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When evaluating an `ExprKind::Call`, we first have to `check_expr` on it's callee. When this one is a `ExprKind::Path`, we had to evaluate the bounds introduced for its arguments, but by the time we evaluated them we no longer had access to the argument spans. Now we special case this so that we can point at the right place on unsatisfied bounds. This also allows the E0277 deduplication to kick in correctly, so we now emit fewer errors.
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.