4445: Correctly fill default type parameters r=flodiebold a=montekki
Fixes#3877
So, basically even if the parameters are omitted from the `impl` block, check the parameters in `trait` if they have a default type, and if they do go from `hir` to `ast::TypeArg`. I've added a helper for that but I am not sure that it's a proper way to go from `hir` to `ast` here.
Co-authored-by: Fedor Sakharov <fedor.sakharov@gmail.com>
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