Auto merge of #119163 - fmease:refactor-ast-trait-bound-modifiers, r=compiler-errors

Refactor AST trait bound modifiers

Instead of having two types to represent trait bound modifiers in the parser / the AST (`parser::ty::BoundModifiers` & `ast::TraitBoundModifier`), only to map one to the other later, just use `parser::ty::BoundModifiers` (moved & renamed to `ast::TraitBoundModifiers`).

The struct type is more extensible and easier to deal with (see [here](https://github.com/rust-lang/rust/pull/119099/files#r1430749981) and [here](https://github.com/rust-lang/rust/pull/119099/files#r1430752116) for context) since it more closely models what it represents: A compound of two kinds of modifiers, constness and polarity. Modeling this as an enum (the now removed `ast::TraitBoundModifier`) meant one had to add a new variant per *combination* of modifier kind, which simply isn't scalable and which lead to a lot of explicit non-DRY matches.

NB: `hir::TraitBoundModifier` being an enum is fine since HIR doesn't need to worry representing invalid modifier kind combinations as those get rejected during AST validation thereby immensely cutting down the number of possibilities.

compiler/rustc_ast/src/ast.rs

@@ -286,41 +286,16 @@ impl ParenthesizedArgs {
 
 pub use crate::node_id::{NodeId, CRATE_NODE_ID, DUMMY_NODE_ID};
 
-/// A modifier on a bound, e.g., `?Trait` or `~const Trait`.
-///
-/// Negative bounds should also be handled here.
+/// Modifiers on a trait bound like `~const`, `?` and `!`.
 #[derive(Copy, Clone, PartialEq, Eq, Encodable, Decodable, Debug)]
-pub enum TraitBoundModifier {
-    /// No modifiers
-    None,
-
-    /// `!Trait`
-    Negative,
-
-    /// `?Trait`
-    Maybe,
-
-    /// `~const Trait`
-    MaybeConst(Span),
-
-    /// `~const !Trait`
-    //
-    // This parses but will be rejected during AST validation.
-    MaybeConstNegative,
-
-    /// `~const ?Trait`
-    //
-    // This parses but will be rejected during AST validation.
-    MaybeConstMaybe,
+pub struct TraitBoundModifiers {
+    pub constness: BoundConstness,
+    pub polarity: BoundPolarity,
 }
 
-impl TraitBoundModifier {
-    pub fn to_constness(self) -> Const {
-        match self {
-            Self::MaybeConst(span) => Const::Yes(span),
-            _ => Const::No,
-        }
-    }
+impl TraitBoundModifiers {
+    pub const NONE: Self =
+        Self { constness: BoundConstness::Never, polarity: BoundPolarity::Positive };
 }
 
 /// The AST represents all type param bounds as types.
@@ -329,7 +304,7 @@ impl TraitBoundModifier {
 /// detects `Copy`, `Send` and `Sync`.
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub enum GenericBound {
-    Trait(PolyTraitRef, TraitBoundModifier),
+    Trait(PolyTraitRef, TraitBoundModifiers),
     Outlives(Lifetime),
 }
 
@@ -1193,7 +1168,7 @@ impl Expr {
         match &self.kind {
             ExprKind::Path(None, path) => Some(GenericBound::Trait(
                 PolyTraitRef::new(ThinVec::new(), path.clone(), self.span),
-                TraitBoundModifier::None,
+                TraitBoundModifiers::NONE,
             )),
             _ => None,
         }
@@ -2491,6 +2466,15 @@ pub enum Const {
     No,
 }
 
+impl From<BoundConstness> for Const {
+    fn from(constness: BoundConstness) -> Self {
+        match constness {
+            BoundConstness::Maybe(span) => Self::Yes(span),
+            BoundConstness::Never => Self::No,
+        }
+    }
+}
+
 /// Item defaultness.
 /// For details see the [RFC #2532](https://github.com/rust-lang/rfcs/pull/2532).
 #[derive(Copy, Clone, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
@@ -2516,7 +2500,9 @@ impl fmt::Debug for ImplPolarity {
     }
 }
 
-#[derive(Copy, Clone, PartialEq, Encodable, Decodable, HashStable_Generic)]
+/// The polarity of a trait bound.
+#[derive(Copy, Clone, PartialEq, Eq, Encodable, Decodable, Debug)]
+#[derive(HashStable_Generic)]
 pub enum BoundPolarity {
     /// `Type: Trait`
     Positive,
@@ -2526,6 +2512,35 @@ pub enum BoundPolarity {
     Maybe(Span),
 }
 
+impl BoundPolarity {
+    pub fn as_str(self) -> &'static str {
+        match self {
+            Self::Positive => "",
+            Self::Negative(_) => "!",
+            Self::Maybe(_) => "?",
+        }
+    }
+}
+
+/// The constness of a trait bound.
+#[derive(Copy, Clone, PartialEq, Eq, Encodable, Decodable, Debug)]
+#[derive(HashStable_Generic)]
+pub enum BoundConstness {
+    /// `Type: Trait`
+    Never,
+    /// `Type: ~const Trait`
+    Maybe(Span),
+}
+
+impl BoundConstness {
+    pub fn as_str(self) -> &'static str {
+        match self {
+            Self::Never => "",
+            Self::Maybe(_) => "~const",
+        }
+    }
+}
+
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub enum FnRetTy {
     /// Returns type is not specified.
@@ -3259,7 +3274,7 @@ mod size_asserts {
     static_assert_size!(ForeignItem, 96);
     static_assert_size!(ForeignItemKind, 24);
     static_assert_size!(GenericArg, 24);
-    static_assert_size!(GenericBound, 64);
+    static_assert_size!(GenericBound, 72);
     static_assert_size!(Generics, 40);
     static_assert_size!(Impl, 136);
     static_assert_size!(Item, 136);