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explain why DispatchFromDyn does the check it does

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Ralf Jung 2023-09-11 10:24:53 +02:00
parent e00120906e
commit e68e9d4a14
1 changed files with 8 additions and 0 deletions
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8
compiler/rustc_hir_analysis/src/coherence/builtin.rs
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@ -157,6 +157,14 @@ fn visit_implementation_of_dispatch_from_dyn(tcx: TyCtxt<'_>, impl_did: LocalDef
let infcx = tcx.infer_ctxt().build();
let cause = ObligationCause::misc(span, impl_did);
// Later parts of the compiler rely on all DispatchFromDyn types to be ABI-compatible with raw
// pointers. This is enforced here: we only allow impls for references, raw pointers, and things
// that are effectively repr(transparent) newtypes around types that already hav a
// DispatchedFromDyn impl. We cannot literally use repr(transparent) on those tpyes since some
// of them support an allocator, but we ensure that for the cases where the type implements this
// trait, they *do* satisfy the repr(transparent) rules, and then we assume that everything else
// in the compiler (in particular, all the call ABI logic) will treat them as repr(transparent)
// even if they do not carry that attribute.
use rustc_type_ir::sty::TyKind::*;
match (source.kind(), target.kind()) {
(&Ref(r_a, _, mutbl_a), Ref(r_b, _, mutbl_b))