relaxing a non-`Sized` bounds is a hard-warning

[lcnr]

rust/compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs

Lines 97 to 111 in b605c65

	let mut seen_sized_unbound = false;
	for unbound in unbounds {
	if let Some(sized_def_id) = sized_def_id
	&& unbound.trait_ref.path.res == Res::Def(DefKind::Trait, sized_def_id)
	{
	seen_sized_unbound = true;
	continue;
	}
	// There was a `?Trait` bound, but it was not `?Sized`; warn.
	self.dcx().span_warn(
	unbound.span,
	"relaxing a default bound only does something for `?Sized`; \
	all other traits are not bound by default",
	);
	}

fn foo() -> impl ?Send {}

warning: relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
 --> src/lib.rs:1:18
  |
1 | fn foo() -> impl ?Send {}
  |                  ^^^^^

This should either be a (future-compat) lint or a hard-error.

[lcnr]

this warning exists since pre 1.0 🤔

[theemathas]

This code compiles. Is it supposed to?

trait Generic<T> {}
trait Huh {
    type Assoc;
}
pub fn f<T: ?Generic<<T as Huh>::Assoc>>() {}

[theemathas]

Clearly this isn't supposed to compile, right?

trait Generic<const N: usize> {}

pub fn f<T: ?Generic<{ panic!() }>>() {}

[lcnr]

This code compiles. Is it supposed to?

Clearly this isn't supposed to compile, right?

I don't think they should, which is why I opened this issue. We should move all of these to a hard error