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Auto merge of #88558 - fee1-dead:const-drop, r=oli-obk

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Const drop

The changes are pretty primitive at this point. But at least it works. ^-^

Problems with the current change that I can think of now:
 - [x] `~const Drop` shouldn't change anything in the non-const world.
 - [x] types that do not have drop glues shouldn't fail to satisfy `~const Drop` in const contexts. `struct S { a: u8, b: u16 }` This might not fail for `needs_non_const_drop`, but it will fail in `rustc_trait_selection`.
 - [x] The current change accepts types that have `const Drop` impls but have non-const `Drop` glue.

Fixes #88424.

Significant Changes:

- `~const Drop` is no longer treated as a normal trait bound. In non-const contexts, this bound has no effect, but in const contexts, this restricts the input type and all of its transitive fields to either a) have a `const Drop` impl or b) can be trivially dropped (i.e. no drop glue)
- `T: ~const Drop` will not be linted like `T: Drop`.
- Instead of recursing and iterating through the type in `rustc_mir::transform::check_consts`, we use the trait system to special case `~const Drop`. See [`rustc_trait_selection::...::candidate_assembly#assemble_const_drop_candidates`](https://github.com/fee1-dead/rust/blob/const-drop/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs#L817) and others.

Changes not related to `const Drop`ping and/or changes that are insignificant:

 - `Node.constness_for_typeck` no longer returns `hir::Constness::Const` for type aliases in traits. This was previously used to hack how we determine default bound constness for items. But because we now use an explicit opt-in, it is no longer needed.
 - Removed `is_const_impl_raw` query. We have `impl_constness`, and the only existing use of that query uses `HirId`, which means we can just operate it with hir.
 - `ty::Destructor` now has a field `constness`, which represents the constness of the destructor.

r? `@oli-obk`
This commit is contained in:
bors 2021-09-15 03:51:03 +00:00
commit cdeba02ff7
26 changed files with 554 additions and 108 deletions
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8
compiler/rustc_hir/src/hir.rs
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@ -3234,12 +3234,7 @@ impl<'hir> Node<'hir> {
}
}
/// Returns `Constness::Const` when this node is a const fn/impl/item,
///
/// HACK(fee1-dead): or an associated type in a trait. This works because
/// only typeck cares about const trait predicates, so although the predicates
/// query would return const predicates when it does not need to be const,
/// it wouldn't have any effect.
/// Returns `Constness::Const` when this node is a const fn/impl/item.
pub fn constness_for_typeck(&self) -> Constness {
match self {
Node::Item(Item {
@ -3258,7 +3253,6 @@ impl<'hir> Node<'hir> {
Node::Item(Item { kind: ItemKind::Const(..), .. })
| Node::TraitItem(TraitItem { kind: TraitItemKind::Const(..), .. })
| Node::TraitItem(TraitItem { kind: TraitItemKind::Type(..), .. })
| Node::ImplItem(ImplItem { kind: ImplItemKind::Const(..), .. }) => Constness::Const,
_ => Constness::NotConst,