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.

1 files changed, 27 insertions, 26 deletions

diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index 47b92981626..ed033d86008 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -1425,19 +1425,16 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         this.arena.alloc_from_iter(bounds.iter().filter_map(|bound| match bound {
                             GenericBound::Trait(
                                 ty,
-                                modifier @ (TraitBoundModifier::None
-                                | TraitBoundModifier::MaybeConst(_)
-                                | TraitBoundModifier::Negative),
-                            ) => {
-                                Some(this.lower_poly_trait_ref(ty, itctx, modifier.to_constness()))
-                            }
-                            // `~const ?Bound` will cause an error during AST validation
-                            // anyways, so treat it like `?Bound` as compilation proceeds.
+                                TraitBoundModifiers {
+                                    polarity: BoundPolarity::Positive | BoundPolarity::Negative(_),
+                                    constness,
+                                },
+                            ) => Some(this.lower_poly_trait_ref(ty, itctx, (*constness).into())),
+                            // We can safely ignore constness here, since AST validation
+                            // will take care of invalid modifier combinations.
                             GenericBound::Trait(
                                 _,
-                                TraitBoundModifier::Maybe
-                                | TraitBoundModifier::MaybeConstMaybe
-                                | TraitBoundModifier::MaybeConstNegative,
+                                TraitBoundModifiers { polarity: BoundPolarity::Maybe(_), .. },
                             ) => None,
                             GenericBound::Outlives(lifetime) => {
                                 if lifetime_bound.is_none() {
@@ -2028,9 +2025,9 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         itctx: &ImplTraitContext,
     ) -> hir::GenericBound<'hir> {
         match tpb {
-            GenericBound::Trait(p, modifier) => hir::GenericBound::Trait(
-                self.lower_poly_trait_ref(p, itctx, modifier.to_constness()),
-                self.lower_trait_bound_modifier(*modifier),
+            GenericBound::Trait(p, modifiers) => hir::GenericBound::Trait(
+                self.lower_poly_trait_ref(p, itctx, modifiers.constness.into()),
+                self.lower_trait_bound_modifiers(*modifiers),
             ),
             GenericBound::Outlives(lifetime) => {
                 hir::GenericBound::Outlives(self.lower_lifetime(lifetime))
@@ -2316,25 +2313,29 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         }
     }
 
-    fn lower_trait_bound_modifier(&mut self, f: TraitBoundModifier) -> hir::TraitBoundModifier {
-        match f {
-            TraitBoundModifier::None => hir::TraitBoundModifier::None,
-            TraitBoundModifier::MaybeConst(_) => hir::TraitBoundModifier::MaybeConst,
-
-            TraitBoundModifier::Negative => {
+    fn lower_trait_bound_modifiers(
+        &mut self,
+        modifiers: TraitBoundModifiers,
+    ) -> hir::TraitBoundModifier {
+        match (modifiers.constness, modifiers.polarity) {
+            (BoundConstness::Never, BoundPolarity::Positive) => hir::TraitBoundModifier::None,
+            (BoundConstness::Never, BoundPolarity::Maybe(_)) => hir::TraitBoundModifier::Maybe,
+            (BoundConstness::Never, BoundPolarity::Negative(_)) => {
                 if self.tcx.features().negative_bounds {
                     hir::TraitBoundModifier::Negative
                 } else {
                     hir::TraitBoundModifier::None
                 }
             }
-
-            // `MaybeConstMaybe` will cause an error during AST validation, but we need to pick a
-            // placeholder for compilation to proceed.
-            TraitBoundModifier::MaybeConstMaybe | TraitBoundModifier::Maybe => {
-                hir::TraitBoundModifier::Maybe
+            (BoundConstness::Maybe(_), BoundPolarity::Positive) => {
+                hir::TraitBoundModifier::MaybeConst
+            }
+            // Invalid modifier combinations will cause an error during AST validation.
+            // Arbitrarily pick a placeholder for compilation to proceed.
+            (BoundConstness::Maybe(_), BoundPolarity::Maybe(_)) => hir::TraitBoundModifier::Maybe,
+            (BoundConstness::Maybe(_), BoundPolarity::Negative(_)) => {
+                hir::TraitBoundModifier::MaybeConst
             }
-            TraitBoundModifier::MaybeConstNegative => hir::TraitBoundModifier::MaybeConst,
         }
     }