3960: ellipsis in tuple patterns r=JoshMcguigan a=JoshMcguigan
This PR lowers ellipsis in tuple patterns. It fixes a bug in the way ellipsis were previously lowered (by replacing the ellipsis with a single `Pat::Wild` no matter how many items the `..` was taking the place of).
It also uses this new information to properly handle `..` in tuple struct patterns when perform match statement exhaustiveness checks.
While this PR provides the building blocks for match statement exhaustiveness checks for tuples, there are some additional challenges there, so that is still unimplemented (unlike tuple structs).
Co-authored-by: Josh Mcguigan <joshmcg88@gmail.com>
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