Closed
Description
I tried this code:
pub trait Foo {
type X: PartialEq;
}
pub trait Bar {
type Y: Eq;
}
// this doesn't work for some reason!!!
// fn get_foo<T: Foo>(t: &T) -> &dyn Foo<X = T::X> {
// t
// }
fn get_bar<T: Bar>(t: &T) -> &dyn Bar<Y = T::Y> {
t
}If I uncomment out get_foo, I get an error, but get_bar doesn't error!
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=f20eef39a18a6fdc11cbeea9778ee294
To me this makes no sense since Bar is more strict than Foo, so either: both should not compile or Foo should compile (Bar may not since it has more requirements on it's associated type).
What's more fun is that this used to compile way back in v1.15.1 according to godbolt 😄 https://godbolt.org/z/TvPsjPq5o
But no version after that
Meta
playground is on v1.76.0 when I ran this
rustc --version --verbose:
rustc 1.79.0-nightly (3c85e5624 2024-03-18)
binary: rustc
commit-hash: 3c85e56249b0b1942339a6a989a971bf6f1c9e0f
commit-date: 2024-03-18
host: x86_64-unknown-linux-gnu
release: 1.79.0-nightly
LLVM version: 18.1.2
Compile Error
error[E0038]: the trait `Foo` cannot be made into an object
--> src/lib.rs:10:31
|
10 | fn get_foo<T: Foo>(t: &T) -> &dyn Foo<X = T::X> {
| ^^^^^^^^^^^^^^^^^ `Foo` cannot be made into an object
|
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
--> src/lib.rs:2:13
|
1 | pub trait Foo {
| --- this trait cannot be made into an object...
2 | type X: PartialEq;
| ^^^^^^^^^ ...because it uses `Self` as a type parameter
For more information about this error, try `rustc --explain E0038`.
error: could not compile `playground` (lib) due to 1 previous error