`Candidate` enum has only a single `Ref` variant. Refactor it into a
struct and reduce overall indentation of the code by two levels.
No functional changes.
Changes from #88558 allowed using `~const Drop` in constants by
introducing a new `NeedsNonConstDrop` qualif.
The new qualif was also used for promotion purposes, and allowed
promotion to happen for values that needs to be dropped but which
do have a const drop impl.
Since for promoted the drop implementation is never executed,
this lead to observable change in behaviour. For example:
```rust
struct Panic();
impl const Drop for Panic {
fn drop(&mut self) {
panic!();
}
}
fn main() {
let _ = &Panic();
}
```
Restore the use of `NeedsDrop` qualif during promotion to avoid the issue.
Coerce const FnDefs to implement const Fn traits
You can now pass a FnDef to a function expecting `F` where `F: ~const FnTrait`.
r? ``@oli-obk``
``@rustbot`` label T-compiler F-const_trait_impl
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`
