Implement negative bounds

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -1168,7 +1168,10 @@ 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: "!" });
                 }
                 _ => {}
             }

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 +694,10 @@ 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,
+}

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.