b574c67b93
This adds a new lint to `rustc` that is used in rustdoc when a code block is empty or cannot be parsed as valid Rust code. Previously this was unconditionally a warning. As such some documentation comments were (unknowingly) abusing this to pass despite the `-Dwarnings` used when compiling `rustc`, this should not be the case anymore.
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.