80df4ab403
Implement negative bounds for internal testing purposes Implements partial support the `!` negative polarity on trait bounds. This is incomplete, but should allow us to at least be able to play with the feature. Not even gonna consider them as a public-facing feature, but I'm implementing them because would've been nice to have in UI tests, for example in #110671.
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.