Auto merge of #124388 - compiler-errors:rollup-v17b8fm, r=compiler-errors

Rollup of 4 pull requests

Successful merges:

 - #124076 (Stablise io_error_downcast)
 - #124378 (Keep the LIB env var in the compiler-builtins test)
 - #124379 (Remove special-casing for `SimplifiedType` for next solver)
 - #124381 (Renamed `DerivedObligation` to `WellFormedDeriveObligation`)

r? `@ghost`
`@rustbot` modify labels: rollup