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|
//! This test demonstrates how `LayoutError::NormalizationFailure` can happen and why
//! it is necessary.
//!
//! This code does not cause an immediate normalization error in typeck, because we
//! don't reveal the hidden type returned by `opaque<T>` in the analysis typing mode.
//! Instead, `<{opaque} as Project2>::Assoc2` is a *rigid projection*, because we know
//! that `{opaque}: Project2` holds, due to the opaque type's `impl Project2` bound,
//! but cannot normalize `<{opaque} as Project2>::Assoc2` any further.
//!
//! However, in the post-analysis typing mode, which is used for the layout computation,
//! the opaque's hidden type is revealed to be `PhantomData<T>`, and now we fail to
//! normalize `<PhantomData<T> as Project2>::Assoc2` if there is a `T: Project1` bound
//! in the param env! This happens, because `PhantomData<T>: Project2` only holds if
//! `<T as Project1>::Assoc1 == ()` holds. This would usually be satisfied by the
//! blanket `impl<T> Project1 for T`, but due to the `T: Project1` bound we do not
//! normalize `<T as Project1>::Assoc1` via the impl and treat it as rigid instead.
//! Therefore, `PhantomData<T>: Project2` does NOT hold and normalizing
//! `<PhantomData<T> as Project2>::Assoc2` fails.
//!
//! Note that this layout error can only happen when computing the layout in a generic
//! context, which is not required for codegen, but may happen for lints, MIR optimizations,
//! and the transmute check.
use std::marker::PhantomData;
trait Project1 {
type Assoc1;
}
impl<T> Project1 for T {
type Assoc1 = ();
}
trait Project2 {
type Assoc2;
fn get(self) -> Self::Assoc2;
}
impl<T: Project1<Assoc1 = ()>> Project2 for PhantomData<T> {
type Assoc2 = ();
fn get(self) -> Self::Assoc2 {}
}
fn opaque<T>() -> impl Project2 {
PhantomData::<T>
}
fn check<T: Project1>() {
unsafe {
std::mem::transmute::<_, ()>(opaque::<T>().get());
//~^ ERROR: cannot transmute
//~| NOTE: (unable to determine layout for `<impl Project2 as Project2>::Assoc2` because `<impl Project2 as Project2>::Assoc2` cannot be normalized)
//~| NOTE: (0 bits)
}
}
fn main() {}
|
