Implement negative bounds

compiler/rustc_ast/src/ast.rs

@@ -287,12 +287,20 @@ pub enum TraitBoundModifier {
     /// No modifiers
     None,
 
+    /// `!Trait`
+    Negative,
+
     /// `?Trait`
     Maybe,
 
     /// `~const Trait`
     MaybeConst,
 
+    /// `~const !Trait`
+    //
+    // This parses but will be rejected during AST validation.
+    MaybeConstNegative,
+
     /// `~const ?Trait`
     //
     // This parses but will be rejected during AST validation.
@@ -2446,6 +2454,16 @@ impl fmt::Debug for ImplPolarity {
     }
 }
 
+#[derive(Copy, Clone, PartialEq, Encodable, Decodable, HashStable_Generic)]
+pub enum BoundPolarity {
+    /// `Type: Trait`
+    Positive,
+    /// `Type: !Trait`
+    Negative(Span),
+    /// `Type: ?Trait`
+    Maybe(Span),
+}
+
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub enum FnRetTy {
     /// Returns type is not specified.