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329e570460
Don't report bivariance error when nesting a struct with field errors into another struct
We currently have logic to avoid reporting lifetime bivariance ("lifetime parameter ... is never used") errors when a struct has field resolution errors. However, this doesn't apply transitively. This PR implements a simple visitor to do so.
This was reported [here](https://twitter.com/fasterthanlime/status/1846257921086165033) since a `derive(Deserialize, Serialize)` ends up generating helper structs which have bivariant lifetimes due to containing the offending struct (that's being derived on).
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.