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Otherwise, we can get into a situation where you have a subtype obligation `#1 <: #2` pending, #1 is constrained by `check_casts`, but #2` is unaffected. Co-authored-by: Niko Matsakis <niko@alum.mit.edu>
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.