Fix typeck probing code to use constness from FnCtxt

Author: fee1-dead

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

@@ -118,6 +118,7 @@ pub struct FnCtxt<'a, 'tcx> {
 
     /// True if the return type has an Opaque type
     pub(super) return_type_has_opaque: bool,
+    constness: ty::ConstnessArg,
 }
 
 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
@@ -145,6 +146,19 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             inh,
             return_type_pre_known: true,
             return_type_has_opaque: false,
+            constness: {
+                if body_id.is_owner() {
+                    let did = body_id.expect_owner();
+                    if inh.tcx.is_const_fn_raw(did.to_def_id()) || inh.tcx.is_const_default_method(did.to_def_id()) || inh.tcx.def_kind(did.to_def_id()) == hir::def::DefKind::Const {
+                        ty::ConstnessArg::Param
+                    } else {
+                        ty::ConstnessArg::Not
+                    }
+                } else {
+                    // TODO: check correctness
+                    ty::ConstnessArg::Not
+                }
+            },
         }
     }
 
@@ -163,6 +177,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     pub fn errors_reported_since_creation(&self) -> bool {
         self.tcx.sess.err_count() > self.err_count_on_creation
     }
+
+    pub fn constness(&self) -> ty::ConstnessArg {
+        self.constness
+    }
 }
 
 impl<'a, 'tcx> Deref for FnCtxt<'a, 'tcx> {

compiler/rustc_typeck/src/check/method/probe.rs

@@ -1549,7 +1549,9 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                     let predicate = ty::Binder::dummy(trait_ref).to_predicate(self.tcx);
                     parent_pred = Some(predicate);
                     let obligation = traits::Obligation::new(cause, self.param_env, predicate);
+                    trace!(?obligation);
                     if !self.predicate_may_hold(&obligation) {
+                        trace!("predicate does not hold");
                         result = ProbeResult::NoMatch;
                         if self.probe(|_| {
                             match self.select_trait_candidate(trait_ref) {
@@ -1839,7 +1841,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                 };
                 self.next_const_var(self.tcx.type_of(param.def_id), origin).into()
             }
-            GenericParamDefKind::Constness => ty::ConstnessArg::Param.into(),
+            GenericParamDefKind::Constness => self.fcx.constness().into(),
         })
     }