wrap_on_hostcall has a generic parameter S. When calling the function this parameter is instantiated with some inference variable - _ - Let's call it ?unknown for now.
When calling exec_args.set_on_hostcall we end up equating for<'r> <S as SandBox>::Access<'r> with for<'r> <?unknown as SandBox>::Access<'r>. This currently constrains ?unknown to S, even though this may not strictly be the case.
There could theoretically exist an impl like
impl<S: Sandbox> Sandbox for Wrapper<S> {
type Access<'r> = S::Access<'r>;
}
with this using wrap_on_hostcall::<Wrapper<S>>(on_hostcall); would also compile.
Your code previously compiled due to a shortcoming of rustc when handling higher ranked regions. This will be fixed in https://github.com/rust-lang/rust/pull/119849 at which point your code will fail with ambiguity.
koute
commented
7 months ago
wrap_on_hostcallhas a generic parameterS. When calling the function this parameter is instantiated with some inference variable -_- Let's call it?unknownfor now.When calling
exec_args.set_on_hostcallwe end up equatingfor<'r> <S as SandBox>::Access<'r>withfor<'r> <?unknown as SandBox>::Access<'r>. This currently constrains?unknowntoS, even though this may not strictly be the case.There could theoretically exist an impl like
with this using
wrap_on_hostcall::<Wrapper<S>>(on_hostcall);would also compile.Your code previously compiled due to a shortcoming of rustc when handling higher ranked regions. This will be fixed in https://github.com/rust-lang/rust/pull/119849 at which point your code will fail with ambiguity.