Ensure constants are WF before calling into CTFE

Author: BoxyUwU

compiler/rustc_middle/src/query/mod.rs

@@ -2374,6 +2374,18 @@ rustc_queries! {
         }
     }
 
+    query check_constant_safe_to_evaluate(
+        key: ty::CanonicalQueryInput<
+            TyCtxt<'tcx>,
+            ty::ParamEnvAnd<'tcx, ty::UnevaluatedConst<'tcx>>,
+        >
+    ) -> bool {
+        desc { |tcx|
+            "checking constant `{}` is safe to evaluate",
+            tcx.def_path_str(key.canonical.value.into_parts().1.def)
+        }
+    }
+
     query method_autoderef_steps(
         goal: CanonicalTyGoal<'tcx>
     ) -> MethodAutoderefStepsResult<'tcx> {

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -28,6 +28,7 @@ use std::ops::ControlFlow;
 
 use rustc_errors::ErrorGuaranteed;
 use rustc_hir::def::DefKind;
+use rustc_infer::infer::canonical::OriginalQueryValues;
 pub use rustc_infer::traits::*;
 use rustc_middle::query::Providers;
 use rustc_middle::span_bug;
@@ -545,75 +546,130 @@ pub fn try_evaluate_const<'tcx>(
         | ty::ConstKind::Expr(_) => Err(EvaluateConstErr::HasGenericsOrInfers),
         ty::ConstKind::Unevaluated(uv) => {
             // Postpone evaluation of constants that depend on generic parameters or
-            // inference variables.
+            // inference variables. Also ensure that constants are wf before passing
+            // them onto CTFE.
             //
-            // We use `TypingMode::PostAnalysis`  here which is not *technically* correct
+            // We use `TypingMode::PostAnalysis` here which is not *technically* correct
             // to be revealing opaque types here as borrowcheck has not run yet. However,
             // CTFE itself uses `TypingMode::PostAnalysis` unconditionally even during
             // typeck and not doing so has a lot of (undesirable) fallout (#101478, #119821).
             // As a result we always use a revealed env when resolving the instance to evaluate.
             //
             // FIXME: `const_eval_resolve_for_typeck` should probably just modify the env itself
             // instead of having this logic here
-            let (args, typing_env) = if tcx.features().generic_const_exprs()
-                && uv.has_non_region_infer()
-            {
-                // `feature(generic_const_exprs)` causes anon consts to inherit all parent generics. This can cause
-                // inference variables and generic parameters to show up in `ty::Const` even though the anon const
-                // does not actually make use of them. We handle this case specially and attempt to evaluate anyway.
-                match tcx.thir_abstract_const(uv.def) {
-                    Ok(Some(ct)) => {
-                        let ct = tcx.expand_abstract_consts(ct.instantiate(tcx, uv.args));
-                        if let Err(e) = ct.error_reported() {
-                            return Err(EvaluateConstErr::EvaluationFailure(e));
-                        } else if ct.has_non_region_infer() || ct.has_non_region_param() {
-                            // If the anon const *does* actually use generic parameters or inference variables from
-                            // the generic arguments provided for it, then we should *not* attempt to evaluate it.
-                            return Err(EvaluateConstErr::HasGenericsOrInfers);
-                        } else {
-                            let args = replace_param_and_infer_args_with_placeholder(tcx, uv.args);
-                            let typing_env = infcx
-                                .typing_env(tcx.erase_regions(param_env))
-                                .with_post_analysis_normalized(tcx);
+            let (args, typing_env) = if tcx.features().generic_const_exprs() {
+                // We handle `generic_const_exprs` separately as reasonable ways of handling constants in the type system
+                // completely fall apart under `generic_const_exprs` and makes this whole function Really hard to reason
+                // about if you have to consider gce whatsoever.
+                //
+                // We don't bother trying to ensure GCE constants are WF before passing them to CTFE as it would cause
+                // query cycles on almost every single call to this function.
+
+                if uv.has_non_region_infer() || uv.has_non_region_param() {
+                    // `feature(generic_const_exprs)` causes anon consts to inherit all parent generics. This can cause
+                    // inference variables and generic parameters to show up in `ty::Const` even though the anon const
+                    // does not actually make use of them. We handle this case specially and attempt to evaluate anyway.
+                    match tcx.thir_abstract_const(uv.def) {
+                        Ok(Some(ct)) => {
+                            let ct = tcx.expand_abstract_consts(ct.instantiate(tcx, uv.args));
+                            if let Err(e) = ct.error_reported() {
+                                return Err(EvaluateConstErr::EvaluationFailure(e));
+                            } else if ct.has_non_region_infer() || ct.has_non_region_param() {
+                                // If the anon const *does* actually use generic parameters or inference variables from
+                                // the generic arguments provided for it, then we should *not* attempt to evaluate it.
+                                return Err(EvaluateConstErr::HasGenericsOrInfers);
+                            } else {
+                                let args =
+                                    replace_param_and_infer_args_with_placeholder(tcx, uv.args);
+                                let typing_env = infcx
+                                    .typing_env(tcx.erase_regions(param_env))
+                                    .with_post_analysis_normalized(tcx);
+                                (args, typing_env)
+                            }
+                        }
+                        Err(_) | Ok(None) => {
+                            let args = GenericArgs::identity_for_item(tcx, uv.def);
+                            let typing_env = ty::TypingEnv::post_analysis(tcx, uv.def);
                             (args, typing_env)
                         }
                     }
-                    Err(_) | Ok(None) => {
-                        let args = GenericArgs::identity_for_item(tcx, uv.def);
-                        let typing_env = ty::TypingEnv::post_analysis(tcx, uv.def);
-                        (args, typing_env)
-                    }
+                } else {
+                    let typing_env = infcx
+                        .typing_env(tcx.erase_regions(param_env))
+                        .with_post_analysis_normalized(tcx);
+                    (uv.args, typing_env)
                 }
-            } else if tcx.def_kind(uv.def) == DefKind::AnonConst && uv.has_non_region_infer() {
+            } else if !tcx.features().min_generic_const_args()
+                && !tcx.features().associated_const_equality()
+                // We check for anon consts so that when `associated_const_equality` bounds are
+                // lowered on stable we still handle them correctly to avoid ICEs in CTFE.
+                && tcx.def_kind(uv.def) == DefKind::AnonConst
+            {
                 // FIXME: remove this when `const_evaluatable_unchecked` is a hard error.
                 //
-                // Diagnostics will sometimes replace the identity args of anon consts in
-                // array repeat expr counts with inference variables so we have to handle this
-                // even though it is not something we should ever actually encounter.
-                //
-                // Array repeat expr counts are allowed to syntactically use generic parameters
-                // but must not actually depend on them in order to evalaute successfully. This means
-                // that it is actually fine to evalaute them in their own environment rather than with
-                // the actually provided generic arguments.
-                tcx.dcx().delayed_bug(
-                    "Encountered anon const with inference variable args but no error reported",
-                );
+                // Under `min_const_generics` the only place we can encounter generics in type system
+                // consts is for the `const_evaluatable_unchecked` future compat lint. We don't care
+                // to handle them in important ways (e.g. deferring evaluation) so we handle it separately.
+
+                if uv.has_non_region_infer() {
+                    // Diagnostics will sometimes replace the identity args of anon consts in
+                    // array repeat expr counts with inference variables so we have to handle this
+                    // even though it is not something we should ever actually encounter.
+                    //
+                    // Array repeat expr counts are allowed to syntactically use generic parameters
+                    // but must not actually depend on them in order to evalaute successfully. This means
+                    // that it is actually fine to evalaute them in their own environment rather than with
+                    // the actually provided generic arguments.
+                    tcx.dcx().delayed_bug(
+                        "Encountered anon const with inference variable args but no error reported",
+                    );
+                }
 
+                // Generic arguments to anon consts in the type system are never meaningful under mcg,
+                // there either are no arguments or its a repeat count and the arguments must not be
+                // depended on for evaluation.
                 let args = GenericArgs::identity_for_item(tcx, uv.def);
                 let typing_env = ty::TypingEnv::post_analysis(tcx, uv.def);
                 (args, typing_env)
             } else {
-                // FIXME: This codepath is reachable under `associated_const_equality` and in the
-                // future will be reachable by `min_generic_const_args`. We should handle inference
-                // variables and generic parameters properly instead of doing nothing.
+                // We are only dealing with "truly" generic/uninferred constants here:
+                // - `generic_const_exprs` has been handled separately in the first if
+                // - `min_const_generics` repeat expr count hacks have been handled in the previous if
+                //
+                // So we are free to simply defer evaluation here. This does assume that `uv.args` has
+                // already been normalized.
+                if uv.args.has_non_region_param() || uv.args.has_non_region_infer() {
+                    return Err(EvaluateConstErr::HasGenericsOrInfers);
+                }
+
+                // If we are dealing with a fully monomorphic constant then we should ensure that
+                // it is well formed as otherwise CTFE will ICE. For the same reasons as with
+                // deferring evaluation of generic/uninferred constants, we do not have to worry
+                // about `generic_const_expr`
+                if tcx.def_kind(uv.def) == DefKind::AnonConst
+                    && tcx.predicates_of(uv.def).predicates.is_empty()
+                {
+                    // ... skip doing any work
+                } else {
+                    let input = infcx
+                        .canonicalize_query(param_env.and(uv), &mut OriginalQueryValues::default());
+                    if !tcx.check_constant_safe_to_evaluate(input) {
+                        let e = tcx.dcx().delayed_bug(
+                            "Attempted to evaluate illformed constant but no error emitted",
+                        );
+                        return Err(EvaluateConstErr::EvaluationFailure(e));
+                    }
+                }
+
                 let typing_env = infcx
                     .typing_env(tcx.erase_regions(param_env))
                     .with_post_analysis_normalized(tcx);
                 (uv.args, typing_env)
             };
-            let uv = ty::UnevaluatedConst::new(uv.def, args);
 
+            let uv = ty::UnevaluatedConst::new(uv.def, args);
             let erased_uv = tcx.erase_regions(uv);
+
             use rustc_middle::mir::interpret::ErrorHandled;
             match tcx.const_eval_resolve_for_typeck(typing_env, erased_uv, DUMMY_SP) {
                 Ok(Ok(val)) => Ok(ty::Const::new_value(
@@ -697,7 +753,51 @@ fn replace_param_and_infer_args_with_placeholder<'tcx>(
     args.fold_with(&mut ReplaceParamAndInferWithPlaceholder { tcx, idx: 0 })
 }
 
-/// Normalizes the predicates and checks whether they hold in an empty environment. If this
+#[instrument(level = "debug", skip(tcx), ret)]
+fn check_constant_safe_to_evaluate<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    input: ty::CanonicalQueryInput<TyCtxt<'tcx>, ty::ParamEnvAnd<'tcx, ty::UnevaluatedConst<'tcx>>>,
+) -> bool {
+    let (infcx, param_env_and, _) = tcx.infer_ctxt().build_with_canonical(DUMMY_SP, &input);
+    let (param_env, uv) = param_env_and.into_parts();
+
+    // We can't just register a wf goal for the constant as that would require evaluating the constant
+    // which would result in a query cycle.
+    let mut predicates = tcx.predicates_of(uv.def).instantiate(tcx, uv.args).predicates;
+
+    // Specifically check trait fulfillment to avoid an error when trying to resolve
+    // associated items.
+    if let Some(trait_def_id) = tcx.trait_of_item(uv.def) {
+        let trait_ref = ty::TraitRef::from_method(tcx, trait_def_id, uv.args);
+        predicates.push(trait_ref.upcast(tcx));
+    }
+
+    let ocx = ObligationCtxt::new(&infcx);
+    let predicates = ocx.normalize(&ObligationCause::dummy(), param_env, predicates);
+    for predicate in predicates {
+        let obligation = Obligation::new(tcx, ObligationCause::dummy(), param_env, predicate);
+        ocx.register_obligation(obligation);
+    }
+    let errors = ocx.select_all_or_error();
+
+    if !errors.is_empty() {
+        return false;
+    }
+
+    // Leak check for any higher-ranked trait mismatches.
+    // We only need to do this in the old solver, since the new solver already
+    // leak-checks.
+    if !infcx.next_trait_solver() && infcx.leak_check(ty::UniverseIndex::ROOT, None).is_err() {
+        return false;
+    }
+
+    // We don't check non-higher-ranked region constraints, but we don't need to as region
+    // constraints cant affect coherence and therefore result in overlapping impls in codegen/ctfe
+    // so it shouldn't matter to speculatively evaluate constants with failing region constraints.
+    true
+}
+
+/// Normalizes the predicates and checks whether they hold in a given empty. If this
 /// returns true, then either normalize encountered an error or one of the predicates did not
 /// hold. Used when creating vtables to check for unsatisfiable methods. This should not be
 /// used during analysis.
@@ -840,6 +940,7 @@ pub fn provide(providers: &mut Providers) {
         specialization_enabled_in: specialize::specialization_enabled_in,
         instantiate_and_check_impossible_predicates,
         is_impossible_associated_item,
+        check_constant_safe_to_evaluate,
         ..*providers
     };
 }

tests/crashes/127643.rs

@@ -0,0 +1,20 @@
+#![feature(associated_const_equality)]
+
+trait Owner<K> {
+    const C: K;
+}
+impl<K: ConstDefault> Owner<K> for () {
+    const C: K = K::DEFAULT;
+}
+
+trait ConstDefault {
+    const DEFAULT: Self;
+}
+
+fn user() -> impl Owner<dyn Sized, C = 0> {}
+//~^ ERROR: the trait bound `(dyn Sized + 'static): ConstDefault` is not satisfied
+//~| ERROR: the size for values of type `(dyn Sized + 'static)` cannot be known at compilation time
+//~| ERROR: the trait `Sized` is not dyn compatible
+//~| ERROR: mismatched types
+
+fn main() {}