Rollup merge of #110791 - compiler-errors:negative-bounds, r=oli-obk

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.

src/librustdoc/html/format.rs

@@ -439,6 +439,7 @@ impl clean::GenericBound {
                 let modifier_str = match modifier {
                     hir::TraitBoundModifier::None => "",
                     hir::TraitBoundModifier::Maybe => "?",
+                    hir::TraitBoundModifier::Negative => "!",
                     // ~const is experimental; do not display those bounds in rustdoc
                     hir::TraitBoundModifier::MaybeConst => "",
                 };

src/librustdoc/json/conversions.rs

@@ -533,6 +533,10 @@ pub(crate) fn from_trait_bound_modifier(
         None => TraitBoundModifier::None,
         Maybe => TraitBoundModifier::Maybe,
         MaybeConst => TraitBoundModifier::MaybeConst,
+        // FIXME(negative-bounds): This bound should be rendered negative, but
+        // since that's experimental, maybe let's not add it to the rustdoc json
+        // API just now...
+        Negative => TraitBoundModifier::None,
     }
 }