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

compiler/rustc_parse/messages.ftl

@@ -575,6 +575,9 @@ parse_missing_trait_in_trait_impl = missing trait in a trait impl
 parse_modifier_lifetime = `{$modifier}` may only modify trait bounds, not lifetime bounds
     .suggestion = remove the `{$modifier}`
 
+parse_modifiers_and_polarity = `{$modifiers_concatenated}` trait not allowed with `{$polarity}` trait polarity modifier
+    .label = there is not a well-defined meaning for a `{$modifiers_concatenated} {$polarity}` trait
+
 parse_more_than_one_char = character literal may only contain one codepoint
     .followed_by = this `{$chr}` is followed by the combining {$len ->
         [one] mark

compiler/rustc_parse/src/errors.rs

@@ -3060,3 +3060,14 @@ pub struct BinderAndPolarity {
     pub binder_span: Span,
     pub polarity: &'static str,
 }
+
+#[derive(Diagnostic)]
+#[diag(parse_modifiers_and_polarity)]
+pub struct PolarityAndModifiers {
+    #[primary_span]
+    pub polarity_span: Span,
+    #[label]
+    pub modifiers_span: Span,
+    pub polarity: &'static str,
+    pub modifiers_concatenated: String,
+}

compiler/rustc_parse/src/parser/ty.rs

@@ -930,6 +930,7 @@ impl<'a> Parser<'a> {
     /// TRAIT_BOUND_MODIFIERS = [["~"] "const"] ["async"] ["?" | "!"]
     /// ```
     fn parse_trait_bound_modifiers(&mut self) -> PResult<'a, TraitBoundModifiers> {
+        let modifier_lo = self.token.span;
         let constness = if self.eat(&token::Tilde) {
             let tilde = self.prev_token.span;
             self.expect_keyword(kw::Const)?;
@@ -962,6 +963,7 @@ impl<'a> Parser<'a> {
         } else {
             BoundAsyncness::Normal
         };
+        let modifier_hi = self.prev_token.span;
 
         let polarity = if self.eat(&token::Question) {
             BoundPolarity::Maybe(self.prev_token.span)
@@ -972,6 +974,33 @@ impl<'a> Parser<'a> {
             BoundPolarity::Positive
         };
 
+        // Enforce the mutual-exclusivity of `const`/`async` and `?`/`!`.
+        match polarity {
+            BoundPolarity::Positive => {
+                // All trait bound modifiers allowed to combine with positive polarity
+            }
+            BoundPolarity::Maybe(polarity_span) | BoundPolarity::Negative(polarity_span) => {
+                match (asyncness, constness) {
+                    (BoundAsyncness::Normal, BoundConstness::Never) => {
+                        // Ok, no modifiers.
+                    }
+                    (_, _) => {
+                        let constness = constness.as_str();
+                        let asyncness = asyncness.as_str();
+                        let glue =
+                            if !constness.is_empty() && !asyncness.is_empty() { " " } else { "" };
+                        let modifiers_concatenated = format!("{constness}{glue}{asyncness}");
+                        self.dcx().emit_err(errors::PolarityAndModifiers {
+                            polarity_span,
+                            polarity: polarity.as_str(),
+                            modifiers_span: modifier_lo.to(modifier_hi),
+                            modifiers_concatenated,
+                        });
+                    }
+                }
+            }
+        }
+
         Ok(TraitBoundModifiers { constness, asyncness, polarity })
     }