Disallow specializing on const impls with non-const impls.

Author: BGR360

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;
@@ -117,12 +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`.
@@ -277,7 +299,7 @@ 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).
 /// * A `T: Tr` predicate where `Tr` is an always-applicable trait.
@@ -375,16 +397,19 @@ fn check_predicates<'tcx>(
     }
 }
 
-/// Checks whether two predicates are the same for the purposes of specialization.
+/// Checks if some predicate on the specializing impl (`predicate1`) is the same
+/// as some predicate on the base impl (`predicate2`).
 ///
 /// This is slightly more complicated than simple syntactic equivalence, since
 /// we want to equate `T: Tr` with `T: ~const Tr` so this can work:
 ///
+/// ```ignore (illustrative)
 /// #[rustc_specialization_trait]
 /// trait Specialize { }
 ///
-/// impl<T: ~const Bound> const Tr for T { }
-/// impl<T: Bound + Specialize> Tr for T { }
+/// impl<T: Bound> Tr for T { }
+/// impl<T: ~const Bound + Specialize> const Tr for T { }
+/// ```
 fn trait_predicates_eq<'tcx>(
     predicate1: ty::Predicate<'tcx>,
     predicate2: ty::Predicate<'tcx>,
@@ -400,6 +425,8 @@ fn trait_predicates_eq<'tcx>(
         _ => kind,
     };
 
+    // We rely on `check_constness` above to ensure that pred1 is const if pred2
+    // is const.
     let pred1_kind_not_const = predicate1.kind().map_bound(predicate_kind_without_constness);
     let pred2_kind_not_const = predicate2.kind().map_bound(predicate_kind_without_constness);
 

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

@@ -1,5 +1,5 @@
-// Tests that a const default trait impl can be specialized by a non-const trait
-// impl, but that the specializing impl cannot be used in a const context.
+// Tests that a const default trait impl cannot be specialized by a non-const
+// trait impl.
 
 #![feature(const_trait_impl)]
 #![feature(min_specialization)]
@@ -8,12 +8,6 @@ trait Value {
     fn value() -> u32;
 }
 
-const fn get_value<T: ~const Value>() -> u32 {
-    T::value()
-    //~^ ERROR any use of this value will cause an error [const_err]
-    //~| WARNING this was previously accepted
-}
-
 impl<T> const Value for T {
     default fn value() -> u32 {
         0
@@ -22,16 +16,11 @@ impl<T> const Value for T {
 
 struct FortyTwo;
 
-impl Value for 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
     }
 }
 
-const ZERO: u32 = get_value::<()>();
-
-const FORTY_TWO: u32 =
-    get_value::<FortyTwo>(); // This is the line that causes the error, but it gets reported above
-
 fn main() {}

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

@@ -1,37 +1,8 @@
-error: any use of this value will cause an error
-  --> $DIR/const-default-non-const-specialized.rs:12:5
+error: cannot specialize on const impl with non-const impl
+  --> $DIR/const-default-non-const-specialized.rs:19:1
    |
-LL |     T::value()
-   |     ^^^^^^^^^^
-   |     |
-   |     calling non-const function `<FortyTwo as Value>::value`
-   |     inside `get_value::<FortyTwo>` at $DIR/const-default-non-const-specialized.rs:12:5
-   |     inside `FORTY_TWO` at $DIR/const-default-non-const-specialized.rs:35:5
-...
-LL | const FORTY_TWO: u32 =
-   | --------------------
-   |
-   = note: `#[deny(const_err)]` on by default
-   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
+LL | impl Value for FortyTwo {
+   | ^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to previous error
 
-Future incompatibility report: Future breakage diagnostic:
-error: any use of this value will cause an error
-  --> $DIR/const-default-non-const-specialized.rs:12:5
-   |
-LL |     T::value()
-   |     ^^^^^^^^^^
-   |     |
-   |     calling non-const function `<FortyTwo as Value>::value`
-   |     inside `get_value::<FortyTwo>` at $DIR/const-default-non-const-specialized.rs:12:5
-   |     inside `FORTY_TWO` at $DIR/const-default-non-const-specialized.rs:35:5
-...
-LL | const FORTY_TWO: u32 =
-   | --------------------
-   |
-   = note: `#[deny(const_err)]` on by default
-   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
-

src/test/ui/rfc-2632-const-trait-impl/specialization/issue-95187-same-trait-bound-different-constness.rs

@@ -1,4 +1,4 @@
-// Tests that `T: ~const Foo` and `T: Foo` are treated as equivalent for the
+// Tests that `T: Foo` and `T: ~const Foo` are treated as equivalent for the
 // purposes of min_specialization.
 
 // check-pass
@@ -14,14 +14,14 @@ trait Foo {}
 
 trait Bar {}
 
-impl<T> const Bar for T
+impl<T> Bar for T
 where
-    T: ~const Foo,
+    T: Foo,
 {}
 
-impl<T> Bar for T
+impl<T> const Bar for T
 where
-    T: Foo,
+    T: ~const Foo,
     T: Specialize,
 {}
 

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

@@ -1,6 +1,8 @@
 // Tests that a non-const default impl can be specialized by a const trait impl,
 // but that the default impl cannot be used in a const context.
 
+// run-pass
+
 #![feature(const_trait_impl)]
 #![feature(min_specialization)]
 
@@ -27,8 +29,10 @@ impl const Value for FortyTwo {
     }
 }
 
-const ZERO: u32 = get_value::<()>(); //~ ERROR the trait bound `(): ~const Value` is not satisfied
-
-const FORTY_TWO: u32 = get_value::<FortyTwo>();
+fn main() {
+    let zero = get_value::<()>();
+    assert_eq!(zero, 0);
 
-fn main() {}
+    const FORTY_TWO: u32 = get_value::<FortyTwo>();
+    assert_eq!(FORTY_TWO, 42);
+}