Add anti-fundamental attribute - #160391
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This adds a new compiler attribute that prevents non-local #[fundamental] types from receiving implementations of the marked trait. This is the infrastructure for fixing soundness problems with DerefMut and DispatchFromDyn on fundamental wrappers like Box and Pin. The attribute is parsed, cross-crate encoded, stored in TraitDef, and checked during the orphan check in coherence.
Mark Deref, DerefMut, DispatchFromDyn, CoerceUnsized, and Receiver with #[rustc_anti_fundamental] to prevent downstream crates from implementing these traits on #[fundamental] types like Box and Pin.
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With DerefMut now marked #[rustc_anti_fundamental], the PinHelper/ PinDerefMutHelper indirection is no longer needed to prevent downstream crates from implementing DerefMut on Pin. Replace it with a direct DerefMut impl for Pin and remove the helper module, its diagnostic item, and the special-casing in trait suggestion rendering.
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This PR introduces the internal
#[rustc_anti_fundamental]trait attribute to prevent downstream crates from implementing traits on non-local#[fundamental]types (such asPinandBox).Using this new attribute on
Deref,DerefMut,Receiver,CoerceUnsized, andDispatchFromDyn, we are able to cleanly resolve orphan rule conflicts and completely remove the internalPinDerefMutHelperworkaround trait incore::pin.See also #85099
This description is WIP, and I will expand on it later, but I wanted to create this PR as a starting point for further discussion with some people I was working on this.
r? @dingxiangfei2009