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rust-lang / trait-system-refactor-initiative

The Rustc Trait System Refactor Initiative
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new solver uses a fixpoint when evaluating nested goals #102

Open lcnr opened 7 months ago

lcnr commented 7 months ago

the old solver does not do so during candidate selection, causing the new solver to accept more code:

during coherence

trait Foo {}
impl<T> Foo for (u8, T, T) {}
trait NotU8 {}
trait Bar {}
impl<T, U: NotU8> Bar for (T, T, U) {}

trait NeedsFixpoint {}
impl<T: Foo + Bar> NeedsFixpoint for T {}
impl NeedsFixpoint for (u8, u8, u8) {}

trait Overlap {}
impl<T: NeedsFixpoint> Overlap for T {}
impl<T, U: NotU8, V> Overlap for (T, U, V) {}

and outside of it

trait Foo {}
impl<T> Foo for (u8, T, T) {}
trait NotU8 {}
trait Bar {}
impl<T, U: NotU8> Bar for (T, T, U) {}

trait NeedsFixpoint {}
impl<T: Foo + Bar> NeedsFixpoint for T {}
impl NeedsFixpoint for (u8, u8, u8) {}

fn impls_needs_fixpoint<T: NeedsFixpoint>() {}

fn main() {
    impls_needs_fixpoint::<(_, _, _)>()
}