Add impl_trait_in_bindings to INCOMPLETE_FEATURES

[matthewjasper]

cc #60473, #60407, #58956

[varkor]

If you want to fix this, all you need to do is add the string "impl_trait_in_bindings" to:

rust/src/libsyntax/feature_gate.rs

Lines 565 to 568 in 0df1e57

	const INCOMPLETE_FEATURES: &[&str] = &[
	"generic_associated_types",
	"const_generics"
	];

And update the tests that make use of that feature to catch the new warning that is emitted.