Rollup merge of #110791 - compiler-errors:negative-bounds, r=oli-obk

Implement negative bounds for internal testing purposes

Implements partial support the `!` negative polarity on trait bounds. This is incomplete, but should allow us to at least be able to play with the feature.

Not even gonna consider them as a public-facing feature, but I'm implementing them because would've been nice to have in UI tests, for example in #110671.

compiler/rustc_ast/src/ast.rs

@@ -287,12 +287,20 @@ pub enum TraitBoundModifier {
     /// No modifiers
     None,
 
+    /// `!Trait`
+    Negative,
+
     /// `?Trait`
     Maybe,
 
     /// `~const Trait`
     MaybeConst,
 
+    /// `~const !Trait`
+    //
+    // This parses but will be rejected during AST validation.
+    MaybeConstNegative,
+
     /// `~const ?Trait`
     //
     // This parses but will be rejected during AST validation.
@@ -2446,6 +2454,16 @@ impl fmt::Debug for ImplPolarity {
     }
 }
 
+#[derive(Copy, Clone, PartialEq, Encodable, Decodable, HashStable_Generic)]
+pub enum BoundPolarity {
+    /// `Type: Trait`
+    Positive,
+    /// `Type: !Trait`
+    Negative(Span),
+    /// `Type: ?Trait`
+    Maybe(Span),
+}
+
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub enum FnRetTy {
     /// Returns type is not specified.

compiler/rustc_ast_lowering/src/lib.rs

@@ -1368,13 +1368,17 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         this.arena.alloc_from_iter(bounds.iter().filter_map(|bound| match bound {
                             GenericBound::Trait(
                                 ty,
-                                TraitBoundModifier::None | TraitBoundModifier::MaybeConst,
+                                TraitBoundModifier::None
+                                | TraitBoundModifier::MaybeConst
+                                | TraitBoundModifier::Negative,
                             ) => Some(this.lower_poly_trait_ref(ty, itctx)),
                             // `~const ?Bound` will cause an error during AST validation
                             // anyways, so treat it like `?Bound` as compilation proceeds.
                             GenericBound::Trait(
                                 _,
-                                TraitBoundModifier::Maybe | TraitBoundModifier::MaybeConstMaybe,
+                                TraitBoundModifier::Maybe
+                                | TraitBoundModifier::MaybeConstMaybe
+                                | TraitBoundModifier::MaybeConstNegative,
                             ) => None,
                             GenericBound::Outlives(lifetime) => {
                                 if lifetime_bound.is_none() {
@@ -2421,11 +2425,20 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             TraitBoundModifier::None => hir::TraitBoundModifier::None,
             TraitBoundModifier::MaybeConst => hir::TraitBoundModifier::MaybeConst,
 
+            TraitBoundModifier::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
             }
+            TraitBoundModifier::MaybeConstNegative => hir::TraitBoundModifier::MaybeConst,
         }
     }
 

compiler/rustc_ast_passes/messages.ftl

@@ -206,7 +206,7 @@ ast_passes_tilde_const_disallowed = `~const` is not allowed here
     .closure = closures cannot have `~const` trait bounds
     .function = this function is not `const`, so it cannot have `~const` trait bounds
 
-ast_passes_optional_const_exclusive = `~const` and `?` are mutually exclusive
+ast_passes_optional_const_exclusive = `~const` and `{$modifier}` are mutually exclusive
 
 ast_passes_const_and_async = functions cannot be both `const` and `async`
     .const = `const` because of this
@@ -235,3 +235,9 @@ ast_passes_incompatible_features = `{$f1}` and `{$f2}` are incompatible, using t
     .help = remove one of these features
 
 ast_passes_show_span = {$msg}
+
+ast_passes_negative_bound_not_supported =
+    negative bounds are not supported
+
+ast_passes_constraint_on_negative_bound =
+    associated type constraints not allowed on negative bounds

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -1168,12 +1168,27 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                     });
                 }
                 (_, TraitBoundModifier::MaybeConstMaybe) => {
-                    self.err_handler().emit_err(errors::OptionalConstExclusive {span: bound.span()});
+                    self.err_handler().emit_err(errors::OptionalConstExclusive {span: bound.span(), modifier: "?" });
+                }
+                (_, TraitBoundModifier::MaybeConstNegative) => {
+                    self.err_handler().emit_err(errors::OptionalConstExclusive {span: bound.span(), modifier: "!" });
                 }
                 _ => {}
             }
         }
 
+        // Negative trait bounds are not allowed to have associated constraints
+        if let GenericBound::Trait(trait_ref, TraitBoundModifier::Negative) = bound
+            && let Some(segment) = trait_ref.trait_ref.path.segments.last()
+            && let Some(ast::GenericArgs::AngleBracketed(args)) = segment.args.as_deref()
+        {
+            for arg in &args.args {
+                if let ast::AngleBracketedArg::Constraint(constraint) = arg {
+                    self.err_handler().emit_err(errors::ConstraintOnNegativeBound { span: constraint.span });
+                }
+            }
+        }
+
         visit::walk_param_bound(self, bound)
     }
 

compiler/rustc_ast_passes/src/errors.rs

@@ -567,6 +567,7 @@ pub enum TildeConstReason {
 pub struct OptionalConstExclusive {
     #[primary_span]
     pub span: Span,
+    pub modifier: &'static str,
 }
 
 #[derive(Diagnostic)]
@@ -693,3 +694,17 @@ pub struct ShowSpan {
     pub span: Span,
     pub msg: &'static str,
 }
+
+#[derive(Diagnostic)]
+#[diag(ast_passes_negative_bound_not_supported)]
+pub struct NegativeBoundUnsupported {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(ast_passes_constraint_on_negative_bound)]
+pub struct ConstraintOnNegativeBound {
+    #[primary_span]
+    pub span: Span,
+}

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -603,6 +603,12 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session) {
     gate_all!(dyn_star, "`dyn*` trait objects are experimental");
     gate_all!(const_closures, "const closures are experimental");
 
+    if !visitor.features.negative_bounds {
+        for &span in spans.get(&sym::negative_bounds).iter().copied().flatten() {
+            sess.emit_err(errors::NegativeBoundUnsupported { span });
+        }
+    }
+
     // All uses of `gate_all!` below this point were added in #65742,
     // and subsequently disabled (with the non-early gating readded).
     // We emit an early future-incompatible warning for these.

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -1570,12 +1570,19 @@ impl<'a> State<'a> {
                 GenericBound::Trait(tref, modifier) => {
                     match modifier {
                         TraitBoundModifier::None => {}
+                        TraitBoundModifier::Negative => {
+                            self.word("!");
+                        }
                         TraitBoundModifier::Maybe => {
                             self.word("?");
                         }
                         TraitBoundModifier::MaybeConst => {
                             self.word_space("~const");
                         }
+                        TraitBoundModifier::MaybeConstNegative => {
+                            self.word_space("~const");
+                            self.word("!");
+                        }
                         TraitBoundModifier::MaybeConstMaybe => {
                             self.word_space("~const");
                             self.word("?");

compiler/rustc_feature/src/active.rs

@@ -164,6 +164,8 @@ declare_features! (
     (active, link_cfg, "1.14.0", None, None),
     /// Allows the `multiple_supertrait_upcastable` lint.
     (active, multiple_supertrait_upcastable, "1.69.0", None, None),
+    /// Allow negative trait bounds. This is an internal-only feature for testing the trait solver!
+    (incomplete, negative_bounds, "CURRENT_RUSTC_VERSION", None, None),
     /// Allows using `#[omit_gdb_pretty_printer_section]`.
     (active, omit_gdb_pretty_printer_section, "1.5.0", None, None),
     /// Allows using `#[prelude_import]` on glob `use` items.

compiler/rustc_hir/src/hir.rs

@@ -435,6 +435,7 @@ pub enum GenericArgsParentheses {
 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug, HashStable_Generic)]
 pub enum TraitBoundModifier {
     None,
+    Negative,
     Maybe,
     MaybeConst,
 }

compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -665,6 +665,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         span: Span,
         binding_span: Option<Span>,
         constness: ty::BoundConstness,
+        polarity: ty::ImplPolarity,
         bounds: &mut Bounds<'tcx>,
         speculative: bool,
         trait_ref_span: Span,
@@ -696,10 +697,20 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             ty::Binder::bind_with_vars(tcx.mk_trait_ref(trait_def_id, substs), bound_vars);
 
         debug!(?poly_trait_ref, ?assoc_bindings);
-        bounds.push_trait_bound(tcx, poly_trait_ref, span, constness);
+        bounds.push_trait_bound(tcx, poly_trait_ref, span, constness, polarity);
 
         let mut dup_bindings = FxHashMap::default();
         for binding in &assoc_bindings {
+            // Don't register additional associated type bounds for negative bounds,
+            // since we should have emitten an error for them earlier, and they will
+            // not be well-formed!
+            if polarity == ty::ImplPolarity::Negative {
+                self.tcx()
+                    .sess
+                    .delay_span_bug(binding.span, "negative trait bounds should not have bindings");
+                continue;
+            }
+
             // Specify type to assert that error was already reported in `Err` case.
             let _: Result<_, ErrorGuaranteed> = self.add_predicates_for_ast_type_binding(
                 hir_id,
@@ -711,6 +722,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 binding_span.unwrap_or(binding.span),
                 constness,
                 only_self_bounds,
+                polarity,
             );
             // Okay to ignore `Err` because of `ErrorGuaranteed` (see above).
         }
@@ -743,6 +755,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         trait_ref: &hir::TraitRef<'_>,
         span: Span,
         constness: ty::BoundConstness,
+        polarity: ty::ImplPolarity,
         self_ty: Ty<'tcx>,
         bounds: &mut Bounds<'tcx>,
         speculative: bool,
@@ -764,6 +777,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             span,
             binding_span,
             constness,
+            polarity,
             bounds,
             speculative,
             trait_ref_span,
@@ -799,6 +813,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             span,
             binding_span,
             constness,
+            ty::ImplPolarity::Positive,
             bounds,
             speculative,
             trait_ref_span,
@@ -961,16 +976,23 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         for ast_bound in ast_bounds {
             match ast_bound {
                 hir::GenericBound::Trait(poly_trait_ref, modifier) => {
-                    let constness = match modifier {
-                        hir::TraitBoundModifier::MaybeConst => ty::BoundConstness::ConstIfConst,
-                        hir::TraitBoundModifier::None => ty::BoundConstness::NotConst,
+                    let (constness, polarity) = match modifier {
+                        hir::TraitBoundModifier::MaybeConst => {
+                            (ty::BoundConstness::ConstIfConst, ty::ImplPolarity::Positive)
+                        }
+                        hir::TraitBoundModifier::None => {
+                            (ty::BoundConstness::NotConst, ty::ImplPolarity::Positive)
+                        }
+                        hir::TraitBoundModifier::Negative => {
+                            (ty::BoundConstness::NotConst, ty::ImplPolarity::Negative)
+                        }
                         hir::TraitBoundModifier::Maybe => continue,
                     };
-
                     let _ = self.instantiate_poly_trait_ref(
                         &poly_trait_ref.trait_ref,
                         poly_trait_ref.span,
                         constness,
+                        polarity,
                         param_ty,
                         bounds,
                         false,
@@ -1088,6 +1110,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         path_span: Span,
         constness: ty::BoundConstness,
         only_self_bounds: OnlySelfBounds,
+        polarity: ty::ImplPolarity,
     ) -> Result<(), ErrorGuaranteed> {
         // Given something like `U: SomeTrait<T = X>`, we want to produce a
         // predicate like `<U as SomeTrait>::T = X`. This is somewhat
@@ -1438,6 +1461,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 &trait_bound.trait_ref,
                 trait_bound.span,
                 ty::BoundConstness::NotConst,
+                ty::ImplPolarity::Positive,
                 dummy_self,
                 &mut bounds,
                 false,

compiler/rustc_hir_analysis/src/bounds.rs

@@ -42,8 +42,14 @@ impl<'tcx> Bounds<'tcx> {
         trait_ref: ty::PolyTraitRef<'tcx>,
         span: Span,
         constness: ty::BoundConstness,
+        polarity: ty::ImplPolarity,
     ) {
-        self.predicates.push((trait_ref.with_constness(constness).to_predicate(tcx), span));
+        self.predicates.push((
+            trait_ref
+                .map_bound(|trait_ref| ty::TraitPredicate { trait_ref, constness, polarity })
+                .to_predicate(tcx),
+            span,
+        ));
     }
 
     pub fn push_projection_bound(

compiler/rustc_hir_analysis/src/lib.rs

@@ -528,6 +528,7 @@ pub fn hir_trait_to_predicates<'tcx>(
         hir_trait,
         DUMMY_SP,
         ty::BoundConstness::NotConst,
+        ty::ImplPolarity::Positive,
         self_ty,
         &mut bounds,
         true,

compiler/rustc_infer/src/traits/util.rs

@@ -200,6 +200,10 @@ impl<'tcx, O: Elaboratable<'tcx>> Elaborator<'tcx, O> {
         let bound_predicate = elaboratable.predicate().kind();
         match bound_predicate.skip_binder() {
             ty::PredicateKind::Clause(ty::Clause::Trait(data)) => {
+                // Negative trait bounds do not imply any supertrait bounds
+                if data.polarity == ty::ImplPolarity::Negative {
+                    return;
+                }
                 // Get predicates implied by the trait, or only super predicates if we only care about self predicates.
                 let predicates = if self.only_self {
                     tcx.super_predicates_of(data.def_id())

compiler/rustc_middle/src/ty/print/pretty.rs

@@ -2816,6 +2816,9 @@ define_print_and_forward_display! {
         if let ty::BoundConstness::ConstIfConst = self.constness && cx.tcx().features().const_trait_impl {
             p!("~const ");
         }
+        if let ty::ImplPolarity::Negative = self.polarity {
+            p!("!");
+        }
         p!(print(self.trait_ref.print_only_trait_path()))
     }
 

compiler/rustc_parse/messages.ftl

@@ -615,13 +615,6 @@ parse_invalid_dyn_keyword = invalid `dyn` keyword
     .help = `dyn` is only needed at the start of a trait `+`-separated list
     .suggestion = remove this keyword
 
-parse_negative_bounds_not_supported = negative bounds are not supported
-    .label = negative bounds are not supported
-    .suggestion = {$num_bounds ->
-            [one] remove the bound
-           *[other] remove the bounds
-        }
-
 parse_help_set_edition_cargo = set `edition = "{$edition}"` in `Cargo.toml`
 parse_help_set_edition_standalone = pass `--edition {$edition}` to `rustc`
 parse_note_edition_guide = for more on editions, read https://doc.rust-lang.org/edition-guide
@@ -772,7 +765,8 @@ parse_assoc_lifetime = associated lifetimes are not supported
 
 parse_tilde_const_lifetime = `~const` may only modify trait bounds, not lifetime bounds
 
-parse_maybe_lifetime = `?` may only modify trait bounds, not lifetime bounds
+parse_modifier_lifetime = `{$sigil}` may only modify trait bounds, not lifetime bounds
+    .suggestion = remove the `{$sigil}`
 
 parse_parenthesized_lifetime = parenthesized lifetime bounds are not supported
     .suggestion = remove the parentheses