Filter and test predicates using normalize_and_test_predicates for const-prop

src/librustc/mir/mono.rs

@@ -1,7 +1,6 @@
 use crate::dep_graph::{DepConstructor, DepNode, WorkProduct, WorkProductId};
 use crate::ich::{Fingerprint, NodeIdHashingMode, StableHashingContext};
 use crate::session::config::OptLevel;
-use crate::traits::TraitQueryMode;
 use crate::ty::print::obsolete::DefPathBasedNames;
 use crate::ty::{subst::InternalSubsts, Instance, InstanceDef, SymbolName, TyCtxt};
 use rustc_data_structures::base_n;
@@ -168,9 +167,7 @@ impl<'tcx> MonoItem<'tcx> {
             MonoItem::GlobalAsm(..) => return true,
         };
 
-        // We shouldn't encounter any overflow here, so we use TraitQueryMode::Standard\
-        // to report an error if overflow somehow occurs.
-        tcx.substitute_normalize_and_test_predicates((def_id, &substs, TraitQueryMode::Standard))
+        tcx.substitute_normalize_and_test_predicates((def_id, &substs))
     }
 
     pub fn to_string(&self, tcx: TyCtxt<'tcx>, debug: bool) -> String {

src/librustc/query/mod.rs

@@ -1148,11 +1148,11 @@ rustc_queries! {
             desc { "normalizing `{:?}`", goal }
         }
 
-        query substitute_normalize_and_test_predicates(key: (DefId, SubstsRef<'tcx>, traits::TraitQueryMode)) -> bool {
+        query substitute_normalize_and_test_predicates(key: (DefId, SubstsRef<'tcx>)) -> bool {
             no_force
             desc { |tcx|
-                "testing substituted normalized predicates in mode {:?}:`{}`",
-                key.2, tcx.def_path_str(key.0)
+                "testing substituted normalized predicates:`{}`",
+                tcx.def_path_str(key.0)
             }
         }
 

src/librustc/traits/fulfill.rs

@@ -16,7 +16,6 @@ use super::CodeSelectionError;
 use super::{ConstEvalFailure, Unimplemented};
 use super::{FulfillmentError, FulfillmentErrorCode};
 use super::{ObligationCause, PredicateObligation};
-use crate::traits::TraitQueryMode;
 
 impl<'tcx> ForestObligation for PendingPredicateObligation<'tcx> {
     type Predicate = ty::Predicate<'tcx>;
@@ -63,9 +62,6 @@ pub struct FulfillmentContext<'tcx> {
     // a snapshot (they don't *straddle* a snapshot, so there
     // is no trouble there).
     usable_in_snapshot: bool,
-
-    // The `TraitQueryMode` used when constructing a `SelectionContext`
-    query_mode: TraitQueryMode,
 }
 
 #[derive(Clone, Debug)]
@@ -79,26 +75,12 @@ pub struct PendingPredicateObligation<'tcx> {
 static_assert_size!(PendingPredicateObligation<'_>, 136);
 
 impl<'a, 'tcx> FulfillmentContext<'tcx> {
-    /// Creates a new fulfillment context with `TraitQueryMode::Standard`
-    /// You almost always want to use this instead of `with_query_mode`
+    /// Creates a new fulfillment context.
     pub fn new() -> FulfillmentContext<'tcx> {
         FulfillmentContext {
             predicates: ObligationForest::new(),
             register_region_obligations: true,
             usable_in_snapshot: false,
-            query_mode: TraitQueryMode::Standard,
-        }
-    }
-
-    /// Creates a new fulfillment context with the specified query mode.
-    /// This should only be used when you want to ignore overflow,
-    /// rather than reporting it as an error.
-    pub fn with_query_mode(query_mode: TraitQueryMode) -> FulfillmentContext<'tcx> {
-        FulfillmentContext {
-            predicates: ObligationForest::new(),
-            register_region_obligations: true,
-            usable_in_snapshot: false,
-            query_mode,
         }
     }
 
@@ -107,7 +89,6 @@ impl<'a, 'tcx> FulfillmentContext<'tcx> {
             predicates: ObligationForest::new(),
             register_region_obligations: true,
             usable_in_snapshot: true,
-            query_mode: TraitQueryMode::Standard,
         }
     }
 
@@ -116,7 +97,6 @@ impl<'a, 'tcx> FulfillmentContext<'tcx> {
             predicates: ObligationForest::new(),
             register_region_obligations: false,
             usable_in_snapshot: false,
-            query_mode: TraitQueryMode::Standard,
         }
     }
 
@@ -237,7 +217,7 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
         &mut self,
         infcx: &InferCtxt<'_, 'tcx>,
     ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
-        let mut selcx = SelectionContext::with_query_mode(infcx, self.query_mode);
+        let mut selcx = SelectionContext::new(infcx);
         self.select(&mut selcx)
     }
 

src/librustc/traits/mod.rs

@@ -95,7 +95,7 @@ pub enum IntercrateMode {
 }
 
 /// The mode that trait queries run in.
-#[derive(Copy, Clone, PartialEq, Eq, Debug, Hash, HashStable)]
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
 pub enum TraitQueryMode {
     // Standard/un-canonicalized queries get accurate
     // spans etc. passed in and hence can do reasonable
@@ -1014,17 +1014,16 @@ where
 /// environment. If this returns false, then either normalize
 /// encountered an error or one of the predicates did not hold. Used
 /// when creating vtables to check for unsatisfiable methods.
-fn normalize_and_test_predicates<'tcx>(
+pub fn normalize_and_test_predicates<'tcx>(
     tcx: TyCtxt<'tcx>,
     predicates: Vec<ty::Predicate<'tcx>>,
-    mode: TraitQueryMode,
 ) -> bool {
-    debug!("normalize_and_test_predicates(predicates={:?}, mode={:?})", predicates, mode);
+    debug!("normalize_and_test_predicates(predicates={:?})", predicates);
 
     let result = tcx.infer_ctxt().enter(|infcx| {
         let param_env = ty::ParamEnv::reveal_all();
-        let mut selcx = SelectionContext::with_query_mode(&infcx, mode);
-        let mut fulfill_cx = FulfillmentContext::with_query_mode(mode);
+        let mut selcx = SelectionContext::new(&infcx);
+        let mut fulfill_cx = FulfillmentContext::new();
         let cause = ObligationCause::dummy();
         let Normalized { value: predicates, obligations } =
             normalize(&mut selcx, param_env, cause.clone(), &predicates);
@@ -1044,12 +1043,12 @@ fn normalize_and_test_predicates<'tcx>(
 
 fn substitute_normalize_and_test_predicates<'tcx>(
     tcx: TyCtxt<'tcx>,
-    key: (DefId, SubstsRef<'tcx>, TraitQueryMode),
+    key: (DefId, SubstsRef<'tcx>),
 ) -> bool {
     debug!("substitute_normalize_and_test_predicates(key={:?})", key);
 
     let predicates = tcx.predicates_of(key.0).instantiate(tcx, key.1).predicates;
-    let result = normalize_and_test_predicates(tcx, predicates, key.2);
+    let result = normalize_and_test_predicates(tcx, predicates);
 
     debug!("substitute_normalize_and_test_predicates(key={:?}) = {:?}", key, result);
     result
@@ -1102,10 +1101,7 @@ fn vtable_methods<'tcx>(
             // Note that this method could then never be called, so we
             // do not want to try and codegen it, in that case (see #23435).
             let predicates = tcx.predicates_of(def_id).instantiate_own(tcx, substs);
-            // We don't expect overflow here, so report an error if it somehow ends
-            // up happening.
-            if !normalize_and_test_predicates(tcx, predicates.predicates, TraitQueryMode::Standard)
-            {
+            if !normalize_and_test_predicates(tcx, predicates.predicates) {
                 debug!("vtable_methods: predicates do not hold");
                 return None;
             }

src/librustc/ty/query/keys.rs

@@ -125,15 +125,6 @@ impl<'tcx> Key for (DefId, SubstsRef<'tcx>) {
     }
 }
 
-impl<'tcx> Key for (DefId, SubstsRef<'tcx>, traits::TraitQueryMode) {
-    fn query_crate(&self) -> CrateNum {
-        self.0.krate
-    }
-    fn default_span(&self, tcx: TyCtxt<'_>) -> Span {
-        self.0.default_span(tcx)
-    }
-}
-
 impl<'tcx> Key for (ty::ParamEnv<'tcx>, ty::PolyTraitRef<'tcx>) {
     fn query_crate(&self) -> CrateNum {
         self.1.def_id().krate

src/librustc_mir/transform/const_prop.rs

@@ -14,7 +14,7 @@ use rustc::mir::{
     SourceInfo, SourceScope, SourceScopeData, Statement, StatementKind, Terminator, TerminatorKind,
     UnOp, RETURN_PLACE,
 };
-use rustc::traits::TraitQueryMode;
+use rustc::traits;
 use rustc::ty::layout::{
     HasDataLayout, HasTyCtxt, LayoutError, LayoutOf, Size, TargetDataLayout, TyLayout,
 };
@@ -90,28 +90,28 @@ impl<'tcx> MirPass<'tcx> for ConstProp {
         // If there are unsatisfiable where clauses, then all bets are
         // off, and we just give up.
         //
-        // Note that we use TraitQueryMode::Canonical here, which causes
-        // us to treat overflow like any other error. This is because we
-        // are "speculatively" evaluating this item with the default substs.
-        // While this usually succeeds, it may fail with tricky impls
-        // (e.g. the typenum crate). Const-propagation is fundamentally
-        // "best-effort", and does not affect correctness in any way.
-        // Therefore, it's perfectly fine to just "give up" if we're
-        // unable to check the bounds with the default substs.
+        // We manually filter the predicates, skipping anything that's not
+        // "global". We are in a potentially generic context
+        // (e.g. we are evaluating a function without substituting generic
+        // parameters, so this filtering serves two purposes:
         //
-        // False negatives (failing to run const-prop on something when we actually
-        // could) are fine. However, false positives (running const-prop on
-        // an item with unsatisfiable bounds) can lead to us generating invalid
-        // MIR.
-        if !tcx.substitute_normalize_and_test_predicates((
-            source.def_id(),
-            InternalSubsts::identity_for_item(tcx, source.def_id()),
-            TraitQueryMode::Canonical,
-        )) {
-            trace!(
-                "ConstProp skipped for item with unsatisfiable predicates: {:?}",
-                source.def_id()
-            );
+        // 1. We skip evaluating any predicates that we would
+        // never be able prove are unsatisfiable (e.g. `<T as Foo>`
+        // 2. We avoid trying to normalize predicates involving generic
+        // parameters (e.g. `<T as Foo>::MyItem`). This can confuse
+        // the normalization code (leading to cycle errors), since
+        // it's usually never invoked in this way.
+        let predicates = tcx
+            .predicates_of(source.def_id())
+            .predicates
+            .iter()
+            .filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None })
+            .collect();
+        if !traits::normalize_and_test_predicates(
+            tcx,
+            traits::elaborate_predicates(tcx, predicates).collect(),
+        ) {
+            trace!("ConstProp skipped for {:?}: found unsatisfiable predicates", source.def_id());
             return;
         }
 

src/test/ui/consts/issue-68264-overflow.rs

@@ -0,0 +1,43 @@
+// check-pass
+// compile-flags: --emit=mir,link
+// Regression test for issue #68264
+// Checks that we don't encounter overflow
+// when running const-prop on functions with
+// complicated bounds
+pub trait Query {}
+
+pub trait AsQuery {
+    type Query: Query;
+}
+pub trait Table: AsQuery + Sized {}
+
+pub trait LimitDsl {
+    type Output;
+}
+
+pub(crate) trait LoadQuery<Conn, U>: RunQueryDsl<Conn> {}
+
+impl<T: Query> AsQuery for T {
+    type Query = Self;
+}
+
+impl<T> LimitDsl for T
+where
+    T: Table,
+    T::Query: LimitDsl,
+{
+    type Output = <T::Query as LimitDsl>::Output;
+}
+
+pub(crate) trait RunQueryDsl<Conn>: Sized {
+    fn first<U>(self, _conn: &Conn) -> U
+    where
+        Self: LimitDsl,
+        Self::Output: LoadQuery<Conn, U>,
+    {
+        // Overflow is caused by this function body
+        unimplemented!()
+    }
+}
+
+fn main() {}