Pass constness to SelectionContext

compiler/rustc_infer/src/traits/engine.rs

@@ -1,5 +1,6 @@
 use crate::infer::InferCtxt;
 use crate::traits::Obligation;
+use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_middle::ty::{self, ToPredicate, Ty, WithConstness};
 
@@ -49,11 +50,28 @@ pub trait TraitEngine<'tcx>: 'tcx {
         infcx: &InferCtxt<'_, 'tcx>,
     ) -> Result<(), Vec<FulfillmentError<'tcx>>>;
 
+    fn select_all_with_constness_or_error(
+        &mut self,
+        infcx: &InferCtxt<'_, 'tcx>,
+        _constness: hir::Constness,
+    ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
+        self.select_all_or_error(infcx)
+    }
+
     fn select_where_possible(
         &mut self,
         infcx: &InferCtxt<'_, 'tcx>,
     ) -> Result<(), Vec<FulfillmentError<'tcx>>>;
 
+    // FIXME 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,
+    ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
+        self.select_where_possible(infcx)
+    }
+
     fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>>;
 }
 

compiler/rustc_trait_selection/src/traits/fulfill.rs

@@ -3,6 +3,7 @@ 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::abstract_const::NotConstEvaluatable;
 use rustc_middle::mir::interpret::ErrorHandled;
@@ -228,6 +229,22 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
         if errors.is_empty() { Ok(()) } else { Err(errors) }
     }
 
+    fn select_all_with_constness_or_error(
+        &mut self,
+        infcx: &InferCtxt<'_, 'tcx>,
+        constness: rustc_hir::Constness,
+    ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
+        self.select_with_constness_where_possible(infcx, constness)?;
+
+        let errors: Vec<_> = self
+            .predicates
+            .to_errors(CodeAmbiguity)
+            .into_iter()
+            .map(to_fulfillment_error)
+            .collect();
+        if errors.is_empty() { Ok(()) } else { Err(errors) }
+    }
+
     fn select_where_possible(
         &mut self,
         infcx: &InferCtxt<'_, 'tcx>,
@@ -236,6 +253,15 @@ 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,
+    ) -> Result<(), 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())
     }

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

@@ -130,6 +130,9 @@ pub struct SelectionContext<'cx, 'tcx> {
     /// and a negative impl
     allow_negative_impls: bool,
 
+    /// Do we only want const impls when we have a const trait predicate?
+    const_impls_required: 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.
@@ -221,6 +224,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             intercrate: false,
             intercrate_ambiguity_causes: None,
             allow_negative_impls: false,
+            const_impls_required: false,
             query_mode: TraitQueryMode::Standard,
         }
     }
@@ -232,6 +236,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             intercrate: true,
             intercrate_ambiguity_causes: None,
             allow_negative_impls: false,
+            const_impls_required: false,
             query_mode: TraitQueryMode::Standard,
         }
     }
@@ -247,6 +252,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             intercrate: false,
             intercrate_ambiguity_causes: None,
             allow_negative_impls,
+            const_impls_required: false,
             query_mode: TraitQueryMode::Standard,
         }
     }
@@ -262,10 +268,26 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             intercrate: false,
             intercrate_ambiguity_causes: None,
             allow_negative_impls: false,
+            const_impls_required: 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,
+            const_impls_required: 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
@@ -1024,6 +1046,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
         let tcx = self.tcx();
         // Respect const trait obligations
+        if self.const_impls_required {
             if let hir::Constness::Const = obligation.predicate.skip_binder().constness {
                 if Some(obligation.predicate.skip_binder().trait_ref.def_id)
                     != tcx.lang_items().sized_trait()
@@ -1035,7 +1058,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                             if tcx.impl_constness(def_id) == hir::Constness::Const => {}
                         // const param
                         ParamCandidate(ty::ConstnessAnd {
-                        constness: hir::Constness::Const, ..
+                            constness: hir::Constness::Const,
+                            ..
                         }) => {}
                         // auto trait impl
                         AutoImplCandidate(..) => {}
@@ -1048,6 +1072,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     }
                 }
             }
+        }
         // Treat negative impls as unimplemented, and reservation impls as ambiguity.
         if let ImplCandidate(def_id) = candidate {
             match tcx.impl_polarity(def_id) {

compiler/rustc_typeck/src/check/compare_method.rs

@@ -1354,7 +1354,9 @@ pub fn check_type_bounds<'tcx>(
 
         // Check that all obligations are satisfied by the implementation's
         // version.
-        if let Err(ref errors) = inh.fulfillment_cx.borrow_mut().select_all_or_error(&infcx) {
+        if let Err(ref errors) =
+            inh.fulfillment_cx.borrow_mut().select_all_with_constness_or_error(&infcx, constness)
+        {
             infcx.report_fulfillment_errors(errors, None, false);
             return Err(ErrorReported);
         }

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

@@ -714,7 +714,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     pub(in super::super) fn select_all_obligations_or_error(&self) {
         debug!("select_all_obligations_or_error");
-        if let Err(errors) = self.fulfillment_cx.borrow_mut().select_all_or_error(&self) {
+        if let Err(errors) = self
+            .fulfillment_cx
+            .borrow_mut()
+            .select_all_with_constness_or_error(&self, self.inh.constness)
+        {
             self.report_fulfillment_errors(&errors, self.inh.body_id, false);
         }
     }
@@ -725,7 +729,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         fallback_has_occurred: bool,
         mutate_fulfillment_errors: impl Fn(&mut Vec<traits::FulfillmentError<'tcx>>),
     ) {
-        let result = self.fulfillment_cx.borrow_mut().select_where_possible(self);
+        let result = self
+            .fulfillment_cx
+            .borrow_mut()
+            .select_with_constness_where_possible(self, self.inh.constness);
         if let Err(mut errors) = result {
             mutate_fulfillment_errors(&mut errors);
             self.report_fulfillment_errors(&errors, self.inh.body_id, fallback_has_occurred);