Rollup merge of #95292 - BGR360:const-trait-specialize, r=lcnr

Allow specialized const trait impls.

Fixes #95186.
Fixes #95187.

I've done my best to create a comprehensive test suite for the interaction between `min_specialization` and `const_trait_impls`. I wouldn't be surprised if there are interesting cases I haven't tested, please let me know.

Author: Manishearth

compiler/rustc_hir_analysis/src/impl_wf_check/min_specialization.rs

@@ -69,6 +69,7 @@ use crate::constrained_generic_params as cgp;
 use crate::errors::SubstsOnOverriddenImpl;
 
 use rustc_data_structures::fx::FxHashSet;
+use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_infer::infer::outlives::env::OutlivesEnvironment;
 use rustc_infer::infer::TyCtxtInferExt;
@@ -80,6 +81,7 @@ use rustc_span::Span;
 use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt;
 use rustc_trait_selection::traits::outlives_bounds::InferCtxtExt as _;
 use rustc_trait_selection::traits::{self, translate_substs, wf, ObligationCtxt};
+use tracing::instrument;
 
 pub(super) fn check_min_specialization(tcx: TyCtxt<'_>, impl_def_id: LocalDefId) {
     if let Some(node) = parent_specialization_node(tcx, impl_def_id) {
@@ -103,13 +105,11 @@ fn parent_specialization_node(tcx: TyCtxt<'_>, impl1_def_id: LocalDefId) -> Opti
 }
 
 /// Check that `impl1` is a sound specialization
+#[instrument(level = "debug", skip(tcx))]
 fn check_always_applicable(tcx: TyCtxt<'_>, impl1_def_id: LocalDefId, impl2_node: Node) {
     if let Some((impl1_substs, impl2_substs)) = get_impl_substs(tcx, impl1_def_id, impl2_node) {
         let impl2_def_id = impl2_node.def_id();
-        debug!(
-            "check_always_applicable(\nimpl1_def_id={:?},\nimpl2_def_id={:?},\nimpl2_substs={:?}\n)",
-            impl1_def_id, impl2_def_id, impl2_substs
-        );
+        debug!(?impl2_def_id, ?impl2_substs);
 
         let parent_substs = if impl2_node.is_from_trait() {
             impl2_substs.to_vec()
@@ -118,12 +118,33 @@ fn check_always_applicable(tcx: TyCtxt<'_>, impl1_def_id: LocalDefId, impl2_node
         };
 
         let span = tcx.def_span(impl1_def_id);
+        check_constness(tcx, impl1_def_id, impl2_node, span);
         check_static_lifetimes(tcx, &parent_substs, span);
         check_duplicate_params(tcx, impl1_substs, &parent_substs, span);
         check_predicates(tcx, impl1_def_id, impl1_substs, impl2_node, impl2_substs, span);
     }
 }
 
+/// Check that the specializing impl `impl1` is at least as const as the base
+/// impl `impl2`
+fn check_constness(tcx: TyCtxt<'_>, impl1_def_id: LocalDefId, impl2_node: Node, span: Span) {
+    if impl2_node.is_from_trait() {
+        // This isn't a specialization
+        return;
+    }
+
+    let impl1_constness = tcx.constness(impl1_def_id.to_def_id());
+    let impl2_constness = tcx.constness(impl2_node.def_id());
+
+    if let hir::Constness::Const = impl2_constness {
+        if let hir::Constness::NotConst = impl1_constness {
+            tcx.sess
+                .struct_span_err(span, "cannot specialize on const impl with non-const impl")
+                .emit();
+        }
+    }
+}
+
 /// Given a specializing impl `impl1`, and the base impl `impl2`, returns two
 /// substitutions `(S1, S2)` that equate their trait references. The returned
 /// types are expressed in terms of the generics of `impl1`.
@@ -278,15 +299,15 @@ fn check_static_lifetimes<'tcx>(
 
 /// Check whether predicates on the specializing impl (`impl1`) are allowed.
 ///
-/// Each predicate `P` must be:
+/// Each predicate `P` must be one of:
 ///
-/// * global (not reference any parameters)
-/// * `T: Tr` predicate where `Tr` is an always-applicable trait
-/// * on the base `impl impl2`
-///     * Currently this check is done using syntactic equality, which is
-///       conservative but generally sufficient.
-/// * a well-formed predicate of a type argument of the trait being implemented,
+/// * Global (not reference any parameters).
+/// * A `T: Tr` predicate where `Tr` is an always-applicable trait.
+/// * Present on the base impl `impl2`.
+///     * This check is done using the `trait_predicates_eq` function below.
+/// * A well-formed predicate of a type argument of the trait being implemented,
 ///   including the `Self`-type.
+#[instrument(level = "debug", skip(tcx))]
 fn check_predicates<'tcx>(
     tcx: TyCtxt<'tcx>,
     impl1_def_id: LocalDefId,
@@ -322,10 +343,7 @@ fn check_predicates<'tcx>(
         .map(|obligation| obligation.predicate)
         .collect()
     };
-    debug!(
-        "check_always_applicable(\nimpl1_predicates={:?},\nimpl2_predicates={:?}\n)",
-        impl1_predicates, impl2_predicates,
-    );
+    debug!(?impl1_predicates, ?impl2_predicates);
 
     // Since impls of always applicable traits don't get to assume anything, we
     // can also assume their supertraits apply.
@@ -373,25 +391,83 @@ fn check_predicates<'tcx>(
     );
 
     for (predicate, span) in impl1_predicates {
-        if !impl2_predicates.contains(&predicate) {
+        if !impl2_predicates.iter().any(|pred2| trait_predicates_eq(tcx, predicate, *pred2, span)) {
             check_specialization_on(tcx, predicate, span)
         }
     }
 }
 
+/// Checks if some predicate on the specializing impl (`predicate1`) is the same
+/// as some predicate on the base impl (`predicate2`).
+///
+/// This basically just checks syntactic equivalence, but is a little more
+/// forgiving since we want to equate `T: Tr` with `T: ~const Tr` so this can work:
+///
+/// ```ignore (illustrative)
+/// #[rustc_specialization_trait]
+/// trait Specialize { }
+///
+/// impl<T: Bound> Tr for T { }
+/// impl<T: ~const Bound + Specialize> const Tr for T { }
+/// ```
+///
+/// However, we *don't* want to allow the reverse, i.e., when the bound on the
+/// specializing impl is not as const as the bound on the base impl:
+///
+/// ```ignore (illustrative)
+/// impl<T: ~const Bound> const Tr for T { }
+/// impl<T: Bound + Specialize> const Tr for T { } // should be T: ~const Bound
+/// ```
+///
+/// So we make that check in this function and try to raise a helpful error message.
+fn trait_predicates_eq<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    predicate1: ty::Predicate<'tcx>,
+    predicate2: ty::Predicate<'tcx>,
+    span: Span,
+) -> bool {
+    let pred1_kind = predicate1.kind().skip_binder();
+    let pred2_kind = predicate2.kind().skip_binder();
+    let (trait_pred1, trait_pred2) = match (pred1_kind, pred2_kind) {
+        (ty::PredicateKind::Trait(pred1), ty::PredicateKind::Trait(pred2)) => (pred1, pred2),
+        // Just use plain syntactic equivalence if either of the predicates aren't
+        // trait predicates or have bound vars.
+        _ => return predicate1 == predicate2,
+    };
+
+    let predicates_equal_modulo_constness = {
+        let pred1_unconsted =
+            ty::TraitPredicate { constness: ty::BoundConstness::NotConst, ..trait_pred1 };
+        let pred2_unconsted =
+            ty::TraitPredicate { constness: ty::BoundConstness::NotConst, ..trait_pred2 };
+        pred1_unconsted == pred2_unconsted
+    };
+
+    if !predicates_equal_modulo_constness {
+        return false;
+    }
+
+    // Check that the predicate on the specializing impl is at least as const as
+    // the one on the base.
+    match (trait_pred2.constness, trait_pred1.constness) {
+        (ty::BoundConstness::ConstIfConst, ty::BoundConstness::NotConst) => {
+            tcx.sess.struct_span_err(span, "missing `~const` qualifier for specialization").emit();
+        }
+        _ => {}
+    }
+
+    true
+}
+
+#[instrument(level = "debug", skip(tcx))]
 fn check_specialization_on<'tcx>(tcx: TyCtxt<'tcx>, predicate: ty::Predicate<'tcx>, span: Span) {
-    debug!("can_specialize_on(predicate = {:?})", predicate);
     match predicate.kind().skip_binder() {
         // Global predicates are either always true or always false, so we
         // are fine to specialize on.
         _ if predicate.is_global() => (),
         // We allow specializing on explicitly marked traits with no associated
         // items.
-        ty::PredicateKind::Trait(ty::TraitPredicate {
-            trait_ref,
-            constness: ty::BoundConstness::NotConst,
-            polarity: _,
-        }) => {
+        ty::PredicateKind::Trait(ty::TraitPredicate { trait_ref, constness: _, polarity: _ }) => {
             if !matches!(
                 trait_predicate_kind(tcx, predicate),
                 Some(TraitSpecializationKind::Marker)

src/test/ui/rfc-2632-const-trait-impl/specialization/const-default-bound-non-const-specialized-bound.rs

@@ -0,0 +1,46 @@
+// Tests that trait bounds on specializing trait impls must be `~const` if the
+// same bound is present on the default impl and is `~const` there.
+
+#![feature(const_trait_impl)]
+#![feature(rustc_attrs)]
+#![feature(min_specialization)]
+
+#[rustc_specialization_trait]
+trait Specialize {}
+
+#[const_trait]
+trait Foo {}
+
+#[const_trait]
+trait Bar {}
+
+// bgr360: I was only able to exercise the code path that raises the
+// "missing ~const qualifier" error by making this base impl non-const, even
+// though that doesn't really make sense to do. As seen below, if the base impl
+// is made const, rustc fails earlier with an overlapping impl failure.
+impl<T> Bar for T
+where
+    T: ~const Foo,
+{}
+
+impl<T> Bar for T
+where
+    T: Foo, //~ ERROR missing `~const` qualifier
+    T: Specialize,
+{}
+
+#[const_trait]
+trait Baz {}
+
+impl<T> const Baz for T
+where
+    T: ~const Foo,
+{}
+
+impl<T> const Baz for T //~ ERROR conflicting implementations of trait `Baz`
+where
+    T: Foo,
+    T: Specialize,
+{}
+
+fn main() {}

src/test/ui/rfc-2632-const-trait-impl/specialization/const-default-bound-non-const-specialized-bound.stderr

@@ -0,0 +1,18 @@
+error: missing `~const` qualifier for specialization
+  --> $DIR/const-default-bound-non-const-specialized-bound.rs:28:8
+   |
+LL |     T: Foo,
+   |        ^^^
+
+error[E0119]: conflicting implementations of trait `Baz`
+  --> $DIR/const-default-bound-non-const-specialized-bound.rs:40:1
+   |
+LL | impl<T> const Baz for T
+   | ----------------------- first implementation here
+...
+LL | impl<T> const Baz for T
+   | ^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0119`.

src/test/ui/rfc-2632-const-trait-impl/specialization/const-default-const-specialized.rs

@@ -0,0 +1,39 @@
+// Tests that a const default trait impl can be specialized by another const
+// trait impl and that the specializing impl will be used during const-eval.
+
+// run-pass
+
+#![feature(const_trait_impl)]
+#![feature(min_specialization)]
+
+#[const_trait]
+trait Value {
+    fn value() -> u32;
+}
+
+const fn get_value<T: ~const Value>() -> u32 {
+    T::value()
+}
+
+impl<T> const Value for T {
+    default fn value() -> u32 {
+        0
+    }
+}
+
+struct FortyTwo;
+
+impl const Value for FortyTwo {
+    fn value() -> u32 {
+        42
+    }
+}
+
+const ZERO: u32 = get_value::<()>();
+
+const FORTY_TWO: u32 = get_value::<FortyTwo>();
+
+fn main() {
+    assert_eq!(ZERO, 0);
+    assert_eq!(FORTY_TWO, 42);
+}

src/test/ui/rfc-2632-const-trait-impl/specialization/const-default-impl-non-const-specialized-impl.rs

@@ -0,0 +1,26 @@
+// Tests that specializing trait impls must be at least as const as the default impl.
+
+#![feature(const_trait_impl)]
+#![feature(min_specialization)]
+
+#[const_trait]
+trait Value {
+    fn value() -> u32;
+}
+
+impl<T> const Value for T {
+    default fn value() -> u32 {
+        0
+    }
+}
+
+struct FortyTwo;
+
+impl Value for FortyTwo { //~ ERROR cannot specialize on const impl with non-const impl
+    fn value() -> u32 {
+        println!("You can't do that (constly)");
+        42
+    }
+}
+
+fn main() {}

src/test/ui/rfc-2632-const-trait-impl/specialization/const-default-impl-non-const-specialized-impl.stderr

@@ -0,0 +1,8 @@
+error: cannot specialize on const impl with non-const impl
+  --> $DIR/const-default-impl-non-const-specialized-impl.rs:19:1
+   |
+LL | impl Value for FortyTwo {
+   | ^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/rfc-2632-const-trait-impl/specialization/default-keyword.rs

@@ -0,0 +1,15 @@
+// check-pass
+
+#![feature(const_trait_impl)]
+#![feature(min_specialization)]
+
+#[const_trait]
+trait Foo {
+    fn foo();
+}
+
+impl const Foo for u32 {
+    default fn foo() {}
+}
+
+fn main() {}

src/test/ui/rfc-2632-const-trait-impl/specialization/issue-95186-specialize-on-tilde-const.rs

@@ -0,0 +1,37 @@
+// Tests that `~const` trait bounds can be used to specialize const trait impls.
+
+// check-pass
+
+#![feature(const_trait_impl)]
+#![feature(rustc_attrs)]
+#![feature(min_specialization)]
+
+#[const_trait]
+#[rustc_specialization_trait]
+trait Specialize {}
+
+#[const_trait]
+trait Foo {}
+
+impl<T> const Foo for T {}
+
+impl<T> const Foo for T
+where
+    T: ~const Specialize,
+{}
+
+#[const_trait]
+trait Bar {}
+
+impl<T> const Bar for T
+where
+    T: ~const Foo,
+{}
+
+impl<T> const Bar for T
+where
+    T: ~const Foo,
+    T: ~const Specialize,
+{}
+
+fn main() {}