`-Znext-solver` ambiguous opaque types in HIR typeck result in poor errors

[lcnr]

fn foo() -> impl Sized {
    foo()
}

results in

error[E0282]: type annotations needed
 --> src/main.rs:2:5
  |
2 |     foo()
  |     ^^^^^ cannot infer type

error: aborting due to 1 previous error

we should mention that this ambiguity is in the hidden type of an opaque

rust/compiler/rustc_hir_typeck/src/opaque_types.rs

Lines 120 to 134 in ad85bc5

	let infer_var = hidden_type
	.ty
	.walk()
	.filter_map(ty::GenericArg::as_term)
	.find(|term| term.is_infer())
	.unwrap_or_else(|| hidden_type.ty.into());
	self.err_ctxt()
	.emit_inference_failure_err(
	self.body_id,
	hidden_type.span,
	infer_var,
	TypeAnnotationNeeded::E0282,
	false,
	)
	.emit()