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Explain pointer and dyn Trait handling in const_to_valtree

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This commit is contained in:
Oli Scherer 2021-03-15 11:53:37 +00:00
parent c01c49430c
commit 0dd5a1b622
1 changed files with 7 additions and 9 deletions
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16
compiler/rustc_mir/src/const_eval/mod.rs
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@ -84,19 +84,18 @@ fn const_to_valtree_inner<'tcx>(
Some(ty::ValTree::Leaf(val.assert_int()))
}
// Raw pointers are not allowed in type level constants, as raw pointers cannot be treated
// like references. If we looked behind the raw pointer, we may be breaking the meaning of
// the raw pointer. Equality on raw pointers is performed on the pointer and not on the pointee,
// and we cannot guarantee any kind of pointer stability in the type system.
// Raw pointers are not allowed in type level constants, as raw pointers compare equal if
// their addresses are equal. Since we cannot guarantee any kind of pointer stability in
// the type system.
// Technically we could allow function pointers, but they are not guaranteed to be the
// same as the function pointers at runtime.
ty::FnPtr(_) | ty::RawPtr(_) => None,
ty::Ref(..) => unimplemented!("need to use deref_const"),
// Trait objects are not allowed in type level constants, as we have no concept for
// resolving their backing type, even if we can do that at const eval time. We may want to consider
// adding a `ValTree::DownCast(Ty<'tcx>, Box<ValTree>)` in the future, but I don't even know the
// questions such a concept would open up, so an RFC would probably be good for this.
// resolving their backing type, even if we can do that at const eval time. We may
// hypothetically be able to allow `dyn StructuralEq` trait objects in the future,
// but it is unclear if this is useful.
ty::Dynamic(..) => None,
ty::Slice(_) | ty::Str => {
@ -107,8 +106,7 @@ fn const_to_valtree_inner<'tcx>(
ty::Adt(def, _) => {
if def.variants.is_empty() {
// Uninhabited
return None;
bug!("uninhabited types should have errored and never gotten converted to valtree")
}
let variant = ecx.read_discriminant(&place.into()).unwrap().1;