@nikomatsakis: could you give your opinion on refactoring
TypeVariableOrigin here: https://github.com/rust-lang/rust/pull/59331#discussion_r267711284 ?
it's essentially just pulling out the common
span into a separate field
but alternatively we could use a pair, or a
(there's a similar question about whether to do the same for the region variable origins, though it means introducing some dummy spans for the variants that currently have none)