add polarity

Author: Bryanskiy

compiler/rustc_ast_lowering/src/lib.rs

@@ -1255,6 +1255,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     itctx,
                     TraitBoundModifiers::NONE,
                 );
+                let bound = (bound, hir::TraitBoundModifier::None);
                 let bounds = this.arena.alloc_from_iter([bound]);
                 let lifetime_bound = this.elided_dyn_bound(t.span);
                 (bounds, lifetime_bound)
@@ -1386,21 +1387,17 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             // We can safely ignore constness here since AST validation
                             // takes care of rejecting invalid modifier combinations and
                             // const trait bounds in trait object types.
-                            GenericBound::Trait(ty, modifiers) => match modifiers.polarity {
-                                BoundPolarity::Positive | BoundPolarity::Negative(_) => {
-                                    Some(this.lower_poly_trait_ref(
-                                        ty,
-                                        itctx,
-                                        // Still, don't pass along the constness here; we don't want to
-                                        // synthesize any host effect args, it'd only cause problems.
-                                        TraitBoundModifiers {
-                                            constness: BoundConstness::Never,
-                                            ..*modifiers
-                                        },
-                                    ))
-                                }
-                                BoundPolarity::Maybe(_) => None,
-                            },
+                            GenericBound::Trait(ty, modifiers) => {
+                                // Still, don't pass along the constness here; we don't want to
+                                // synthesize any host effect args, it'd only cause problems.
+                                let modifiers = TraitBoundModifiers {
+                                    constness: BoundConstness::Never,
+                                    ..*modifiers
+                                };
+                                let trait_ref = this.lower_poly_trait_ref(ty, itctx, modifiers);
+                                let polarity = this.lower_trait_bound_modifiers(modifiers);
+                                Some((trait_ref, polarity))
+                            }
                             GenericBound::Outlives(lifetime) => {
                                 if lifetime_bound.is_none() {
                                     lifetime_bound = Some(this.lower_lifetime(lifetime));
@@ -2502,6 +2499,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             trait_ref: hir::TraitRef { path, hir_ref_id: hir_id },
                             span: self.lower_span(span),
                         };
+                        let principal = (principal, hir::TraitBoundModifier::None);
 
                         // The original ID is taken by the `PolyTraitRef`,
                         // so the `Ty` itself needs a different one.

compiler/rustc_ast_passes/messages.ftl

@@ -202,11 +202,6 @@ ast_passes_nomangle_ascii = `#[no_mangle]` requires ASCII identifier
 ast_passes_obsolete_auto = `impl Trait for .. {"{}"}` is an obsolete syntax
     .help = use `auto trait Trait {"{}"}` instead
 
-ast_passes_optional_trait_object = `?Trait` is not permitted in trait object types
-
-ast_passes_optional_trait_supertrait = `?Trait` is not permitted in supertraits
-    .note = traits are `?{$path_str}` by default
-
 ast_passes_out_of_order_params = {$param_ord} parameters must be declared prior to {$max_param} parameters
     .suggestion = reorder the parameters: lifetimes, then consts and types
 

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -1260,16 +1260,8 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
 
     fn visit_param_bound(&mut self, bound: &'a GenericBound, ctxt: BoundKind) {
         if let GenericBound::Trait(poly, modifiers) = bound {
+            // Some of the arms are feature-gated. See `feature_gate::PostExpansionVisitor`.
             match (ctxt, modifiers.constness, modifiers.polarity) {
-                (BoundKind::SuperTraits, BoundConstness::Never, BoundPolarity::Maybe(_)) => {
-                    self.dcx().emit_err(errors::OptionalTraitSupertrait {
-                        span: poly.span,
-                        path_str: pprust::path_to_string(&poly.trait_ref.path),
-                    });
-                }
-                (BoundKind::TraitObject, BoundConstness::Never, BoundPolarity::Maybe(_)) => {
-                    self.dcx().emit_err(errors::OptionalTraitObject { span: poly.span });
-                }
                 (BoundKind::TraitObject, BoundConstness::Always(_), BoundPolarity::Positive) => {
                     self.dcx().emit_err(errors::ConstBoundTraitObject { span: poly.span });
                 }

compiler/rustc_ast_passes/src/errors.rs

@@ -543,22 +543,6 @@ pub struct NestedLifetimes {
     pub span: Span,
 }
 
-#[derive(Diagnostic)]
-#[diag(ast_passes_optional_trait_supertrait)]
-#[note]
-pub struct OptionalTraitSupertrait {
-    #[primary_span]
-    pub span: Span,
-    pub path_str: String,
-}
-
-#[derive(Diagnostic)]
-#[diag(ast_passes_optional_trait_object)]
-pub struct OptionalTraitObject {
-    #[primary_span]
-    pub span: Span,
-}
-
 #[derive(Diagnostic)]
 #[diag(ast_passes_const_bound_trait_object)]
 pub struct ConstBoundTraitObject {

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -169,6 +169,27 @@ impl<'a> PostExpansionVisitor<'a> {
 }
 
 impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
+    fn visit_param_bound(&mut self, bound: &'a ast::GenericBound, ctxt: visit::BoundKind) {
+        use ast::visit::BoundKind;
+        use ast::{BoundConstness, BoundPolarity};
+        if let ast::GenericBound::Trait(poly, modifiers) = bound {
+            let gate = |descr| {
+                gate!(&self, allow_maybe_polarity, poly.span, descr);
+            };
+
+            match (ctxt, modifiers.constness, modifiers.polarity) {
+                (BoundKind::TraitObject, BoundConstness::Never, BoundPolarity::Maybe(_)) => {
+                    gate("`?Trait` is not permitted in trait object types");
+                }
+                (BoundKind::SuperTraits, BoundConstness::Never, BoundPolarity::Maybe(_)) => {
+                    gate("`?Trait` is not permitted in supertraits");
+                }
+                _ => {}
+            }
+        }
+        visit::walk_param_bound(self, bound);
+    }
+
     fn visit_attribute(&mut self, attr: &ast::Attribute) {
         let attr_info = attr.ident().and_then(|ident| BUILTIN_ATTRIBUTE_MAP.get(&ident.name));
         // Check feature gates for built-in attributes.

compiler/rustc_feature/src/unstable.rs

@@ -191,6 +191,8 @@ declare_features! (
     /// below (it has to be checked before expansion possibly makes
     /// macros disappear).
     (internal, allow_internal_unstable, "1.0.0", None),
+    /// Allows using `?Trait` trait bound.
+    (internal, allow_maybe_polarity, "CURRENT_RUSTC_VERSION", None),
     /// Allows using anonymous lifetimes in argument-position impl-trait.
     (unstable, anonymous_lifetime_in_impl_trait, "1.63.0", None),
     /// Allows identifying the `compiler_builtins` crate.

compiler/rustc_hir/src/hir.rs

@@ -2603,7 +2603,11 @@ pub enum TyKind<'hir> {
     OpaqueDef(ItemId, &'hir [GenericArg<'hir>], bool),
     /// A trait object type `Bound1 + Bound2 + Bound3`
     /// where `Bound` is a trait or a lifetime.
-    TraitObject(&'hir [PolyTraitRef<'hir>], &'hir Lifetime, TraitObjectSyntax),
+    TraitObject(
+        &'hir [(PolyTraitRef<'hir>, TraitBoundModifier)],
+        &'hir Lifetime,
+        TraitObjectSyntax,
+    ),
     /// Unused for now.
     Typeof(AnonConst),
     /// `TyKind::Infer` means the type should be inferred instead of it having been

compiler/rustc_hir/src/intravisit.rs

@@ -873,7 +873,9 @@ pub fn walk_ty<'v, V: Visitor<'v>>(visitor: &mut V, typ: &'v Ty<'v>) -> V::Resul
             try_visit!(visitor.visit_array_length(length));
         }
         TyKind::TraitObject(bounds, ref lifetime, _syntax) => {
-            walk_list!(visitor, visit_poly_trait_ref, bounds);
+            for (bound, _) in bounds {
+                try_visit!(visitor.visit_poly_trait_ref(bound));
+            }
             try_visit!(visitor.visit_lifetime(lifetime));
         }
         TyKind::Typeof(ref expression) => try_visit!(visitor.visit_anon_const(expression)),

compiler/rustc_hir_analysis/src/astconv/bounds.rs

@@ -66,7 +66,7 @@ impl<'tcx> dyn AstConv<'tcx> + '_ {
             }
         }
 
-        if unbounds.len() > 1 {
+        if unbounds.len() > 1 && !tcx.features().allow_maybe_polarity {
             tcx.dcx().emit_err(errors::MultipleRelaxedDefaultBounds {
                 spans: unbounds.iter().map(|ptr| ptr.span).collect(),
             });
@@ -80,12 +80,14 @@ impl<'tcx> dyn AstConv<'tcx> + '_ {
                 seen_sized_unbound = true;
                 continue;
             }
-            // There was a `?Trait` bound, but it was not `?Sized`; warn.
-            tcx.dcx().span_warn(
-                unbound.span,
-                "relaxing a default bound only does something for `?Sized`; \
+            if !tcx.features().allow_maybe_polarity {
+                // There was a `?Trait` bound, but it was not `?Sized`; warn.
+                tcx.dcx().span_warn(
+                    unbound.span,
+                    "relaxing a default bound only does something for `?Sized`; \
                 all other traits are not bound by default",
-            );
+                );
+            }
         }
 
         if seen_sized_unbound || seen_negative_sized_bound || seen_positive_sized_bound {

compiler/rustc_hir_analysis/src/astconv/errors.rs

@@ -592,7 +592,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         &self,
         associated_types: FxIndexMap<Span, FxIndexSet<DefId>>,
         potential_assoc_types: Vec<Span>,
-        trait_bounds: &[hir::PolyTraitRef<'_>],
+        trait_bounds: &[(hir::PolyTraitRef<'_>, hir::TraitBoundModifier)],
     ) {
         if associated_types.values().all(|v| v.is_empty()) {
             return;
@@ -634,7 +634,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         if object_safety_violations {
             return;
         }
-        if let ([], [bound]) = (&potential_assoc_types[..], &trait_bounds) {
+        if let ([], [(bound, _)]) = (&potential_assoc_types[..], &trait_bounds) {
             match bound.trait_ref.path.segments {
                 // FIXME: `trait_ref.path.span` can point to a full path with multiple
                 // segments, even though `trait_ref.path.segments` is of length `1`. Work
@@ -676,7 +676,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         // and we can then use their span to indicate this to the user.
         let bound_names = trait_bounds
             .iter()
-            .filter_map(|poly_trait_ref| {
+            .filter_map(|(poly_trait_ref, _)| {
                 let path = poly_trait_ref.trait_ref.path.segments.last()?;
                 let args = path.args?;