Auto merge of rust-lang#132329 - compiler-errors:fn-and-destruct, r=lcnr

bors · bors · commit a9398015dc4b · 2024-11-23T02:03:50.000Z
Implement `~const Destruct` effect goal in the new solver

This also fixed a subtle bug/limitation of the `NeedsConstDrop` check. Specifically, the "`Qualif`" API basically treats const drops as totally structural, even though dropping something that has an explicit `Drop` implementation cannot be structurally decomposed. For example:

```rust
#![feature(const_trait_impl)]

#[const_trait] trait Foo {
    fn foo();
}

struct Conditional&lt;T: Foo&gt;(T);

impl Foo for () {
    fn foo() {
        println!("uh oh");
    }
}

impl&lt;T&gt; const Drop for Conditional&lt;T&gt; where T: ~const Foo {
    fn drop(&amp;mut self) {
        T::foo();
    }
}

const FOO: () = {
    let _ = Conditional(());
    //~^ This should error.
};

fn main() {}
```

In this example, when checking if the `Conditional(())` rvalue is const-drop, since `Conditional` has a const destructor, we would previously recurse into the `()` value and determine it has nothing to drop, which means that it is considered to *not* need a const drop -- even though dropping `Conditional(())` would mean evaluating the destructor which relies on that `T: const Foo` bound to hold!

This could be fixed alternatively by banning any const conditions on `const Drop` impls, but that really sucks -- that means that basically no *interesting* const drop impls could be written. We have the capability to totally and intuitively support the right behavior, which I've implemented here.
diff --git a/core/src/marker.rs b/core/src/marker.rs
@@ -953,7 +953,7 @@ marker_impls! {
 ///
 /// This should be used for `~const` bounds,
 /// as non-const bounds will always hold for every type.
-#[unstable(feature = "const_trait_impl", issue = "67792")]
+#[unstable(feature = "const_destruct", issue = "133214")]
 #[lang = "destruct"]
 #[rustc_on_unimplemented(message = "can't drop `{Self}`", append_const_msg)]
 #[rustc_deny_explicit_impl(implement_via_object = false)]
