Implement ~const opaques

compiler/rustc_hir_analysis/src/check/check.rs

@@ -339,6 +339,8 @@ fn check_opaque_meets_bounds<'tcx>(
 
     let misc_cause = ObligationCause::misc(span, def_id);
     // FIXME: We should just register the item bounds here, rather than equating.
+    // FIXME(const_trait_impl): When we do that, please make sure to also register
+    // the `~const` bounds.
     match ocx.eq(&misc_cause, param_env, opaque_ty, hidden_ty) {
         Ok(()) => {}
         Err(ty_err) => {

compiler/rustc_hir_analysis/src/collect.rs

@@ -339,6 +339,10 @@ impl<'tcx> Visitor<'tcx> for CollectItemTypesVisitor<'tcx> {
         self.tcx.ensure().explicit_item_super_predicates(def_id);
         self.tcx.ensure().item_bounds(def_id);
         self.tcx.ensure().item_super_predicates(def_id);
+        if self.tcx.is_conditionally_const(def_id) {
+            self.tcx.ensure().explicit_implied_const_bounds(def_id);
+            self.tcx.ensure().const_conditions(def_id);
+        }
         intravisit::walk_opaque_ty(self, opaque);
     }
 
@@ -681,6 +685,10 @@ fn lower_item(tcx: TyCtxt<'_>, item_id: hir::ItemId) {
                 tcx.ensure().generics_of(item.owner_id);
                 tcx.ensure().type_of(item.owner_id);
                 tcx.ensure().predicates_of(item.owner_id);
+                if tcx.is_conditionally_const(def_id) {
+                    tcx.ensure().explicit_implied_const_bounds(def_id);
+                    tcx.ensure().const_conditions(def_id);
+                }
                 match item.kind {
                     hir::ForeignItemKind::Fn(..) => {
                         tcx.ensure().codegen_fn_attrs(item.owner_id);

compiler/rustc_hir_analysis/src/collect/predicates_of.rs

@@ -958,6 +958,12 @@ pub(super) fn const_conditions<'tcx>(
             hir::ForeignItemKind::Fn(_, _, generics) => (generics, None, false),
             _ => bug!("const_conditions called on wrong item: {def_id:?}"),
         },
+        Node::OpaqueTy(opaque) => match opaque.origin {
+            hir::OpaqueTyOrigin::FnReturn { parent, .. } => return tcx.const_conditions(parent),
+            hir::OpaqueTyOrigin::AsyncFn { .. } | hir::OpaqueTyOrigin::TyAlias { .. } => {
+                unreachable!()
+            }
+        },
         // N.B. Tuple ctors are unconditionally constant.
         Node::Ctor(hir::VariantData::Tuple { .. }) => return Default::default(),
         _ => bug!("const_conditions called on wrong item: {def_id:?}"),
@@ -1033,7 +1039,8 @@ pub(super) fn explicit_implied_const_bounds<'tcx>(
                 PredicateFilter::SelfConstIfConst,
             )
         }
-        Node::TraitItem(hir::TraitItem { kind: hir::TraitItemKind::Type(..), .. }) => {
+        Node::TraitItem(hir::TraitItem { kind: hir::TraitItemKind::Type(..), .. })
+        | Node::OpaqueTy(_) => {
             explicit_item_bounds_with_filter(tcx, def_id, PredicateFilter::ConstIfConst)
         }
         _ => bug!("explicit_implied_const_bounds called on wrong item: {def_id:?}"),

compiler/rustc_infer/src/infer/opaque_types/mod.rs

@@ -574,9 +574,8 @@ impl<'tcx> InferCtxt<'tcx> {
         // unexpected region errors.
         goals.push(Goal::new(tcx, param_env, ty::ClauseKind::WellFormed(hidden_ty.into())));
 
-        let item_bounds = tcx.explicit_item_bounds(def_id);
-        for (predicate, _) in item_bounds.iter_instantiated_copied(tcx, args) {
-            let predicate = predicate.fold_with(&mut BottomUpFolder {
+        let replace_opaques_in = |clause: ty::Clause<'tcx>, goals: &mut Vec<_>| {
+            clause.fold_with(&mut BottomUpFolder {
                 tcx,
                 ty_op: |ty| match *ty.kind() {
                     // We can't normalize associated types from `rustc_infer`,
@@ -612,11 +611,31 @@ impl<'tcx> InferCtxt<'tcx> {
                 },
                 lt_op: |lt| lt,
                 ct_op: |ct| ct,
-            });
+            })
+        };
+
+        let item_bounds = tcx.explicit_item_bounds(def_id);
+        for (predicate, _) in item_bounds.iter_instantiated_copied(tcx, args) {
+            let predicate = replace_opaques_in(predicate, goals);
 
             // Require that the predicate holds for the concrete type.
             debug!(?predicate);
             goals.push(Goal::new(self.tcx, param_env, predicate));
         }
+
+        // If this opaque is being defined and it's conditionally const,
+        if self.tcx.is_conditionally_const(def_id) {
+            let item_bounds = tcx.explicit_implied_const_bounds(def_id);
+            for (predicate, _) in item_bounds.iter_instantiated_copied(tcx, args) {
+                let predicate = replace_opaques_in(
+                    predicate.to_host_effect_clause(self.tcx, ty::BoundConstness::Maybe),
+                    goals,
+                );
+
+                // Require that the predicate holds for the concrete type.
+                debug!(?predicate);
+                goals.push(Goal::new(self.tcx, param_env, predicate));
+            }
+        }
     }
 }

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -1532,6 +1532,10 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
                 self.encode_explicit_item_super_predicates(def_id);
                 record!(self.tables.opaque_ty_origin[def_id] <- self.tcx.opaque_ty_origin(def_id));
                 self.encode_precise_capturing_args(def_id);
+                if tcx.is_conditionally_const(def_id) {
+                    record_defaulted_array!(self.tables.explicit_implied_const_bounds[def_id]
+                        <- tcx.explicit_implied_const_bounds(def_id).skip_binder());
+                }
             }
             if tcx.impl_method_has_trait_impl_trait_tys(def_id)
                 && let Ok(table) = self.tcx.collect_return_position_impl_trait_in_trait_tys(def_id)

compiler/rustc_middle/src/ty/mod.rs

@@ -2117,7 +2117,13 @@ impl<'tcx> TyCtxt<'tcx> {
                     _ => bug!("unexpected parent item of associated item: {parent_def_id:?}"),
                 }
             }
-            DefKind::Closure | DefKind::OpaqueTy => {
+            DefKind::OpaqueTy => match self.opaque_ty_origin(def_id) {
+                hir::OpaqueTyOrigin::FnReturn { parent, .. } => self.is_conditionally_const(parent),
+                hir::OpaqueTyOrigin::AsyncFn { .. } => false,
+                // FIXME(const_trait_impl): ATPITs could be conditionally const?
+                hir::OpaqueTyOrigin::TyAlias { .. } => false,
+            },
+            DefKind::Closure => {
                 // Closures and RPITs will eventually have const conditions
                 // for `~const` bounds.
                 false

tests/ui/traits/const-traits/const-closure-parse-not-item.stderr

@@ -12,5 +12,13 @@ LL | const fn test() -> impl ~const Fn() {
    |
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
-error: aborting due to 2 previous errors
+error: `~const` can only be applied to `#[const_trait]` traits
+  --> $DIR/const-closure-parse-not-item.rs:7:25
+   |
+LL | const fn test() -> impl ~const Fn() {
+   |                         ^^^^^^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error: aborting due to 3 previous errors
 

tests/ui/traits/const-traits/const-opaque.no.stderr

@@ -0,0 +1,15 @@
+error[E0277]: the trait bound `(): const Foo` is not satisfied
+  --> $DIR/const-opaque.rs:31:18
+   |
+LL |     let opaque = bar(());
+   |                  ^^^^^^^
+
+error[E0277]: the trait bound `(): const Foo` is not satisfied
+  --> $DIR/const-opaque.rs:33:5
+   |
+LL |     opaque.method();
+   |     ^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/traits/const-traits/const-opaque.rs

@@ -0,0 +1,38 @@
+//@ revisions: yes no
+//@ compile-flags: -Znext-solver
+//@[yes] check-pass
+
+#![feature(const_trait_impl)]
+
+#[const_trait]
+trait Foo {
+    fn method(&self);
+}
+
+impl<T: ~const Foo> const Foo for (T,) {
+    fn method(&self) {}
+}
+
+#[cfg(yes)]
+impl const Foo for () {
+    fn method(&self) {}
+}
+
+#[cfg(no)]
+impl Foo for () {
+    fn method(&self) {}
+}
+
+const fn bar<T: ~const Foo>(t: T) -> impl ~const Foo {
+    (t,)
+}
+
+const _: () = {
+    let opaque = bar(());
+    //[no]~^ ERROR the trait bound `(): const Foo` is not satisfied
+    opaque.method();
+    //[no]~^ ERROR the trait bound `(): const Foo` is not satisfied
+    std::mem::forget(opaque);
+};
+
+fn main() {}

tests/ui/traits/const-traits/effects/ice-112822-expected-type-for-param.rs

@@ -3,6 +3,7 @@
 const fn test() -> impl ~const Fn() {
     //~^ ERROR `~const` can only be applied to `#[const_trait]` traits
     //~| ERROR `~const` can only be applied to `#[const_trait]` traits
+    //~| ERROR `~const` can only be applied to `#[const_trait]` traits
     const move || { //~ ERROR const closures are experimental
         let sl: &[u8] = b"foo";
 

tests/ui/traits/const-traits/effects/ice-112822-expected-type-for-param.stderr

@@ -1,5 +1,5 @@
 error[E0658]: const closures are experimental
-  --> $DIR/ice-112822-expected-type-for-param.rs:6:5
+  --> $DIR/ice-112822-expected-type-for-param.rs:7:5
    |
 LL |     const move || {
    |     ^^^^^
@@ -22,8 +22,16 @@ LL | const fn test() -> impl ~const Fn() {
    |
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
+error: `~const` can only be applied to `#[const_trait]` traits
+  --> $DIR/ice-112822-expected-type-for-param.rs:3:25
+   |
+LL | const fn test() -> impl ~const Fn() {
+   |                         ^^^^^^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
 error[E0015]: cannot call non-const operator in constant functions
-  --> $DIR/ice-112822-expected-type-for-param.rs:11:17
+  --> $DIR/ice-112822-expected-type-for-param.rs:12:17
    |
 LL |                 assert_eq!(first, &b'f');
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^
@@ -32,15 +40,15 @@ LL |                 assert_eq!(first, &b'f');
    = note: this error originates in the macro `assert_eq` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0015]: cannot call non-const fn `core::panicking::assert_failed::<&u8, &u8>` in constant functions
-  --> $DIR/ice-112822-expected-type-for-param.rs:11:17
+  --> $DIR/ice-112822-expected-type-for-param.rs:12:17
    |
 LL |                 assert_eq!(first, &b'f');
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
    = note: this error originates in the macro `assert_eq` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: aborting due to 5 previous errors
+error: aborting due to 6 previous errors
 
 Some errors have detailed explanations: E0015, E0658.
 For more information about an error, try `rustc --explain E0015`.