Use the constness from the param env instead of having a separate dimension for it

This breaks a ~const test that will be fixed in a follow up commit of this PR

Author: oli-obk

compiler/rustc_const_eval/src/transform/check_consts/check.rs

@@ -817,8 +817,7 @@ impl Visitor<'tcx> for Checker<'mir, 'tcx> {
                     );
 
                     let implsrc = tcx.infer_ctxt().enter(|infcx| {
-                        let mut selcx =
-                            SelectionContext::with_constness(&infcx, hir::Constness::Const);
+                        let mut selcx = SelectionContext::new(&infcx);
                         selcx.select(&obligation)
                     });
 

compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs

@@ -3,7 +3,6 @@
 //! See the `Qualif` trait for more info.
 
 use rustc_errors::ErrorReported;
-use rustc_hir as hir;
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::mir::*;
 use rustc_middle::ty::{self, subst::SubstsRef, AdtDef, Ty};
@@ -167,7 +166,7 @@ impl Qualif for NeedsNonConstDrop {
         );
 
         let implsrc = cx.tcx.infer_ctxt().enter(|infcx| {
-            let mut selcx = SelectionContext::with_constness(&infcx, hir::Constness::Const);
+            let mut selcx = SelectionContext::new(&infcx);
             selcx.select(&obligation)
         });
         !matches!(

compiler/rustc_infer/src/traits/engine.rs

@@ -1,7 +1,6 @@
 use crate::infer::InferCtxt;
 use crate::traits::Obligation;
 use rustc_data_structures::fx::FxHashMap;
-use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_middle::ty::{self, ToPredicate, Ty, WithConstness};
 
@@ -48,25 +47,10 @@ pub trait TraitEngine<'tcx>: 'tcx {
 
     fn select_all_or_error(&mut self, infcx: &InferCtxt<'_, 'tcx>) -> Vec<FulfillmentError<'tcx>>;
 
-    fn select_all_with_constness_or_error(
+    fn select_where_possible(
         &mut self,
         infcx: &InferCtxt<'_, 'tcx>,
-        _constness: hir::Constness,
-    ) -> Vec<FulfillmentError<'tcx>> {
-        self.select_all_or_error(infcx)
-    }
-
-    fn select_where_possible(&mut self, infcx: &InferCtxt<'_, 'tcx>)
-    -> Vec<FulfillmentError<'tcx>>;
-
-    // FIXME(fee1-dead) this should not provide a default body for chalk as chalk should be updated
-    fn select_with_constness_where_possible(
-        &mut self,
-        infcx: &InferCtxt<'_, 'tcx>,
-        _constness: hir::Constness,
-    ) -> Vec<FulfillmentError<'tcx>> {
-        self.select_where_possible(infcx)
-    }
+    ) -> Vec<FulfillmentError<'tcx>>;
 
     fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>>;
 

compiler/rustc_infer/src/traits/mod.rs

@@ -69,6 +69,16 @@ impl PredicateObligation<'tcx> {
     }
 }
 
+impl TraitObligation<'tcx> {
+    /// Returns `true` if the trait predicate is considered `const` in its ParamEnv.
+    pub fn is_const(&self) -> bool {
+        match (self.predicate.skip_binder().constness, self.param_env.constness()) {
+            (ty::BoundConstness::ConstIfConst, hir::Constness::Const) => true,
+            _ => false,
+        }
+    }
+}
+
 // `PredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger.
 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
 static_assert_size!(PredicateObligation<'_>, 32);

compiler/rustc_trait_selection/src/traits/fulfill.rs

@@ -4,7 +4,6 @@ use rustc_data_structures::obligation_forest::ProcessResult;
 use rustc_data_structures::obligation_forest::{Error, ForestObligation, Outcome};
 use rustc_data_structures::obligation_forest::{ObligationForest, ObligationProcessor};
 use rustc_errors::ErrorReported;
-use rustc_hir as hir;
 use rustc_infer::traits::{SelectionError, TraitEngine, TraitEngineExt as _, TraitObligation};
 use rustc_middle::mir::interpret::ErrorHandled;
 use rustc_middle::thir::abstract_const::NotConstEvaluatable;
@@ -231,21 +230,6 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
         self.predicates.to_errors(CodeAmbiguity).into_iter().map(to_fulfillment_error).collect()
     }
 
-    fn select_all_with_constness_or_error(
-        &mut self,
-        infcx: &InferCtxt<'_, 'tcx>,
-        constness: rustc_hir::Constness,
-    ) -> Vec<FulfillmentError<'tcx>> {
-        {
-            let errors = self.select_with_constness_where_possible(infcx, constness);
-            if !errors.is_empty() {
-                return errors;
-            }
-        }
-
-        self.predicates.to_errors(CodeAmbiguity).into_iter().map(to_fulfillment_error).collect()
-    }
-
     fn select_where_possible(
         &mut self,
         infcx: &InferCtxt<'_, 'tcx>,
@@ -254,15 +238,6 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
         self.select(&mut selcx)
     }
 
-    fn select_with_constness_where_possible(
-        &mut self,
-        infcx: &InferCtxt<'_, 'tcx>,
-        constness: hir::Constness,
-    ) -> Vec<FulfillmentError<'tcx>> {
-        let mut selcx = SelectionContext::with_constness(infcx, constness);
-        self.select(&mut selcx)
-    }
-
     fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>> {
         self.predicates.map_pending_obligations(|o| o.obligation.clone())
     }
@@ -679,12 +654,7 @@ impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
         if obligation.predicate.is_known_global() {
             // no type variables present, can use evaluation for better caching.
             // FIXME: consider caching errors too.
-            //
-            // If the predicate is considered const, then we cannot use this because
-            // it will cause false negatives in the ui tests.
-            if !self.selcx.is_predicate_const(obligation.predicate)
-                && infcx.predicate_must_hold_considering_regions(obligation)
-            {
+            if infcx.predicate_must_hold_considering_regions(obligation) {
                 debug!(
                     "selecting trait at depth {} evaluated to holds",
                     obligation.recursion_depth
@@ -738,12 +708,7 @@ impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
         if obligation.predicate.is_global(tcx) {
             // no type variables present, can use evaluation for better caching.
             // FIXME: consider caching errors too.
-            //
-            // If the predicate is considered const, then we cannot use this because
-            // it will cause false negatives in the ui tests.
-            if !self.selcx.is_predicate_const(obligation.predicate)
-                && self.selcx.infcx().predicate_must_hold_considering_regions(obligation)
-            {
+            if self.selcx.infcx().predicate_must_hold_considering_regions(obligation) {
                 return ProcessResult::Changed(vec![]);
             } else {
                 tracing::debug!("Does NOT hold: {:?}", obligation);

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -303,7 +303,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             } else if lang_items.drop_trait() == Some(def_id)
                 && obligation.predicate.skip_binder().constness == ty::BoundConstness::ConstIfConst
             {
-                if self.is_in_const_context {
+                if obligation.param_env.constness() == hir::Constness::Const {
                     self.assemble_const_drop_candidates(obligation, stack, &mut candidates)?;
                 } else {
                     debug!("passing ~const Drop bound; in non-const context");

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -128,9 +128,6 @@ pub struct SelectionContext<'cx, 'tcx> {
     /// and a negative impl
     allow_negative_impls: bool,
 
-    /// Are we in a const context that needs `~const` bounds to be const?
-    is_in_const_context: bool,
-
     /// The mode that trait queries run in, which informs our error handling
     /// policy. In essence, canonicalized queries need their errors propagated
     /// rather than immediately reported because we do not have accurate spans.
@@ -222,7 +219,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             intercrate: false,
             intercrate_ambiguity_causes: None,
             allow_negative_impls: false,
-            is_in_const_context: false,
             query_mode: TraitQueryMode::Standard,
         }
     }
@@ -234,7 +230,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             intercrate: true,
             intercrate_ambiguity_causes: None,
             allow_negative_impls: false,
-            is_in_const_context: false,
             query_mode: TraitQueryMode::Standard,
         }
     }
@@ -250,7 +245,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             intercrate: false,
             intercrate_ambiguity_causes: None,
             allow_negative_impls,
-            is_in_const_context: false,
             query_mode: TraitQueryMode::Standard,
         }
     }
@@ -266,26 +260,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             intercrate: false,
             intercrate_ambiguity_causes: None,
             allow_negative_impls: false,
-            is_in_const_context: false,
             query_mode,
         }
     }
 
-    pub fn with_constness(
-        infcx: &'cx InferCtxt<'cx, 'tcx>,
-        constness: hir::Constness,
-    ) -> SelectionContext<'cx, 'tcx> {
-        SelectionContext {
-            infcx,
-            freshener: infcx.freshener_keep_static(),
-            intercrate: false,
-            intercrate_ambiguity_causes: None,
-            allow_negative_impls: false,
-            is_in_const_context: matches!(constness, hir::Constness::Const),
-            query_mode: TraitQueryMode::Standard,
-        }
-    }
-
     /// Enables tracking of intercrate ambiguity causes. These are
     /// used in coherence to give improved diagnostics. We don't do
     /// this until we detect a coherence error because it can lead to
@@ -318,20 +296,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         self.intercrate
     }
 
-    /// Returns `true` if the trait predicate is considerd `const` to this selection context.
-    pub fn is_trait_predicate_const(&self, pred: ty::TraitPredicate<'_>) -> bool {
-        matches!(pred.constness, ty::BoundConstness::ConstIfConst) && self.is_in_const_context
-    }
-
-    /// Returns `true` if the predicate is considered `const` to
-    /// this selection context.
-    pub fn is_predicate_const(&self, pred: ty::Predicate<'_>) -> bool {
-        match pred.kind().skip_binder() {
-            ty::PredicateKind::Trait(pred) => self.is_trait_predicate_const(pred),
-            _ => false,
-        }
-    }
-
     ///////////////////////////////////////////////////////////////////////////
     // Selection
     //
@@ -1138,7 +1102,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         for candidate in candidates {
             // Respect const trait obligations
-            if self.is_trait_predicate_const(obligation.predicate.skip_binder()) {
+            if obligation.is_const() {
                 match candidate {
                     // const impl
                     ImplCandidate(def_id)

compiler/rustc_typeck/src/check/compare_method.rs

@@ -1388,13 +1388,7 @@ pub fn check_type_bounds<'tcx>(
         impl_ty_substs.rebase_onto(tcx, impl_ty.container.id(), impl_trait_ref.substs);
 
     tcx.infer_ctxt().enter(move |infcx| {
-        let constness = impl_ty
-            .container
-            .impl_def_id()
-            .map(|did| tcx.impl_constness(did))
-            .unwrap_or(hir::Constness::NotConst);
-
-        let inh = Inherited::with_constness(infcx, impl_ty.def_id.expect_local(), constness);
+        let inh = Inherited::new(infcx, impl_ty.def_id.expect_local());
         let infcx = &inh.infcx;
         let mut selcx = traits::SelectionContext::new(&infcx);
 
@@ -1439,7 +1433,7 @@ pub fn check_type_bounds<'tcx>(
         // Check that all obligations are satisfied by the implementation's
         // version.
         let errors =
-            inh.fulfillment_cx.borrow_mut().select_all_with_constness_or_error(&infcx, constness);
+            inh.fulfillment_cx.borrow_mut().select_all_or_error(&infcx);
         if !errors.is_empty() {
             infcx.report_fulfillment_errors(&errors, None, false);
             return Err(ErrorReported);

compiler/rustc_typeck/src/check/fn_ctxt/_impl.rs

@@ -613,7 +613,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let errors = self
             .fulfillment_cx
             .borrow_mut()
-            .select_all_with_constness_or_error(&self, self.inh.constness);
+            .select_all_or_error(&self);
 
         if !errors.is_empty() {
             self.report_fulfillment_errors(&errors, self.inh.body_id, false);
@@ -629,7 +629,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let mut result = self
             .fulfillment_cx
             .borrow_mut()
-            .select_with_constness_where_possible(self, self.inh.constness);
+            .select_where_possible(self);
         if !result.is_empty() {
             mutate_fulfillment_errors(&mut result);
             self.report_fulfillment_errors(&result, self.inh.body_id, fallback_has_occurred);

compiler/rustc_typeck/src/check/inherited.rs

@@ -53,9 +53,6 @@ pub struct Inherited<'a, 'tcx> {
     pub(super) deferred_generator_interiors:
         RefCell<Vec<(hir::BodyId, Ty<'tcx>, hir::GeneratorKind)>>,
 
-    /// Reports whether this is in a const context.
-    pub(super) constness: hir::Constness,
-
     pub(super) body_id: Option<hir::BodyId>,
 
     /// Whenever we introduce an adjustment from `!` into a type variable,
@@ -102,16 +99,6 @@ impl<'tcx> InheritedBuilder<'tcx> {
 
 impl Inherited<'a, 'tcx> {
     pub(super) fn new(infcx: InferCtxt<'a, 'tcx>, def_id: LocalDefId) -> Self {
-        let tcx = infcx.tcx;
-        let item_id = tcx.hir().local_def_id_to_hir_id(def_id);
-        Self::with_constness(infcx, def_id, tcx.hir().get(item_id).constness_for_typeck())
-    }
-
-    pub(super) fn with_constness(
-        infcx: InferCtxt<'a, 'tcx>,
-        def_id: LocalDefId,
-        constness: hir::Constness,
-    ) -> Self {
         let tcx = infcx.tcx;
         let item_id = tcx.hir().local_def_id_to_hir_id(def_id);
         let body_id = tcx.hir().maybe_body_owned_by(item_id);
@@ -128,7 +115,6 @@ impl Inherited<'a, 'tcx> {
             deferred_cast_checks: RefCell::new(Vec::new()),
             deferred_generator_interiors: RefCell::new(Vec::new()),
             diverging_type_vars: RefCell::new(Default::default()),
-            constness,
             body_id,
         }
     }