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.

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -1168,12 +1168,27 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                     });
                 }
                 (_, TraitBoundModifier::MaybeConstMaybe) => {
-                    self.err_handler().emit_err(errors::OptionalConstExclusive {span: bound.span()});
+                    self.err_handler().emit_err(errors::OptionalConstExclusive {span: bound.span(), modifier: "?" });
+                }
+                (_, TraitBoundModifier::MaybeConstNegative) => {
+                    self.err_handler().emit_err(errors::OptionalConstExclusive {span: bound.span(), modifier: "!" });
                 }
                 _ => {}
             }
         }
 
+        // Negative trait bounds are not allowed to have associated constraints
+        if let GenericBound::Trait(trait_ref, TraitBoundModifier::Negative) = bound
+            && let Some(segment) = trait_ref.trait_ref.path.segments.last()
+            && let Some(ast::GenericArgs::AngleBracketed(args)) = segment.args.as_deref()
+        {
+            for arg in &args.args {
+                if let ast::AngleBracketedArg::Constraint(constraint) = arg {
+                    self.err_handler().emit_err(errors::ConstraintOnNegativeBound { span: constraint.span });
+                }
+            }
+        }
+
         visit::walk_param_bound(self, bound)
     }
 

compiler/rustc_ast_passes/src/errors.rs

@@ -567,6 +567,7 @@ pub enum TildeConstReason {
 pub struct OptionalConstExclusive {
     #[primary_span]
     pub span: Span,
+    pub modifier: &'static str,
 }
 
 #[derive(Diagnostic)]
@@ -693,3 +694,17 @@ pub struct ShowSpan {
     pub span: Span,
     pub msg: &'static str,
 }
+
+#[derive(Diagnostic)]
+#[diag(ast_passes_negative_bound_not_supported)]
+pub struct NegativeBoundUnsupported {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(ast_passes_constraint_on_negative_bound)]
+pub struct ConstraintOnNegativeBound {
+    #[primary_span]
+    pub span: Span,
+}

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -603,6 +603,12 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session) {
     gate_all!(dyn_star, "`dyn*` trait objects are experimental");
     gate_all!(const_closures, "const closures are experimental");
 
+    if !visitor.features.negative_bounds {
+        for &span in spans.get(&sym::negative_bounds).iter().copied().flatten() {
+            sess.emit_err(errors::NegativeBoundUnsupported { span });
+        }
+    }
+
     // All uses of `gate_all!` below this point were added in #65742,
     // and subsequently disabled (with the non-early gating readded).
     // We emit an early future-incompatible warning for these.