Rollup merge of #67820 - ecstatic-morse:const-trait, r=oli-obk

Parse the syntax described in RFC 2632

This adds support for both `impl const Trait for Ty` and `?const Trait` bound syntax from rust-lang/rfcs#2632 to the parser. For now, both modifiers end up in a newly-added `constness` field on `ast::TraitRef`, although this may change once the implementation is fleshed out.

I was planning on using `delay_span_bug` when this syntax is encountered during lowering, but I can't write `should-ice` UI tests. I emit a normal error instead, which causes duplicates when the feature gate is not enabled (see the `.stderr` files for the feature gate tests). Not sure what the desired approach is; Maybe just do nothing when the syntax is encountered with the feature gate is enabled?

@oli-obk I went with `const_trait_impl` and `const_trait_bound_opt_out` for the names of these features. Are these to your liking?

cc #67792 #67794

r? @Centril

3 files changed, 21 insertions, 4 deletions

diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index 47070261385..1d3bb7d8768 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -1033,7 +1033,7 @@ 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(), self.span),
+                PolyTraitRef::new(Vec::new(), path.clone(), None, self.span),
                 TraitBoundModifier::None,
             )),
             _ => None,
@@ -2376,6 +2376,15 @@ 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)]
@@ -2390,10 +2399,15 @@ pub struct PolyTraitRef {
 }
 
 impl PolyTraitRef {
-    pub fn new(generic_params: Vec<GenericParam>, path: Path, span: Span) -> Self {
+    pub fn new(
+        generic_params: Vec<GenericParam>,
+        path: Path,
+        constness: Option<Constness>,
+        span: Span,
+    ) -> Self {
         PolyTraitRef {
             bound_generic_params: generic_params,
-            trait_ref: TraitRef { path, ref_id: DUMMY_NODE_ID },
+            trait_ref: TraitRef { path, constness, ref_id: DUMMY_NODE_ID },
             span,
         }
     }
diff --git a/src/libsyntax/feature_gate/check.rs b/src/libsyntax/feature_gate/check.rs
index 26545bfa61b..52eb20d320f 100644
--- a/src/libsyntax/feature_gate/check.rs
+++ b/src/libsyntax/feature_gate/check.rs
@@ -909,6 +909,8 @@ pub fn check_crate(
     gate_all!(or_patterns, "or-patterns syntax is experimental");
     gate_all!(const_extern_fn, "`const extern fn` definitions are unstable");
     gate_all!(raw_ref_op, "raw address of syntax is experimental");
+    gate_all!(const_trait_bound_opt_out, "`?const` on trait bounds is experimental");
+    gate_all!(const_trait_impl, "const trait impls are experimental");
 
     // All uses of `gate_all!` below this point were added in #65742,
     // and subsequently disabled (with the non-early gating readded).
diff --git a/src/libsyntax/mut_visit.rs b/src/libsyntax/mut_visit.rs
index 1413f1566d0..264ba25cede 100644
--- a/src/libsyntax/mut_visit.rs
+++ b/src/libsyntax/mut_visit.rs
@@ -838,7 +838,8 @@ pub fn noop_visit_variant_data<T: MutVisitor>(vdata: &mut VariantData, vis: &mut
     }
 }
 
-pub fn noop_visit_trait_ref<T: MutVisitor>(TraitRef { path, ref_id }: &mut TraitRef, vis: &mut T) {
+pub fn noop_visit_trait_ref<T: MutVisitor>(tr: &mut TraitRef, vis: &mut T) {
+    let TraitRef { path, ref_id, constness: _ } = tr;
     vis.visit_path(path);
     vis.visit_id(ref_id);
 }