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 @@ fn visit_param_bound(&mut self, bound: &'a GenericBound, ctxt: BoundKind) {
                         };
                         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 {

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 @@ fn recover_paren_lifetime(&mut self, lo: Span, lt_span: Span) -> PResult<'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 @@ fn parse_trait_bound_modifiers(&mut self) -> PResult<'a, TraitBoundModifiers> {
         } 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 @@ fn parse_trait_bound_modifiers(&mut self) -> PResult<'a, TraitBoundModifiers> {
             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 })
     }
 

tests/ui/parser/bounds-type.rs

@@ -1,4 +1,5 @@
 //@ compile-flags: -Z parse-only
+//@ edition: 2021
 
 struct S<
     T: 'a + Tr, // OK
@@ -10,10 +11,20 @@ struct S<
     T: ?'a, //~ ERROR `?` may only modify trait bounds, not lifetime bounds
 
     T: ~const Tr, // OK
-    T: ~const ?Tr, // OK
+    T: ~const ?Tr, //~ ERROR `~const` trait not allowed with `?` trait polarity modifier
     T: ~const Tr + 'a, // OK
     T: ~const 'a, //~ ERROR `~const` may only modify trait bounds, not lifetime bounds
     T: const 'a, //~ ERROR `const` may only modify trait bounds, not lifetime bounds
+
+    T: async Tr, // OK
+    T: async ?Tr, //~ ERROR `async` trait not allowed with `?` trait polarity modifier
+    T: async Tr + 'a, // OK
+    T: async 'a, //~ ERROR `async` may only modify trait bounds, not lifetime bounds
+
+    T: const async Tr, // OK
+    T: const async ?Tr, //~ ERROR `const async` trait not allowed with `?` trait polarity modifier
+    T: const async Tr + 'a, // OK
+    T: const async 'a, //~ ERROR `const` may only modify trait bounds, not lifetime bounds
 >;
 
 fn main() {}

tests/ui/parser/bounds-type.stderr

@@ -1,5 +1,5 @@
 error: `for<...>` binder not allowed with `?` trait polarity modifier
-  --> $DIR/bounds-type.rs:8:16
+  --> $DIR/bounds-type.rs:9:16
    |
 LL |     T: for<'a> ?Trait,
    |           ---- ^
@@ -7,22 +7,58 @@ LL |     T: for<'a> ?Trait,
    |           there is not a well-defined meaning for a higher-ranked `?` trait
 
 error: `?` may only modify trait bounds, not lifetime bounds
-  --> $DIR/bounds-type.rs:10:8
+  --> $DIR/bounds-type.rs:11:8
    |
 LL |     T: ?'a,
    |        ^
 
+error: `~const` trait not allowed with `?` trait polarity modifier
+  --> $DIR/bounds-type.rs:14:15
+   |
+LL |     T: ~const ?Tr,
+   |        ------ ^
+   |        |
+   |        there is not a well-defined meaning for a `~const ?` trait
+
 error: `~const` may only modify trait bounds, not lifetime bounds
-  --> $DIR/bounds-type.rs:15:8
+  --> $DIR/bounds-type.rs:16:8
    |
 LL |     T: ~const 'a,
    |        ^^^^^^
 
 error: `const` may only modify trait bounds, not lifetime bounds
-  --> $DIR/bounds-type.rs:16:8
+  --> $DIR/bounds-type.rs:17:8
    |
 LL |     T: const 'a,
    |        ^^^^^
 
-error: aborting due to 4 previous errors
+error: `async` trait not allowed with `?` trait polarity modifier
+  --> $DIR/bounds-type.rs:20:14
+   |
+LL |     T: async ?Tr,
+   |        ----- ^
+   |        |
+   |        there is not a well-defined meaning for a `async ?` trait
+
+error: `async` may only modify trait bounds, not lifetime bounds
+  --> $DIR/bounds-type.rs:22:8
+   |
+LL |     T: async 'a,
+   |        ^^^^^
+
+error: `const async` trait not allowed with `?` trait polarity modifier
+  --> $DIR/bounds-type.rs:25:20
+   |
+LL |     T: const async ?Tr,
+   |        ----------- ^
+   |        |
+   |        there is not a well-defined meaning for a `const async ?` trait
+
+error: `const` may only modify trait bounds, not lifetime bounds
+  --> $DIR/bounds-type.rs:27:8
+   |
+LL |     T: const async 'a,
+   |        ^^^^^
+
+error: aborting due to 9 previous errors
 

tests/ui/rfcs/rfc-2632-const-trait-impl/mutually-exclusive-trait-bound-modifiers.rs

@@ -1,17 +1,17 @@
 #![feature(const_trait_impl)]
 
 const fn maybe_const_maybe<T: ~const ?Sized>() {}
-//~^ ERROR `~const` and `?` are mutually exclusive
+//~^ ERROR `~const` trait not allowed with `?` trait polarity modifier
 
 fn const_maybe<T: const ?Sized>() {}
-//~^ ERROR `const` and `?` are mutually exclusive
+//~^ ERROR `const` trait not allowed with `?` trait polarity modifier
 
 const fn maybe_const_negative<T: ~const !Trait>() {}
-//~^ ERROR `~const` and `!` are mutually exclusive
+//~^ ERROR `~const` trait not allowed with `!` trait polarity modifier
 //~| ERROR negative bounds are not supported
 
 fn const_negative<T: const !Trait>() {}
-//~^ ERROR `const` and `!` are mutually exclusive
+//~^ ERROR `const` trait not allowed with `!` trait polarity modifier
 //~| ERROR negative bounds are not supported
 
 #[const_trait]

tests/ui/rfcs/rfc-2632-const-trait-impl/mutually-exclusive-trait-bound-modifiers.stderr

@@ -1,26 +1,34 @@
-error: `~const` and `?` are mutually exclusive
+error: `~const` trait not allowed with `?` trait polarity modifier
-  --> $DIR/mutually-exclusive-trait-bound-modifiers.rs:3:31
+  --> $DIR/mutually-exclusive-trait-bound-modifiers.rs:3:38
    |
 LL | const fn maybe_const_maybe<T: ~const ?Sized>() {}
-   |                               ^^^^^^^^^^^^^
+   |                               ------ ^
+   |                               |
+   |                               there is not a well-defined meaning for a `~const ?` trait
 
-error: `const` and `?` are mutually exclusive
+error: `const` trait not allowed with `?` trait polarity modifier
-  --> $DIR/mutually-exclusive-trait-bound-modifiers.rs:6:19
+  --> $DIR/mutually-exclusive-trait-bound-modifiers.rs:6:25
    |
 LL | fn const_maybe<T: const ?Sized>() {}
-   |                   ^^^^^^^^^^^^
+   |                   ----- ^
+   |                   |
+   |                   there is not a well-defined meaning for a `const ?` trait
 
-error: `~const` and `!` are mutually exclusive
+error: `~const` trait not allowed with `!` trait polarity modifier
-  --> $DIR/mutually-exclusive-trait-bound-modifiers.rs:9:34
+  --> $DIR/mutually-exclusive-trait-bound-modifiers.rs:9:41
    |
 LL | const fn maybe_const_negative<T: ~const !Trait>() {}
-   |                                  ^^^^^^^^^^^^^
+   |                                  ------ ^
+   |                                  |
+   |                                  there is not a well-defined meaning for a `~const !` trait
 
-error: `const` and `!` are mutually exclusive
+error: `const` trait not allowed with `!` trait polarity modifier
-  --> $DIR/mutually-exclusive-trait-bound-modifiers.rs:13:22
+  --> $DIR/mutually-exclusive-trait-bound-modifiers.rs:13:28
    |
 LL | fn const_negative<T: const !Trait>() {}
-   |                      ^^^^^^^^^^^^
+   |                      ----- ^
+   |                      |
+   |                      there is not a well-defined meaning for a `const !` trait
 
 error: negative bounds are not supported
   --> $DIR/mutually-exclusive-trait-bound-modifiers.rs:9:41