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Add a debug assertion in codegen that unsize casts of the same principal trait def id are truly NOPs

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This commit is contained in:
Michael Goulet 2024-09-23 13:52:02 -04:00
parent 8fc8e03150
commit 3209943604
3 changed files with 38 additions and 18 deletions
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15
compiler/rustc_mir_transform/src/validate.rs
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@ -1233,21 +1233,6 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
)) {
self.fail(location, format!("Unsize coercion, but `{op_ty}` isn't coercible to `{target_type}`"));
}
// FIXME: Codegen has an additional assumption, where if the
// principal trait def id of what's being casted doesn't change,
// then we don't need to adjust the vtable at all. This
// corresponds to the fact that `dyn Tr<A>: Unsize<dyn Tr<B>>`
// requires that `A = B`; we don't allow *upcasting* objects
// between the same trait with different args. Nothing actually
// validates this, though. While it's true right now, if we for
// some reason were to relax the `Unsize` trait, it could become
// unsound. We should eventually validate that, but it would
// require peeling `&Box<Struct<.., dyn Tr<A>, ..>>` down to
// the trait object that's being unsized, and that's rather
// annoying, and also it would need to be opportunistic since
// this MIR is not yet fully monomorphized, so we may bottom
// out in an alias or a projection or something.
}
CastKind::PointerCoercion(PointerCoercion::DynStar, _) => {
// FIXME(dyn-star): make sure nothing needs to be done here.