Auto merge of rust-lang#90104 - spastorino:coherence-for-negative-tra…

…it, r=nikomatsakis

Implement coherence checks for negative trait impls

The main purpose of this PR is to be able to [move Error trait to core](rust-lang/project-error-handling#3).

This feature is necessary to handle the following from impl on box.

```rust
impl From<&str> for Box<dyn Error> { ... }
```

Without having negative traits affect coherence moving the error trait into `core` and moving that `From` impl to `alloc` will cause the from impl to no longer compiler because of a potential future incompatibility. The compiler indicates that `&str` _could_ introduce an `Error` impl in the future, and thus prevents the `From` impl in `alloc` that would cause overlap with `From<E: Error> for Box<dyn Error>`. Adding `impl !Error for &str {}` with the negative trait coherence feature will disable this error by encoding a stability guarantee that `&str` will never implement `Error`, making the `From` impl compile.

We would have this in `alloc`:

```rust
impl From<&str> for Box<dyn Error> {} // A
impl<E> From<E> for Box<dyn Error> where E: Error {} // B
```

and this in `core`:

```rust
trait Error {}
impl !Error for &str {}
```

r? `@nikomatsakis`

This PR was built on top of `@yaahc` PR rust-lang#85764.

Language team proposal: to rust-lang/lang-team#96

compiler/rustc_borrowck/src/type_check/canonical.rs

@@ -94,6 +94,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
             Some(ty::Binder::dummy(ty::PredicateKind::Trait(ty::TraitPredicate {
                 trait_ref,
                 constness: ty::BoundConstness::NotConst,
+                polarity: ty::ImplPolarity::Positive,
             }))),
             locations,
             category,

compiler/rustc_const_eval/src/transform/check_consts/check.rs

@@ -854,6 +854,7 @@ impl Visitor<'tcx> for Checker<'mir, 'tcx> {
                         Binder::dummy(TraitPredicate {
                             trait_ref,
                             constness: ty::BoundConstness::ConstIfConst,
+                            polarity: ty::ImplPolarity::Positive,
                         }),
                     );
 

compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs

@@ -162,6 +162,7 @@ impl Qualif for NeedsNonConstDrop {
             ty::Binder::dummy(ty::TraitPredicate {
                 trait_ref,
                 constness: ty::BoundConstness::ConstIfConst,
+                polarity: ty::ImplPolarity::Positive,
             }),
         );
 

compiler/rustc_feature/src/builtin_attrs.rs

@@ -556,6 +556,7 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     rustc_attr!(TEST, rustc_outlives, Normal, template!(Word)),
     rustc_attr!(TEST, rustc_capture_analysis, Normal, template!(Word)),
     rustc_attr!(TEST, rustc_insignificant_dtor, Normal, template!(Word)),
+    rustc_attr!(TEST, rustc_strict_coherence, Normal, template!(Word)),
     rustc_attr!(TEST, rustc_variance, Normal, template!(Word)),
     rustc_attr!(TEST, rustc_layout, Normal, template!(List: "field1, field2, ...")),
     rustc_attr!(TEST, rustc_regions, Normal, template!(Word)),

compiler/rustc_infer/src/traits/mod.rs

@@ -10,7 +10,7 @@ pub mod util;
 
 use rustc_hir as hir;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
-use rustc_middle::ty::{self, Const, Ty};
+use rustc_middle::ty::{self, Const, Ty, TyCtxt};
 use rustc_span::Span;
 
 pub use self::FulfillmentErrorCode::*;
@@ -55,6 +55,20 @@ pub struct Obligation<'tcx, T> {
 pub type PredicateObligation<'tcx> = Obligation<'tcx, ty::Predicate<'tcx>>;
 pub type TraitObligation<'tcx> = Obligation<'tcx, ty::PolyTraitPredicate<'tcx>>;
 
+impl PredicateObligation<'tcx> {
+    /// Flips the polarity of the inner predicate.
+    ///
+    /// Given `T: Trait` predicate it returns `T: !Trait` and given `T: !Trait` returns `T: Trait`.
+    pub fn flip_polarity(&self, tcx: TyCtxt<'tcx>) -> Option<PredicateObligation<'tcx>> {
+        Some(PredicateObligation {
+            cause: self.cause.clone(),
+            param_env: self.param_env,
+            predicate: self.predicate.flip_polarity(tcx)?,
+            recursion_depth: self.recursion_depth,
+        })
+    }
+}
+
 // `PredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger.
 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
 static_assert_size!(PredicateObligation<'_>, 32);
@@ -129,6 +143,10 @@ impl<'tcx> FulfillmentError<'tcx> {
 }
 
 impl<'tcx> TraitObligation<'tcx> {
+    pub fn polarity(&self) -> ty::ImplPolarity {
+        self.predicate.skip_binder().polarity
+    }
+
     pub fn self_ty(&self) -> ty::Binder<'tcx, Ty<'tcx>> {
         self.predicate.map_bound(|p| p.self_ty())
     }

compiler/rustc_middle/src/traits/select.rs

@@ -12,12 +12,14 @@ use rustc_hir::def_id::DefId;
 use rustc_query_system::cache::Cache;
 
 pub type SelectionCache<'tcx> = Cache<
-    ty::ConstnessAnd<ty::ParamEnvAnd<'tcx, ty::TraitRef<'tcx>>>,
+    (ty::ConstnessAnd<ty::ParamEnvAnd<'tcx, ty::TraitRef<'tcx>>>, ty::ImplPolarity),
     SelectionResult<'tcx, SelectionCandidate<'tcx>>,
 >;
 
-pub type EvaluationCache<'tcx> =
-    Cache<ty::ParamEnvAnd<'tcx, ty::ConstnessAnd<ty::PolyTraitRef<'tcx>>>, EvaluationResult>;
+pub type EvaluationCache<'tcx> = Cache<
+    (ty::ParamEnvAnd<'tcx, ty::ConstnessAnd<ty::PolyTraitRef<'tcx>>>, ty::ImplPolarity),
+    EvaluationResult,
+>;
 
 /// The selection process begins by considering all impls, where
 /// clauses, and so forth that might resolve an obligation. Sometimes
@@ -101,7 +103,7 @@ pub enum SelectionCandidate<'tcx> {
         /// `false` if there are no *further* obligations.
         has_nested: bool,
     },
-    ParamCandidate(ty::ConstnessAnd<ty::PolyTraitRef<'tcx>>),
+    ParamCandidate((ty::ConstnessAnd<ty::PolyTraitRef<'tcx>>, ty::ImplPolarity)),
     ImplCandidate(DefId),
     AutoImplCandidate(DefId),
 

compiler/rustc_middle/src/ty/error.rs

@@ -34,6 +34,7 @@ impl<T> ExpectedFound<T> {
 pub enum TypeError<'tcx> {
     Mismatch,
     ConstnessMismatch(ExpectedFound<ty::BoundConstness>),
+    PolarityMismatch(ExpectedFound<ty::ImplPolarity>),
     UnsafetyMismatch(ExpectedFound<hir::Unsafety>),
     AbiMismatch(ExpectedFound<abi::Abi>),
     Mutability,
@@ -104,6 +105,9 @@ impl<'tcx> fmt::Display for TypeError<'tcx> {
             ConstnessMismatch(values) => {
                 write!(f, "expected {} bound, found {} bound", values.expected, values.found)
             }
+            PolarityMismatch(values) => {
+                write!(f, "expected {} polarity, found {} polarity", values.expected, values.found)
+            }
             UnsafetyMismatch(values) => {
                 write!(f, "expected {} fn, found {} fn", values.expected, values.found)
             }
@@ -212,10 +216,9 @@ impl<'tcx> TypeError<'tcx> {
         use self::TypeError::*;
         match self {
             CyclicTy(_) | CyclicConst(_) | UnsafetyMismatch(_) | ConstnessMismatch(_)
-            | Mismatch | AbiMismatch(_) | FixedArraySize(_) | ArgumentSorts(..) | Sorts(_)
-            | IntMismatch(_) | FloatMismatch(_) | VariadicMismatch(_) | TargetFeatureCast(_) => {
-                false
-            }
+            | PolarityMismatch(_) | Mismatch | AbiMismatch(_) | FixedArraySize(_)
+            | ArgumentSorts(..) | Sorts(_) | IntMismatch(_) | FloatMismatch(_)
+            | VariadicMismatch(_) | TargetFeatureCast(_) => false,
 
             Mutability
             | ArgumentMutability(_)

compiler/rustc_middle/src/ty/mod.rs

@@ -164,7 +164,18 @@ pub struct ImplHeader<'tcx> {
     pub predicates: Vec<Predicate<'tcx>>,
 }
 
-#[derive(Copy, Clone, PartialEq, TyEncodable, TyDecodable, HashStable, Debug)]
+#[derive(
+    Copy,
+    Clone,
+    PartialEq,
+    Eq,
+    Hash,
+    TyEncodable,
+    TyDecodable,
+    HashStable,
+    Debug,
+    TypeFoldable
+)]
 pub enum ImplPolarity {
     /// `impl Trait for Type`
     Positive,
@@ -177,6 +188,27 @@ pub enum ImplPolarity {
     Reservation,
 }
 
+impl ImplPolarity {
+    /// Flips polarity by turning `Positive` into `Negative` and `Negative` into `Positive`.
+    pub fn flip(&self) -> Option<ImplPolarity> {
+        match self {
+            ImplPolarity::Positive => Some(ImplPolarity::Negative),
+            ImplPolarity::Negative => Some(ImplPolarity::Positive),
+            ImplPolarity::Reservation => None,
+        }
+    }
+}
+
+impl fmt::Display for ImplPolarity {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::Positive => f.write_str("positive"),
+            Self::Negative => f.write_str("negative"),
+            Self::Reservation => f.write_str("reservation"),
+        }
+    }
+}
+
 #[derive(Clone, Debug, PartialEq, Eq, Copy, Hash, TyEncodable, TyDecodable, HashStable)]
 pub enum Visibility {
     /// Visible everywhere (including in other crates).
@@ -459,6 +491,29 @@ impl<'tcx> Predicate<'tcx> {
     pub fn kind(self) -> Binder<'tcx, PredicateKind<'tcx>> {
         self.inner.kind
     }
+
+    /// Flips the polarity of a Predicate.
+    ///
+    /// Given `T: Trait` predicate it returns `T: !Trait` and given `T: !Trait` returns `T: Trait`.
+    pub fn flip_polarity(&self, tcx: TyCtxt<'tcx>) -> Option<Predicate<'tcx>> {
+        let kind = self
+            .inner
+            .kind
+            .map_bound(|kind| match kind {
+                PredicateKind::Trait(TraitPredicate { trait_ref, constness, polarity }) => {
+                    Some(PredicateKind::Trait(TraitPredicate {
+                        trait_ref,
+                        constness,
+                        polarity: polarity.flip()?,
+                    }))
+                }
+
+                _ => None,
+            })
+            .transpose()?;
+
+        Some(tcx.mk_predicate(kind))
+    }
 }
 
 impl<'a, 'tcx> HashStable<StableHashingContext<'a>> for Predicate<'tcx> {
@@ -654,6 +709,8 @@ pub struct TraitPredicate<'tcx> {
     pub trait_ref: TraitRef<'tcx>,
 
     pub constness: BoundConstness,
+
+    pub polarity: ImplPolarity,
 }
 
 pub type PolyTraitPredicate<'tcx> = ty::Binder<'tcx, TraitPredicate<'tcx>>;
@@ -788,7 +845,11 @@ impl<'tcx> ToPredicate<'tcx> for ConstnessAnd<PolyTraitRef<'tcx>> {
     fn to_predicate(self, tcx: TyCtxt<'tcx>) -> Predicate<'tcx> {
         self.value
             .map_bound(|trait_ref| {
-                PredicateKind::Trait(ty::TraitPredicate { trait_ref, constness: self.constness })
+                PredicateKind::Trait(ty::TraitPredicate {
+                    trait_ref,
+                    constness: self.constness,
+                    polarity: ty::ImplPolarity::Positive,
+                })
             })
             .to_predicate(tcx)
     }

compiler/rustc_middle/src/ty/relate.rs

@@ -797,6 +797,20 @@ impl<'tcx> Relate<'tcx> for GenericArg<'tcx> {
     }
 }
 
+impl<'tcx> Relate<'tcx> for ty::ImplPolarity {
+    fn relate<R: TypeRelation<'tcx>>(
+        relation: &mut R,
+        a: ty::ImplPolarity,
+        b: ty::ImplPolarity,
+    ) -> RelateResult<'tcx, ty::ImplPolarity> {
+        if a != b {
+            Err(TypeError::PolarityMismatch(expected_found(relation, a, b)))
+        } else {
+            Ok(a)
+        }
+    }
+}
+
 impl<'tcx> Relate<'tcx> for ty::TraitPredicate<'tcx> {
     fn relate<R: TypeRelation<'tcx>>(
         relation: &mut R,
@@ -806,6 +820,7 @@ impl<'tcx> Relate<'tcx> for ty::TraitPredicate<'tcx> {
         Ok(ty::TraitPredicate {
             trait_ref: relation.relate(a.trait_ref, b.trait_ref)?,
             constness: relation.relate(a.constness, b.constness)?,
+            polarity: relation.relate(a.polarity, b.polarity)?,
         })
     }
 }

compiler/rustc_middle/src/ty/structural_impls.rs

@@ -157,7 +157,7 @@ impl fmt::Debug for ty::TraitPredicate<'tcx> {
         if let ty::BoundConstness::ConstIfConst = self.constness {
             write!(f, "~const ")?;
         }
-        write!(f, "TraitPredicate({:?})", self.trait_ref)
+        write!(f, "TraitPredicate({:?}, polarity:{:?})", self.trait_ref, self.polarity)
     }
 }
 
@@ -365,8 +365,11 @@ impl<'a, 'tcx> Lift<'tcx> for ty::ExistentialPredicate<'a> {
 impl<'a, 'tcx> Lift<'tcx> for ty::TraitPredicate<'a> {
     type Lifted = ty::TraitPredicate<'tcx>;
     fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<ty::TraitPredicate<'tcx>> {
-        tcx.lift(self.trait_ref)
-            .map(|trait_ref| ty::TraitPredicate { trait_ref, constness: self.constness })
+        tcx.lift(self.trait_ref).map(|trait_ref| ty::TraitPredicate {
+            trait_ref,
+            constness: self.constness,
+            polarity: self.polarity,
+        })
     }
 }
 
@@ -591,6 +594,7 @@ impl<'a, 'tcx> Lift<'tcx> for ty::error::TypeError<'a> {
         Some(match self {
             Mismatch => Mismatch,
             ConstnessMismatch(x) => ConstnessMismatch(x),
+            PolarityMismatch(x) => PolarityMismatch(x),
             UnsafetyMismatch(x) => UnsafetyMismatch(x),
             AbiMismatch(x) => AbiMismatch(x),
             Mutability => Mutability,

compiler/rustc_middle/src/ty/sty.rs

@@ -882,6 +882,7 @@ impl<'tcx> PolyTraitRef<'tcx> {
         self.map_bound(|trait_ref| ty::TraitPredicate {
             trait_ref,
             constness: ty::BoundConstness::NotConst,
+            polarity: ty::ImplPolarity::Positive,
         })
     }
 }

compiler/rustc_privacy/src/lib.rs

@@ -124,9 +124,11 @@ where
 
     fn visit_predicate(&mut self, predicate: ty::Predicate<'tcx>) -> ControlFlow<V::BreakTy> {
         match predicate.kind().skip_binder() {
-            ty::PredicateKind::Trait(ty::TraitPredicate { trait_ref, constness: _ }) => {
-                self.visit_trait(trait_ref)
-            }
+            ty::PredicateKind::Trait(ty::TraitPredicate {
+                trait_ref,
+                constness: _,
+                polarity: _,
+            }) => self.visit_trait(trait_ref),
             ty::PredicateKind::Projection(ty::ProjectionPredicate { projection_ty, ty }) => {
                 ty.visit_with(self)?;
                 self.visit_projection_ty(projection_ty)

compiler/rustc_span/src/symbol.rs

@@ -1141,6 +1141,7 @@ symbols! {
         rustc_specialization_trait,
         rustc_stable,
         rustc_std_internal_symbol,
+        rustc_strict_coherence,
         rustc_symbol_name,
         rustc_synthetic,
         rustc_test_marker,

compiler/rustc_trait_selection/src/traits/auto_trait.rs

@@ -286,6 +286,8 @@ impl AutoTraitFinder<'tcx> {
                 substs: infcx.tcx.mk_substs_trait(ty, &[]),
             },
             constness: ty::BoundConstness::NotConst,
+            // Auto traits are positive
+            polarity: ty::ImplPolarity::Positive,
         }));
 
         let computed_preds = param_env.caller_bounds().iter();