Rollup merge of #68140 - ecstatic-morse:const-trait-bound-opt-out, r=oli-obk

Implement `?const` opt-out for trait bounds

For now, such bounds are treated exactly the same as unprefixed ones in all contexts. [RFC 2632](https://github.com/rust-lang/rfcs/pull/2632) does not specify whether such bounds are forbidden outside of `const` contexts, so they are allowed at the moment.

Prior to this PR, the constness of a trait bound/impl was stored in `TraitRef`. Now, the constness of an `impl` is stored in `ast::ItemKind::Impl` and the constness of a bound in `ast::TraitBoundModifer`. Additionally, constness of trait bounds is now stored in an additional field of `ty::Predicate::Trait`, and the combination of the constness of the item along with any `TraitBoundModifier` determines the constness of the bound in accordance with the RFC. Encoding the constness of impls at the `ty` level is left for a later PR.

After a discussion in \#wg-grammar on Discord, it was decided that the grammar should not encode the mutual exclusivity of trait bound modifiers. The grammar for trait bound modifiers remains `[?const] [?]`. To encode this, I add a dummy variant to `ast::TraitBoundModifier` that is used when the syntax `?const ?` appears. This variant causes an error in AST validation and disappears during HIR lowering.

cc #67794

r? @oli-obk

4 files changed, 32 insertions, 22 deletions

diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index a5a4eb1583b..5f38ac4cc0f 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -266,12 +266,24 @@ pub const CRATE_NODE_ID: NodeId = NodeId::from_u32_const(0);
 /// small, positive ids.
 pub const DUMMY_NODE_ID: NodeId = NodeId::MAX;
 
-/// A modifier on a bound, currently this is only used for `?Sized`, where the
-/// modifier is `Maybe`. Negative bounds should also be handled here.
+/// A modifier on a bound, e.g., `?Sized` or `?const Trait`.
+///
+/// Negative bounds should also be handled here.
 #[derive(Copy, Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Debug)]
 pub enum TraitBoundModifier {
+    /// No modifiers
     None,
+
+    /// `?Trait`
     Maybe,
+
+    /// `?const Trait`
+    MaybeConst,
+
+    /// `?const ?Trait`
+    //
+    // This parses but will be rejected during AST validation.
+    MaybeConstMaybe,
 }
 
 /// The AST represents all type param bounds as types.
@@ -1033,7 +1045,7 @@ impl Expr {
     pub fn to_bound(&self) -> Option<GenericBound> {
         match &self.kind {
             ExprKind::Path(None, path) => Some(GenericBound::Trait(
-                PolyTraitRef::new(Vec::new(), path.clone(), None, self.span),
+                PolyTraitRef::new(Vec::new(), path.clone(), self.span),
                 TraitBoundModifier::None,
             )),
             _ => None,
@@ -2158,7 +2170,8 @@ impl IsAsync {
     }
 }
 
-#[derive(Copy, Clone, PartialEq, RustcEncodable, RustcDecodable, Debug, HashStable_Generic)]
+#[derive(Copy, Clone, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable, Debug)]
+#[derive(HashStable_Generic)]
 pub enum Constness {
     Const,
     NotConst,
@@ -2376,15 +2389,6 @@ pub enum AttrKind {
 pub struct TraitRef {
     pub path: Path,
     pub ref_id: NodeId,
-
-    /// The `const` modifier, if any, that appears before this trait.
-    ///
-    /// |                | `constness`                 |
-    /// |----------------|-----------------------------|
-    /// | `Trait`        | `None`                      |
-    /// | `const Trait`  | `Some(Constness::Const)`    |
-    /// | `?const Trait` | `Some(Constness::NotConst)` |
-    pub constness: Option<Constness>,
 }
 
 #[derive(Clone, RustcEncodable, RustcDecodable, Debug)]
@@ -2399,15 +2403,10 @@ pub struct PolyTraitRef {
 }
 
 impl PolyTraitRef {
-    pub fn new(
-        generic_params: Vec<GenericParam>,
-        path: Path,
-        constness: Option<Constness>,
-        span: Span,
-    ) -> Self {
+    pub fn new(generic_params: Vec<GenericParam>, path: Path, span: Span) -> Self {
         PolyTraitRef {
             bound_generic_params: generic_params,
-            trait_ref: TraitRef { path, constness, ref_id: DUMMY_NODE_ID },
+            trait_ref: TraitRef { path, ref_id: DUMMY_NODE_ID },
             span,
         }
     }
@@ -2618,6 +2617,7 @@ pub enum ItemKind {
         unsafety: Unsafety,
         polarity: ImplPolarity,
         defaultness: Defaultness,
+        constness: Constness,
         generics: Generics,
 
         /// The trait being implemented, if any.
diff --git a/src/libsyntax/mut_visit.rs b/src/libsyntax/mut_visit.rs
index 750d054e8a0..4a460c5d7b2 100644
--- a/src/libsyntax/mut_visit.rs
+++ b/src/libsyntax/mut_visit.rs
@@ -838,8 +838,7 @@ pub fn noop_visit_variant_data<T: MutVisitor>(vdata: &mut VariantData, vis: &mut
     }
 }
 
-pub fn noop_visit_trait_ref<T: MutVisitor>(tr: &mut TraitRef, vis: &mut T) {
-    let TraitRef { path, ref_id, constness: _ } = tr;
+pub fn noop_visit_trait_ref<T: MutVisitor>(TraitRef { path, ref_id }: &mut TraitRef, vis: &mut T) {
     vis.visit_path(path);
     vis.visit_id(ref_id);
 }
@@ -922,6 +921,7 @@ pub fn noop_visit_item_kind<T: MutVisitor>(kind: &mut ItemKind, vis: &mut T) {
             unsafety: _,
             polarity: _,
             defaultness: _,
+            constness: _,
             generics,
             of_trait,
             self_ty,
diff --git a/src/libsyntax/print/pprust.rs b/src/libsyntax/print/pprust.rs
index bc67980c454..3927e4f9030 100644
--- a/src/libsyntax/print/pprust.rs
+++ b/src/libsyntax/print/pprust.rs
@@ -1230,6 +1230,7 @@ impl<'a> State<'a> {
                 unsafety,
                 polarity,
                 defaultness,
+                constness,
                 ref generics,
                 ref of_trait,
                 ref self_ty,
@@ -1240,6 +1241,7 @@ impl<'a> State<'a> {
                 self.print_defaultness(defaultness);
                 self.print_unsafety(unsafety);
                 self.word_nbsp("impl");
+                self.print_constness(constness);
 
                 if !generics.params.is_empty() {
                     self.print_generic_params(&generics.params);
@@ -2773,6 +2775,13 @@ impl<'a> State<'a> {
         }
     }
 
+    crate fn print_constness(&mut self, s: ast::Constness) {
+        match s {
+            ast::Constness::Const => self.word_nbsp("const"),
+            ast::Constness::NotConst => {}
+        }
+    }
+
     crate fn print_is_auto(&mut self, s: ast::IsAuto) {
         match s {
             ast::IsAuto::Yes => self.word_nbsp("auto"),
diff --git a/src/libsyntax/visit.rs b/src/libsyntax/visit.rs
index d03a9dfc167..946a0d29cd3 100644
--- a/src/libsyntax/visit.rs
+++ b/src/libsyntax/visit.rs
@@ -312,6 +312,7 @@ pub fn walk_item<'a, V: Visitor<'a>>(visitor: &mut V, item: &'a Item) {
             unsafety: _,
             polarity: _,
             defaultness: _,
+            constness: _,
             ref generics,
             ref of_trait,
             ref self_ty,