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

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`

src/test/ui/rfc-2632-const-trait-impl/const-drop-fail.precise.stderr

@@ -0,0 +1,59 @@
+error: `~const` is not allowed here
+  --> $DIR/const-drop-fail.rs:27:35
+   |
+LL | struct ConstDropImplWithBounds<T: ~const A>(PhantomData<T>);
+   |                                   ^^^^^^^^
+   |
+   = note: only allowed on bounds on traits' associated types and functions, const fns, const impls and its associated functions
+
+error[E0277]: the trait bound `NonTrivialDrop: Drop` is not satisfied
+  --> $DIR/const-drop-fail.rs:45:5
+   |
+LL |     NonTrivialDrop,
+   |     ^^^^^^^^^^^^^^ the trait `Drop` is not implemented for `NonTrivialDrop`
+   |
+note: required by a bound in `check`
+  --> $DIR/const-drop-fail.rs:36:19
+   |
+LL | const fn check<T: ~const Drop>(_: T) {}
+   |                   ^^^^^^^^^^^ required by this bound in `check`
+
+error[E0277]: the trait bound `ConstImplWithDropGlue: Drop` is not satisfied
+  --> $DIR/const-drop-fail.rs:47:5
+   |
+LL |     ConstImplWithDropGlue(NonTrivialDrop),
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Drop` is not implemented for `ConstImplWithDropGlue`
+   |
+note: required by a bound in `check`
+  --> $DIR/const-drop-fail.rs:36:19
+   |
+LL | const fn check<T: ~const Drop>(_: T) {}
+   |                   ^^^^^^^^^^^ required by this bound in `check`
+
+error[E0277]: the trait bound `NonTrivialDrop: A` is not satisfied
+  --> $DIR/const-drop-fail.rs:49:5
+   |
+LL |     ConstDropImplWithBounds::<NonTrivialDrop>(PhantomData),
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `A` is not implemented for `NonTrivialDrop`
+   |
+note: required by `ConstDropImplWithBounds`
+  --> $DIR/const-drop-fail.rs:27:1
+   |
+LL | struct ConstDropImplWithBounds<T: ~const A>(PhantomData<T>);
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0277]: the trait bound `NonTrivialDrop: A` is not satisfied
+  --> $DIR/const-drop-fail.rs:49:5
+   |
+LL |     ConstDropImplWithBounds::<NonTrivialDrop>(PhantomData),
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `A` is not implemented for `NonTrivialDrop`
+   |
+note: required by a bound in `ConstDropImplWithBounds`
+  --> $DIR/const-drop-fail.rs:27:35
+   |
+LL | struct ConstDropImplWithBounds<T: ~const A>(PhantomData<T>);
+   |                                   ^^^^^^^^ required by this bound in `ConstDropImplWithBounds`
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

src/test/ui/rfc-2632-const-trait-impl/const-drop-fail.rs

@@ -0,0 +1,54 @@
+// revisions: stock precise
+#![feature(const_trait_impl)]
+#![feature(const_mut_refs)]
+#![feature(const_fn_trait_bound)]
+#![cfg_attr(precise, feature(const_precise_live_drops))]
+
+use std::marker::PhantomData;
+
+struct NonTrivialDrop;
+
+impl Drop for NonTrivialDrop {
+    fn drop(&mut self) {
+        println!("Non trivial drop");
+    }
+}
+
+struct ConstImplWithDropGlue(NonTrivialDrop);
+
+impl const Drop for ConstImplWithDropGlue {
+    fn drop(&mut self) {}
+}
+
+trait A { fn a() { println!("A"); } }
+
+impl A for NonTrivialDrop {}
+
+struct ConstDropImplWithBounds<T: ~const A>(PhantomData<T>);
+//~^ ERROR `~const` is not allowed
+
+impl<T: ~const A> const Drop for ConstDropImplWithBounds<T> {
+    fn drop(&mut self) {
+        T::a();
+    }
+}
+
+const fn check<T: ~const Drop>(_: T) {}
+
+macro_rules! check_all {
+    ($($exp:expr),*$(,)?) => {$(
+        const _: () = check($exp);
+    )*};
+}
+
+check_all! {
+    NonTrivialDrop,
+    //~^ ERROR the trait bound
+    ConstImplWithDropGlue(NonTrivialDrop),
+    //~^ ERROR the trait bound
+    ConstDropImplWithBounds::<NonTrivialDrop>(PhantomData),
+    //~^ ERROR the trait bound
+    //~| ERROR the trait bound
+}
+
+fn main() {}

src/test/ui/rfc-2632-const-trait-impl/const-drop-fail.stock.stderr

@@ -0,0 +1,59 @@
+error: `~const` is not allowed here
+  --> $DIR/const-drop-fail.rs:27:35
+   |
+LL | struct ConstDropImplWithBounds<T: ~const A>(PhantomData<T>);
+   |                                   ^^^^^^^^
+   |
+   = note: only allowed on bounds on traits' associated types and functions, const fns, const impls and its associated functions
+
+error[E0277]: the trait bound `NonTrivialDrop: Drop` is not satisfied
+  --> $DIR/const-drop-fail.rs:45:5
+   |
+LL |     NonTrivialDrop,
+   |     ^^^^^^^^^^^^^^ the trait `Drop` is not implemented for `NonTrivialDrop`
+   |
+note: required by a bound in `check`
+  --> $DIR/const-drop-fail.rs:36:19
+   |
+LL | const fn check<T: ~const Drop>(_: T) {}
+   |                   ^^^^^^^^^^^ required by this bound in `check`
+
+error[E0277]: the trait bound `ConstImplWithDropGlue: Drop` is not satisfied
+  --> $DIR/const-drop-fail.rs:47:5
+   |
+LL |     ConstImplWithDropGlue(NonTrivialDrop),
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Drop` is not implemented for `ConstImplWithDropGlue`
+   |
+note: required by a bound in `check`
+  --> $DIR/const-drop-fail.rs:36:19
+   |
+LL | const fn check<T: ~const Drop>(_: T) {}
+   |                   ^^^^^^^^^^^ required by this bound in `check`
+
+error[E0277]: the trait bound `NonTrivialDrop: A` is not satisfied
+  --> $DIR/const-drop-fail.rs:49:5
+   |
+LL |     ConstDropImplWithBounds::<NonTrivialDrop>(PhantomData),
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `A` is not implemented for `NonTrivialDrop`
+   |
+note: required by `ConstDropImplWithBounds`
+  --> $DIR/const-drop-fail.rs:27:1
+   |
+LL | struct ConstDropImplWithBounds<T: ~const A>(PhantomData<T>);
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0277]: the trait bound `NonTrivialDrop: A` is not satisfied
+  --> $DIR/const-drop-fail.rs:49:5
+   |
+LL |     ConstDropImplWithBounds::<NonTrivialDrop>(PhantomData),
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `A` is not implemented for `NonTrivialDrop`
+   |
+note: required by a bound in `ConstDropImplWithBounds`
+  --> $DIR/const-drop-fail.rs:27:35
+   |
+LL | struct ConstDropImplWithBounds<T: ~const A>(PhantomData<T>);
+   |                                   ^^^^^^^^ required by this bound in `ConstDropImplWithBounds`
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

src/test/ui/rfc-2632-const-trait-impl/const-drop.rs

@@ -0,0 +1,81 @@
+// run-pass
+// revisions: stock precise
+#![feature(const_trait_impl)]
+#![feature(const_fn_trait_bound)]
+#![feature(const_mut_refs)]
+#![feature(const_panic)]
+#![cfg_attr(precise, feature(const_precise_live_drops))]
+
+struct S<'a>(&'a mut u8);
+
+impl<'a> const Drop for S<'a> {
+    fn drop(&mut self) {
+        *self.0 += 1;
+    }
+}
+
+const fn a<T: ~const Drop>(_: T) {}
+
+const fn b() -> u8 {
+    let mut c = 0;
+    let _ = S(&mut c);
+    a(S(&mut c));
+    c
+}
+
+const C: u8 = b();
+
+macro_rules! implements_const_drop {
+    ($($exp:expr),*$(,)?) => {
+        $(
+            const _: () = a($exp);
+        )*
+    }
+}
+
+#[allow(dead_code)]
+mod t {
+    pub struct Foo;
+    pub enum Bar { A }
+    pub fn foo() {}
+    pub struct ConstDrop;
+
+    impl const Drop for ConstDrop {
+        fn drop(&mut self) {}
+    }
+
+    pub struct HasConstDrop(pub ConstDrop);
+    pub struct TrivialFields(pub u8, pub i8, pub usize, pub isize);
+}
+
+use t::*;
+
+implements_const_drop! {
+    1u8,
+    2,
+    3.0,
+    Foo,
+    Bar::A,
+    foo,
+    ConstDrop,
+    HasConstDrop(ConstDrop),
+    TrivialFields(1, 2, 3, 4),
+    &1,
+    &1 as *const i32,
+}
+
+fn main() {
+    struct HasDropGlue(Box<u8>);
+    struct HasDropImpl;
+    impl Drop for HasDropImpl {
+        fn drop(&mut self) {
+            println!("not trivial drop");
+        }
+    }
+
+    // These types should pass because ~const in a non-const context should have no effect.
+    a(HasDropGlue(Box::new(0)));
+    a(HasDropImpl);
+
+    assert_eq!(C, 2);
+}

src/test/ui/rfc-2632-const-trait-impl/inherent-impl-const-bounds.rs

@@ -0,0 +1,21 @@
+// check-pass
+#![feature(const_trait_impl)]
+#![feature(const_fn_trait_bound)]
+
+struct S;
+
+trait A {}
+trait B {}
+
+impl const A for S {}
+impl const B for S {}
+
+impl S {
+    const fn a<T: ~const A>() where T: ~const B {
+
+    }
+}
+
+const _: () = S::a::<S>();
+
+fn main() {}