Only allow ~const bounds for traits with #[const_trait]

Author: oli-obk

compiler/rustc_trait_selection/src/traits/wf.rs

@@ -308,6 +308,10 @@ impl<'tcx> WfPredicates<'tcx> {
         let obligations = if trait_pred.constness == ty::BoundConstness::NotConst {
             self.nominal_obligations_without_const(trait_ref.def_id, trait_ref.substs)
         } else {
+            if !tcx.has_attr(trait_ref.def_id, rustc_span::sym::const_trait) {
+                tcx.sess
+                    .span_err(self.span, "~const can only be applied to `#[const_trait]` traits");
+            }
             self.nominal_obligations(trait_ref.def_id, trait_ref.substs)
         };
 

library/core/src/iter/traits/collect.rs

@@ -264,7 +264,7 @@ pub trait IntoIterator {
 
 #[rustc_const_unstable(feature = "const_intoiterator_identity", issue = "90603")]
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<I: ~const Iterator> const IntoIterator for I {
+impl<I: Iterator> const IntoIterator for I {
     type Item = I::Item;
     type IntoIter = I;
 

library/core/src/marker.rs

@@ -799,6 +799,7 @@ impl<T: ?Sized> Unpin for *mut T {}
 #[unstable(feature = "const_trait_impl", issue = "67792")]
 #[lang = "destruct"]
 #[rustc_on_unimplemented(message = "can't drop `{Self}`", append_const_msg)]
+#[const_trait]
 pub trait Destruct {}
 
 /// A marker for tuple types.

src/test/ui/consts/const-fn-error.stderr

@@ -22,8 +22,8 @@ LL |     for i in 0..x {
 note: impl defined here, but it is not `const`
   --> $SRC_DIR/core/src/iter/traits/collect.rs:LL:COL
    |
-LL | impl<I: ~const Iterator> const IntoIterator for I {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | impl<I: Iterator> const IntoIterator for I {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
 
 error[E0658]: mutable references are not allowed in constant functions

src/test/ui/consts/const-for.stderr

@@ -7,8 +7,8 @@ LL |     for _ in 0..5 {}
 note: impl defined here, but it is not `const`
   --> $SRC_DIR/core/src/iter/traits/collect.rs:LL:COL
    |
-LL | impl<I: ~const Iterator> const IntoIterator for I {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | impl<I: Iterator> const IntoIterator for I {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: calls in constants are limited to constant functions, tuple structs and tuple variants
 
 error[E0015]: cannot call non-const fn `<std::ops::Range<i32> as Iterator>::next` in constants

src/test/ui/consts/constifconst-call-in-const-position.rs

@@ -2,6 +2,7 @@
 
 #![feature(const_trait_impl, generic_const_exprs)]
 
+#[const_trait]
 pub trait Tr {
     fn a() -> usize;
 }

src/test/ui/consts/constifconst-call-in-const-position.stderr

@@ -8,7 +8,7 @@ LL | #![feature(const_trait_impl, generic_const_exprs)]
    = note: `#[warn(incomplete_features)]` on by default
 
 error[E0080]: evaluation of `foo::<()>::{constant#0}` failed
-  --> $DIR/constifconst-call-in-const-position.rs:15:38
+  --> $DIR/constifconst-call-in-const-position.rs:16:38
    |
 LL | const fn foo<T: ~const Tr>() -> [u8; T::a()] {
    |                                      ^^^^^^ calling non-const function `<() as Tr>::a`

src/test/ui/never_type/issue-52443.stderr

@@ -47,8 +47,8 @@ LL |     [(); { for _ in 0usize.. {}; 0}];
 note: impl defined here, but it is not `const`
   --> $SRC_DIR/core/src/iter/traits/collect.rs:LL:COL
    |
-LL | impl<I: ~const Iterator> const IntoIterator for I {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | impl<I: Iterator> const IntoIterator for I {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: calls in constants are limited to constant functions, tuple structs and tuple variants
 
 error[E0658]: mutable references are not allowed in constants

src/test/ui/rfc-2632-const-trait-impl/call-generic-method-nonconst.rs

@@ -2,13 +2,18 @@
 
 struct S;
 
-impl PartialEq for S {
+#[const_trait]
+trait Foo {
+    fn eq(&self, _: &Self) -> bool;
+}
+
+impl Foo for S {
     fn eq(&self, _: &S) -> bool {
         true
     }
 }
 
-const fn equals_self<T: ~const PartialEq>(t: &T) -> bool {
+const fn equals_self<T: ~const Foo>(t: &T) -> bool {
     true
 }
 

src/test/ui/rfc-2632-const-trait-impl/call-generic-method-nonconst.stderr

@@ -1,26 +1,21 @@
-error[E0277]: can't compare `S` with `S` in const contexts
-  --> $DIR/call-generic-method-nonconst.rs:18:34
+error[E0277]: the trait bound `S: ~const Foo` is not satisfied
+  --> $DIR/call-generic-method-nonconst.rs:23:34
    |
 LL | pub const EQ: bool = equals_self(&S);
-   |                      ----------- ^^ no implementation for `S == S`
+   |                      ----------- ^^ the trait `~const Foo` is not implemented for `S`
    |                      |
    |                      required by a bound introduced by this call
    |
-   = help: the trait `~const PartialEq` is not implemented for `S`
-note: the trait `PartialEq` is implemented for `S`, but that implementation is not `const`
-  --> $DIR/call-generic-method-nonconst.rs:18:34
+note: the trait `Foo` is implemented for `S`, but that implementation is not `const`
+  --> $DIR/call-generic-method-nonconst.rs:23:34
    |
 LL | pub const EQ: bool = equals_self(&S);
    |                                  ^^
 note: required by a bound in `equals_self`
-  --> $DIR/call-generic-method-nonconst.rs:11:25
-   |
-LL | const fn equals_self<T: ~const PartialEq>(t: &T) -> bool {
-   |                         ^^^^^^^^^^^^^^^^ required by this bound in `equals_self`
-help: consider annotating `S` with `#[derive(PartialEq)]`
-   |
-LL | #[derive(PartialEq)]
+  --> $DIR/call-generic-method-nonconst.rs:16:25
    |
+LL | const fn equals_self<T: ~const Foo>(t: &T) -> bool {
+   |                         ^^^^^^^^^^ required by this bound in `equals_self`
 
 error: aborting due to previous error