bjoernager/rust
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Rollup merge of #100208 - RalfJung:dyn-upcast-nop, r=petrochenkov

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make NOP dyn casts not require anything about the vtable

As suggested [on Zulip](https://rust-lang.zulipchat.com/#narrow/stream/144729-t-types/topic/dyn-upcasting.20stabilization/near/292151439). This matches what the codegen backends already do, and what Miri did do until https://github.com/rust-lang/rust/pull/99420 when I made it super extra paranoid.
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Dylan DPC 2022-08-19 12:26:41 +05:30 • committed by GitHub
commit c4707ff8ef
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6 changed files with 46 additions and 21 deletions
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1
compiler/rustc_codegen_cranelift/src/unsize.rs
View file

@ -29,6 +29,7 @@ pub(crate) fn unsized_info<'tcx>(
let old_info =
old_info.expect("unsized_info: missing old info for trait upcasting coercion");
if data_a.principal_def_id() == data_b.principal_def_id() {
// A NOP cast that doesn't actually change anything, should be allowed even with invalid vtables.
return old_info;
}

1
compiler/rustc_codegen_ssa/src/base.rs
View file

@ -151,6 +151,7 @@ pub fn unsized_info<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
let old_info =
old_info.expect("unsized_info: missing old info for trait upcasting coercion");
if data_a.principal_def_id() == data_b.principal_def_id() {
// A NOP cast that doesn't actually change anything, should be allowed even with invalid vtables.
return old_info;
}

7
compiler/rustc_const_eval/src/interpret/cast.rs
View file

@ -298,7 +298,12 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
self.write_immediate(val, dest)
}
(&ty::Dynamic(ref data_a, ..), &ty::Dynamic(ref data_b, ..)) => {
let (old_data, old_vptr) = self.read_immediate(src)?.to_scalar_pair()?;
let val = self.read_immediate(src)?;
if data_a.principal() == data_b.principal() {
// A NOP cast that doesn't actually change anything, should be allowed even with mismatching vtables.
return self.write_immediate(*val, dest);
}
let (old_data, old_vptr) = val.to_scalar_pair()?;
let old_vptr = old_vptr.to_pointer(self)?;
let (ty, old_trait) = self.get_ptr_vtable(old_vptr)?;
if old_trait != data_a.principal() {