And additionally enforce ? and async/const aren't mixed

compiler/rustc_ast_passes/messages.ftl

@@ -152,8 +152,6 @@ ast_passes_impl_trait_path = `impl Trait` is not allowed in path parameters
 ast_passes_incompatible_features = `{$f1}` and `{$f2}` are incompatible, using them at the same time is not allowed
     .help = remove one of these features
 
-ast_passes_incompatible_trait_bound_modifiers = `{$left}` and `{$right}` are mutually exclusive
-
 ast_passes_inherent_cannot_be = inherent impls cannot be {$annotation}
     .because = {$annotation} because of this
     .type = inherent impl for this type

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -1435,17 +1435,6 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                         };
                         self.dcx().emit_err(errors::TildeConstDisallowed { span, reason });
                     }
-                    (
-                        _,
-                        BoundConstness::Always(_) | BoundConstness::Maybe(_),
-                        BoundPolarity::Negative(_) | BoundPolarity::Maybe(_),
-                    ) => {
-                        self.dcx().emit_err(errors::IncompatibleTraitBoundModifiers {
-                            span: bound.span(),
-                            left: modifiers.constness.as_str(),
-                            right: modifiers.polarity.as_str(),
-                        });
-                    }
                     _ => {}
                 }
 

compiler/rustc_ast_passes/src/errors.rs

@@ -657,15 +657,6 @@ pub enum TildeConstReason {
     Item,
 }
 
-#[derive(Diagnostic)]
-#[diag(ast_passes_incompatible_trait_bound_modifiers)]
-pub struct IncompatibleTraitBoundModifiers {
-    #[primary_span]
-    pub span: Span,
-    pub left: &'static str,
-    pub right: &'static str,
-}
-
 #[derive(Diagnostic)]
 #[diag(ast_passes_const_and_async)]
 pub struct ConstAndAsync {