require const_impl_trait gate for all conditional and trait const calls

Author: RalfJung

compiler/rustc_const_eval/messages.ftl

@@ -25,6 +25,9 @@ const_eval_closure_fndef_not_const =
     function defined here, but it is not `const`
 const_eval_closure_non_const =
     cannot call non-const closure in {const_eval_const_context}s
+const_eval_conditionally_const_or_trait_call =
+    cannot call conditionally-const or trait fn `{$def_path_str}` in {const_eval_const_context}s
+
 const_eval_consider_dereferencing =
     consider dereferencing here
 

compiler/rustc_const_eval/src/check_consts/check.rs

@@ -15,7 +15,7 @@ use rustc_middle::mir::visit::Visitor;
 use rustc_middle::mir::*;
 use rustc_middle::span_bug;
 use rustc_middle::ty::adjustment::PointerCoercion;
-use rustc_middle::ty::{self, Instance, InstanceKind, Ty, TypeVisitableExt};
+use rustc_middle::ty::{self, Ty, TypeVisitableExt};
 use rustc_mir_dataflow::Analysis;
 use rustc_mir_dataflow::impls::MaybeStorageLive;
 use rustc_mir_dataflow::storage::always_storage_live_locals;
@@ -361,32 +361,20 @@ impl<'mir, 'tcx> Checker<'mir, 'tcx> {
         !is_transient
     }
 
+    /// Returns whether there are const-conditions.
     fn revalidate_conditional_constness(
         &mut self,
         callee: DefId,
         callee_args: ty::GenericArgsRef<'tcx>,
-        call_source: CallSource,
         call_span: Span,
-    ) {
+    ) -> bool {
         let tcx = self.tcx;
         if !tcx.is_conditionally_const(callee) {
-            return;
+            return false;
         }
 
         let const_conditions = tcx.const_conditions(callee).instantiate(tcx, callee_args);
-        // If there are any const conditions on this fn and `const_trait_impl`
-        // is not enabled, simply bail. We shouldn't be able to call conditionally
-        // const functions on stable.
-        if !const_conditions.is_empty() && !tcx.features().const_trait_impl() {
-            self.check_op(ops::FnCallNonConst {
-                callee,
-                args: callee_args,
-                span: call_span,
-                call_source,
-                feature: Some(sym::const_trait_impl),
-            });
-            return;
-        }
+        let have_const_conditions = !const_conditions.is_empty();
 
         let infcx = tcx.infer_ctxt().build(self.body.typing_mode(tcx));
         let ocx = ObligationCtxt::new_with_diagnostics(&infcx);
@@ -421,6 +409,8 @@ impl<'mir, 'tcx> Checker<'mir, 'tcx> {
             tcx.dcx()
                 .span_delayed_bug(call_span, "this should have reported a ~const error in HIR");
         }
+
+        have_const_conditions
     }
 }
 
@@ -627,11 +617,11 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                     _ => unreachable!(),
                 };
 
-                let ConstCx { tcx, body, param_env, .. } = *self.ccx;
+                let ConstCx { tcx, body, .. } = *self.ccx;
 
                 let fn_ty = func.ty(body, tcx);
 
-                let (mut callee, mut fn_args) = match *fn_ty.kind() {
+                let (callee, fn_args) = match *fn_ty.kind() {
                     ty::FnDef(def_id, fn_args) => (def_id, fn_args),
 
                     ty::FnPtr(..) => {
@@ -645,57 +635,39 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                     }
                 };
 
-                self.revalidate_conditional_constness(callee, fn_args, call_source, *fn_span);
+                let has_const_conditions =
+                    self.revalidate_conditional_constness(callee, fn_args, *fn_span);
 
-                let mut is_trait = false;
                 // Attempting to call a trait method?
                 if let Some(trait_did) = tcx.trait_of_item(callee) {
-                    trace!("attempting to call a trait method");
+                    // We can't determine the actual callee here, so we have to do different checks
+                    // than usual.
 
+                    trace!("attempting to call a trait method");
                     let trait_is_const = tcx.is_const_trait(trait_did);
-                    // trait method calls are only permitted when `effects` is enabled.
-                    // typeck ensures the conditions for calling a const trait method are met,
-                    // so we only error if the trait isn't const. We try to resolve the trait
-                    // into the concrete method, and uses that for const stability checks.
-                    // FIXME(const_trait_impl) we might consider moving const stability checks
-                    // to typeck as well.
-                    if tcx.features().const_trait_impl() && trait_is_const {
-                        // This skips the check below that ensures we only call `const fn`.
-                        is_trait = true;
-
-                        if let Ok(Some(instance)) =
-                            Instance::try_resolve(tcx, param_env, callee, fn_args)
-                            && let InstanceKind::Item(def) = instance.def
-                        {
-                            // Resolve a trait method call to its concrete implementation, which may be in a
-                            // `const` trait impl. This is only used for the const stability check below, since
-                            // we want to look at the concrete impl's stability.
-                            fn_args = instance.args;
-                            callee = def;
-                        }
+
+                    if trait_is_const {
+                        // All trait calls are guarded by this, no matter whether they have
+                        // const-conditions or not.
+                        self.check_op(ops::ConditionallyConstOrTraitCall { callee, args: fn_args });
+                        // FIXME(const_trait_impl): do a more fine-grained check whether this
+                        // particular trait can be const-stably called.
                     } else {
-                        // if the trait is const but the user has not enabled the feature(s),
-                        // suggest them.
-                        let feature = if trait_is_const {
-                            Some(if tcx.features().const_trait_impl() {
-                                sym::effects
-                            } else {
-                                sym::const_trait_impl
-                            })
-                        } else {
-                            None
-                        };
+                        // Not even a const trait.
                         self.check_op(ops::FnCallNonConst {
                             callee,
                             args: fn_args,
                             span: *fn_span,
                             call_source,
-                            feature,
                         });
-                        // If we allowed this, we're in miri-unleashed mode, so we might
-                        // as well skip the remaining checks.
-                        return;
                     }
+                    // That's all we can check here.
+                    return;
+                }
+
+                // Even if we know the callee, ensure we can use conditionally-const calls.
+                if has_const_conditions {
+                    self.check_op(ops::ConditionallyConstOrTraitCall { callee, args: fn_args });
                 }
 
                 // At this point, we are calling a function, `callee`, whose `DefId` is known...
@@ -783,14 +755,12 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                     return;
                 }
 
-                // Trait functions are not `const fn` so we have to skip them here.
-                if !tcx.is_const_fn(callee) && !is_trait {
+                if !tcx.is_const_fn(callee) {
                     self.check_op(ops::FnCallNonConst {
                         callee,
                         args: fn_args,
                         span: *fn_span,
                         call_source,
-                        feature: None,
                     });
                     // If we allowed this, we're in miri-unleashed mode, so we might
                     // as well skip the remaining checks.

compiler/rustc_const_eval/src/check_consts/ops.rs

@@ -70,22 +70,48 @@ impl<'tcx> NonConstOp<'tcx> for FnCallIndirect {
     }
 }
 
+/// A call to a function that is in a trait, or has trait bounds that make it conditionally-const.
+#[derive(Debug)]
+pub(crate) struct ConditionallyConstOrTraitCall<'tcx> {
+    pub callee: DefId,
+    pub args: GenericArgsRef<'tcx>,
+}
+
+impl<'tcx> NonConstOp<'tcx> for ConditionallyConstOrTraitCall<'tcx> {
+    fn status_in_item(&self, _ccx: &ConstCx<'_, 'tcx>) -> Status {
+        // We use the `const_trait_impl` for all conditionally-const calls.
+        Status::Unstable {
+            gate: sym::const_trait_impl,
+            safe_to_expose_on_stable: false,
+            // We don't want the "mark the callee as `#[rustc_const_stable_indirect]`" hint
+            is_function_call: false,
+        }
+    }
+
+    fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> Diag<'tcx> {
+        ccx.dcx().create_err(errors::ConditionallyConstOrTraitCall {
+            span,
+            def_path_str: ccx.tcx.def_path_str_with_args(self.callee, self.args),
+            kind: ccx.const_kind(),
+        })
+    }
+}
+
 /// A function call where the callee is not marked as `const`.
 #[derive(Debug, Clone, Copy)]
 pub(crate) struct FnCallNonConst<'tcx> {
     pub callee: DefId,
     pub args: GenericArgsRef<'tcx>,
     pub span: Span,
     pub call_source: CallSource,
-    pub feature: Option<Symbol>,
 }
 
 impl<'tcx> NonConstOp<'tcx> for FnCallNonConst<'tcx> {
     // FIXME: make this translatable
     #[allow(rustc::diagnostic_outside_of_impl)]
     #[allow(rustc::untranslatable_diagnostic)]
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, _: Span) -> Diag<'tcx> {
-        let FnCallNonConst { callee, args, span, call_source, feature } = *self;
+        let FnCallNonConst { callee, args, span, call_source } = *self;
         let ConstCx { tcx, param_env, .. } = *ccx;
         let caller = ccx.def_id();
 
@@ -285,14 +311,6 @@ impl<'tcx> NonConstOp<'tcx> for FnCallNonConst<'tcx> {
             ccx.const_kind(),
         ));
 
-        if let Some(feature) = feature {
-            ccx.tcx.disabled_nightly_features(
-                &mut err,
-                Some(ccx.tcx.local_def_id_to_hir_id(caller)),
-                [(String::new(), feature)],
-            );
-        }
-
         if let ConstContext::Static(_) = ccx.const_kind() {
             err.note(fluent_generated::const_eval_lazy_lock);
         }

compiler/rustc_const_eval/src/errors.rs

@@ -176,6 +176,15 @@ pub(crate) struct NonConstFmtMacroCall {
     pub kind: ConstContext,
 }
 
+#[derive(Diagnostic)]
+#[diag(const_eval_conditionally_const_or_trait_call)]
+pub(crate) struct ConditionallyConstOrTraitCall {
+    #[primary_span]
+    pub span: Span,
+    pub def_path_str: String,
+    pub kind: ConstContext,
+}
+
 #[derive(Diagnostic)]
 #[diag(const_eval_non_const_fn_call, code = E0015)]
 pub(crate) struct NonConstFnCall {

tests/ui/traits/const-traits/cross-crate.rs

@@ -18,11 +18,9 @@ const fn const_context() {
     #[cfg(any(stocknc, gatednc))]
     NonConst.func();
     //[stocknc]~^ ERROR: cannot call
-    //[stocknc]~| ERROR: cannot call
-    //[gatednc]~^^^ ERROR: the trait bound
+    //[gatednc]~^^ ERROR: the trait bound
     Const.func();
     //[stock,stocknc]~^ ERROR: cannot call
-    //[stock,stocknc]~| ERROR: cannot call
 }
 
 fn main() {}

tests/ui/traits/const-traits/cross-crate.stock.stderr

@@ -1,28 +1,8 @@
-error[E0015]: cannot call non-const fn `<cross_crate::Const as cross_crate::MyTrait>::func` in constant functions
-  --> $DIR/cross-crate.rs:23:11
+error: cannot call conditionally-const or trait fn `<cross_crate::Const as cross_crate::MyTrait>::func` in constant functions
+  --> $DIR/cross-crate.rs:22:5
    |
 LL |     Const.func();
-   |           ^^^^^^
-   |
-   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
-help: add `#![feature(const_trait_impl)]` to the crate attributes to enable
-   |
-LL + #![feature(const_trait_impl)]
-   |
-
-error[E0015]: cannot call non-const fn `<cross_crate::Const as cross_crate::MyTrait>::func` in constant functions
-  --> $DIR/cross-crate.rs:23:11
-   |
-LL |     Const.func();
-   |           ^^^^^^
-   |
-   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
-   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
-help: add `#![feature(const_trait_impl)]` to the crate attributes to enable
-   |
-LL + #![feature(const_trait_impl)]
-   |
+   |     ^^^^^^^^^^^^
 
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0015`.

tests/ui/traits/const-traits/cross-crate.stocknc.stderr

@@ -1,53 +1,14 @@
-error[E0015]: cannot call non-const fn `<cross_crate::NonConst as cross_crate::MyTrait>::func` in constant functions
-  --> $DIR/cross-crate.rs:19:14
+error: cannot call conditionally-const or trait fn `<cross_crate::NonConst as cross_crate::MyTrait>::func` in constant functions
+  --> $DIR/cross-crate.rs:19:5
    |
 LL |     NonConst.func();
-   |              ^^^^^^
-   |
-   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
-help: add `#![feature(const_trait_impl)]` to the crate attributes to enable
-   |
-LL + #![feature(const_trait_impl)]
-   |
-
-error[E0015]: cannot call non-const fn `<cross_crate::NonConst as cross_crate::MyTrait>::func` in constant functions
-  --> $DIR/cross-crate.rs:19:14
-   |
-LL |     NonConst.func();
-   |              ^^^^^^
-   |
-   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
-   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
-help: add `#![feature(const_trait_impl)]` to the crate attributes to enable
-   |
-LL + #![feature(const_trait_impl)]
-   |
+   |     ^^^^^^^^^^^^^^^
 
-error[E0015]: cannot call non-const fn `<cross_crate::Const as cross_crate::MyTrait>::func` in constant functions
-  --> $DIR/cross-crate.rs:23:11
+error: cannot call conditionally-const or trait fn `<cross_crate::Const as cross_crate::MyTrait>::func` in constant functions
+  --> $DIR/cross-crate.rs:22:5
    |
 LL |     Const.func();
-   |           ^^^^^^
-   |
-   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
-help: add `#![feature(const_trait_impl)]` to the crate attributes to enable
-   |
-LL + #![feature(const_trait_impl)]
-   |
-
-error[E0015]: cannot call non-const fn `<cross_crate::Const as cross_crate::MyTrait>::func` in constant functions
-  --> $DIR/cross-crate.rs:23:11
-   |
-LL |     Const.func();
-   |           ^^^^^^
-   |
-   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
-   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
-help: add `#![feature(const_trait_impl)]` to the crate attributes to enable
-   |
-LL + #![feature(const_trait_impl)]
-   |
+   |     ^^^^^^^^^^^^
 
-error: aborting due to 4 previous errors
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0015`.

tests/ui/traits/const-traits/staged-api-user-crate.rs

@@ -10,8 +10,7 @@ fn non_const_context() {
 
 const fn stable_const_context() {
     Unstable::func();
-    //~^ ERROR cannot call non-const fn `<staged_api::Unstable as staged_api::MyTrait>::func` in constant functions
-    //~| ERROR cannot call non-const fn `<staged_api::Unstable as staged_api::MyTrait>::func` in constant functions
+    //~^ ERROR cannot call conditionally-const or trait fn `<staged_api::Unstable as staged_api::MyTrait>::func` in constant functions
 }
 
 fn main() {}