### ROOT CAUSE
The issue is caused by an ICE in the Rust compiler's const evaluator when comparing vtable entries for two different trait objects. The const evaluator is panicking because it is expecting two slices of vtable entries to be equal, but they are not. This likely occurs due to the compiler's handling of trait objects with generic supertraits during constant evaluation.
### CODE FIX
Change the const to a static to avoid the const evaluation that triggers the ICE.
Original code:
```rust
trait Supertrait
{
fn static(mut self) {}
}
trait Trait: Supertrait<> {}
impl
Trait
for () {}
const fn upcast
(x: &dyn Trait
) -> &dyn Trait
{
x
}
const _: &'static dyn Supertrait<()> = foo();
```
Fixed code:
```rust
trait Supertrait {
fn static(mut self) {}
}
trait Trait: Supertrait<> {}
impl
Trait
for () {}
fn upcast
(x: &dyn Trait
) -> &dyn Trait
{
x
}
static UPCASTED: &'static dyn Supertrait<()> = upcast(&());
// Use UPCASTED as needed.
```
This change avoids the const evaluation and thus the ICE.
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