Generics signature help should include type parameter defaults #12729
Description
For example, in std::ops::ControlFlow::<$0, the signature help is rendered as enum ControlFlow<B, C>, which does not tell the user that C is optional and only B needs to be provided. Rendering the default type would fix that, and result in enum ControlFlow<B, C = ()>.
It's possible that a default type would be too long when fully rendered, so the length should probably be limited (the HirDisplay machinery can already do that).