Rollup merge of #145627 - compiler-errors:const-supertrait-dyn-compat, r=fee1-dead

Unconditionally-const supertraits are considered not dyn compatible

Let's save some space in the design of const traits by making `dyn Trait` where `trait Trait: const Super` not dyn compatible.

Such a trait cannot satisfy `dyn Trait: Trait`; we could in the future make this dyn compatible but *NOT* implement `Trait`, but that's a bit weird and seems like it needs to be independently justified moving forward.

Fixes https://github.com/rust-lang/rust/issues/145198

r? fee1-dead

compiler/rustc_middle/src/traits/mod.rs

@@ -760,6 +760,9 @@ pub enum DynCompatibilityViolation {
     // Supertrait has a non-lifetime `for<T>` binder.
     SupertraitNonLifetimeBinder(SmallVec<[Span; 1]>),
 
+    // Trait has a `const Trait` supertrait.
+    SupertraitConst(SmallVec<[Span; 1]>),
+
     /// Method has something illegal.
     Method(Symbol, MethodViolationCode, Span),
 
@@ -785,6 +788,9 @@ impl DynCompatibilityViolation {
             DynCompatibilityViolation::SupertraitNonLifetimeBinder(_) => {
                 "where clause cannot reference non-lifetime `for<...>` variables".into()
             }
+            DynCompatibilityViolation::SupertraitConst(_) => {
+                "it cannot have a `const` supertrait".into()
+            }
             DynCompatibilityViolation::Method(name, MethodViolationCode::StaticMethod(_), _) => {
                 format!("associated function `{name}` has no `self` parameter").into()
             }
@@ -842,7 +848,8 @@ impl DynCompatibilityViolation {
         match self {
             DynCompatibilityViolation::SizedSelf(_)
             | DynCompatibilityViolation::SupertraitSelf(_)
-            | DynCompatibilityViolation::SupertraitNonLifetimeBinder(..) => {
+            | DynCompatibilityViolation::SupertraitNonLifetimeBinder(..)
+            | DynCompatibilityViolation::SupertraitConst(_) => {
                 DynCompatibilityViolationSolution::None
             }
             DynCompatibilityViolation::Method(
@@ -873,15 +880,17 @@ impl DynCompatibilityViolation {
         match self {
             DynCompatibilityViolation::SupertraitSelf(spans)
             | DynCompatibilityViolation::SizedSelf(spans)
-            | DynCompatibilityViolation::SupertraitNonLifetimeBinder(spans) => spans.clone(),
+            | DynCompatibilityViolation::SupertraitNonLifetimeBinder(spans)
+            | DynCompatibilityViolation::SupertraitConst(spans) => spans.clone(),
             DynCompatibilityViolation::AssocConst(_, span)
             | DynCompatibilityViolation::GAT(_, span)
-            | DynCompatibilityViolation::Method(_, _, span)
-                if *span != DUMMY_SP =>
-            {
+            | DynCompatibilityViolation::Method(_, _, span) => {
+                if *span != DUMMY_SP {
                     smallvec![*span]
+                } else {
+                    smallvec![]
+                }
             }
-            _ => smallvec![],
         }
     }
 }

compiler/rustc_trait_selection/src/traits/dyn_compatibility.rs

@@ -106,6 +106,10 @@ fn dyn_compatibility_violations_for_trait(
     if !spans.is_empty() {
         violations.push(DynCompatibilityViolation::SupertraitNonLifetimeBinder(spans));
     }
+    let spans = super_predicates_are_unconditionally_const(tcx, trait_def_id);
+    if !spans.is_empty() {
+        violations.push(DynCompatibilityViolation::SupertraitConst(spans));
+    }
 
     violations
 }
@@ -247,16 +251,31 @@ fn super_predicates_have_non_lifetime_binders(
     tcx: TyCtxt<'_>,
     trait_def_id: DefId,
 ) -> SmallVec<[Span; 1]> {
-    // If non_lifetime_binders is disabled, then exit early
-    if !tcx.features().non_lifetime_binders() {
-        return SmallVec::new();
-    }
     tcx.explicit_super_predicates_of(trait_def_id)
         .iter_identity_copied()
         .filter_map(|(pred, span)| pred.has_non_region_bound_vars().then_some(span))
         .collect()
 }
 
+/// Checks for `const Trait` supertraits. We're okay with `[const] Trait`,
+/// supertraits since for a non-const instantiation of that trait, the
+/// conditionally-const supertrait is also not required to be const.
+fn super_predicates_are_unconditionally_const(
+    tcx: TyCtxt<'_>,
+    trait_def_id: DefId,
+) -> SmallVec<[Span; 1]> {
+    tcx.explicit_super_predicates_of(trait_def_id)
+        .iter_identity_copied()
+        .filter_map(|(pred, span)| {
+            if let ty::ClauseKind::HostEffect(_) = pred.kind().skip_binder() {
+                Some(span)
+            } else {
+                None
+            }
+        })
+        .collect()
+}
+
 fn trait_has_sized_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
     tcx.generics_require_sized_self(trait_def_id)
 }

tests/ui/traits/const-traits/const-supertraits-dyn-compat.rs

@@ -0,0 +1,18 @@
+#![feature(const_trait_impl)]
+
+const trait Super {}
+
+// Not ok
+const trait Unconditionally: const Super {}
+fn test() {
+    let _: &dyn Unconditionally;
+    //~^ ERROR the trait `Unconditionally` is not dyn compatible
+}
+
+// Okay
+const trait Conditionally: [const] Super {}
+fn test2() {
+    let _: &dyn Conditionally;
+}
+
+fn main() {}

tests/ui/traits/const-traits/const-supertraits-dyn-compat.stderr

@@ -0,0 +1,18 @@
+error[E0038]: the trait `Unconditionally` is not dyn compatible
+  --> $DIR/const-supertraits-dyn-compat.rs:8:17
+   |
+LL |     let _: &dyn Unconditionally;
+   |                 ^^^^^^^^^^^^^^^ `Unconditionally` is not dyn compatible
+   |
+note: for a trait to be dyn compatible it needs to allow building a vtable
+      for more information, visit <https://doc.rust-lang.org/reference/items/traits.html#dyn-compatibility>
+  --> $DIR/const-supertraits-dyn-compat.rs:6:30
+   |
+LL | const trait Unconditionally: const Super {}
+   |             ---------------  ^^^^^^^^^^^ ...because it cannot have a `const` supertrait
+   |             |
+   |             this trait is not dyn compatible...
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0038`.