Unsafe derives and attributes

[joshtriplett]

Allow declaring proc macro attributes and derive macros as unsafe, and
requiring unsafe to invoke them.

Rendered

[clarfonthey]

Just my 2¢, but I think that the shorthand for derive(..., unsafe(Trait), ...) is a bit unprecedented.

You have to separate out derive traits any time there's some different requirement, e.g. cfg_attr(feature = "serde", derive(Serialize, Deserialize)), and by keeping unsafe at the top level you can easily grep for #!?\[unsafe\( whereas unsafe( will catch any call to method_unsafe(.

Sure, it's likely it won't make a difference, but I think that only having to check attributes at the top level for unsafe to verify safety is best.

[carbotaniuman]

The greppability is already broken by things like #[cfg_attr(all(), unsafe(no_mangle)], and can be restored by just grepping unsafe( I think putting the unsafe with the trait makes syntactic sense as it discharges the safety obligation of each trait derive.

[clarfonthey]

The greppability is already broken by things like #[cfg_attr(all(), unsafe(no_mangle)], and can be restored by just grepping unsafe( I think putting the unsafe with the trait makes syntactic sense as it discharges the safety obligation of each trait derive.

I feel kind of silly for literally alluding to this point in my post and missing it somehow. You're right and I retract my original claim.

[joshtriplett]

We had a @rust-lang/lang design meeting today on the set of macro RFCs. I've updated the RFC to incorporate the feedback from that design meeting.

Per the feedback in that meeting, I'm starting an FCP to start letting people register consensus for this RFC.

@rfcbot merge

[rfcbot]

Team member @joshtriplett has proposed to merge this. The next step is review by the rest of the tagged team members:

• @joshtriplett
• @nikomatsakis
• @pnkfelix
• @scottmcm
• @tmandry
• @traviscross

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.