`#[deprecated]` has no effect on macros

[sgrif]

The following code compiles successfully with no warnings, when it should be producing a deprecation warning.

https://play.rust-lang.org/?gist=aa4c719a14da2bb5cf3ddf8c3066f452&version=stable

[abonander]

Some discussion here: #48524 (comment)

Deprecation and stability checking is actually done after macro expansion, so a complete solution would probably need to lift both of those into phase 2. or just have a separate implementation for macros like I did with stability checking in that PR.

cc @petrochenkov

[sgrif]

If nothing else, #[deprecated] should be giving an error when placed on an item where it has no effect.

[abonander]

The goal is to support deprecation on macros, it's just that all the infrastructure for deprecation is in a later phase so even a naive implementation would need to redo a lot of the support. We still need stability checking and deprecation after macro expansion as well.

[petrochenkov]

Fixed in #62042