and only allow those.

@rustbot claim

[JonathanBrouwer]

@rustbot label +A-diagnostics +A-attributes +T-compiler

[ehuss]

My understanding is that attributes like link_section on trait functions are not implicitly inherited by impls of those functions. Thus, these attributes on a function with no body has no effect, and should probably not be allowed (long term). I would recommend to start warning on them.

Offhand I don't have an exhaustive list of which attributes those are, but that includes most codegen attributes like no_mangle and friends.