Erase trivial caller-bounds when typechecking MIR after optimizations

compiler/rustc_const_eval/src/transform/validate.rs

@@ -171,9 +171,10 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
             return true;
         }
         // Normalize projections and things like that.
-        // FIXME: We need to reveal_all, as some optimizations change types in ways
-        // that require unfolding opaque types.
-        let param_env = self.param_env.with_reveal_all_normalized(self.tcx);
+        // FIXME: We need to reveal_all and erase any trivial caller bounds (see query description)
+        // due to the fact that certain optimizations (e.g. inlining) change the way that types are
+        // normalized.
+        let param_env = self.tcx.reveal_all_and_erase_trivial_caller_bounds(self.param_env);
         let src = self.tcx.normalize_erasing_regions(param_env, src);
         let dest = self.tcx.normalize_erasing_regions(param_env, dest);
 

compiler/rustc_middle/src/query/mod.rs

@@ -1152,6 +1152,16 @@ rustc_queries! {
         desc { |tcx| "computing revealed normalized predicates of `{}`", tcx.def_path_str(def_id) }
     }
 
+    /// Like `ty::ParamEnv::reveal_all_normalized`, but returns the `ParamEnv` with any
+    /// where clause bounds removed if they hold "trivially" -- that is, if they are
+    /// satisfied by the input param-env with that bound removed, such as if they match a
+    /// global `impl` or are a duplicated bound in the where clause.
+    ///
+    /// This is a query because it is computationally expensive, and we want to cache it.
+    query reveal_all_and_erase_trivial_caller_bounds(key: ty::ParamEnv<'tcx>) -> ty::ParamEnv<'tcx> {
+        desc { |tcx| "normalizing and erasing trivial predicates of `{:?}`", key }
+    }
+
     /// Trait selection queries. These are best used by invoking `ty.is_copy_modulo_regions()`,
     /// `ty.is_copy()`, etc, since that will prune the environment where possible.
     query is_copy_raw(env: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool {

compiler/rustc_middle/src/ty/mod.rs

@@ -1455,6 +1455,12 @@ impl<'tcx> ParamEnv<'tcx> {
         Self::new(List::empty(), self.reveal(), self.constness())
     }
 
+    /// Returns this same environment but with new caller bounds.
+    #[inline]
+    pub fn with_caller_bounds(self, caller_bounds: &'tcx List<Predicate<'tcx>>) -> Self {
+        Self::new(caller_bounds, self.reveal(), self.constness())
+    }
+
     /// Creates a suitable environment in which to perform trait
     /// queries on the given value. When type-checking, this is simply
     /// the pair of the environment plus value. But when reveal is set to

compiler/rustc_ty_utils/src/ty.rs

@@ -1,6 +1,8 @@
 use rustc_data_structures::fx::FxIndexSet;
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
+use rustc_infer::infer::TyCtxtInferExt;
+use rustc_infer::traits::TraitEngine;
 use rustc_middle::ty::subst::Subst;
 use rustc_middle::ty::{self, Binder, Predicate, PredicateKind, ToPredicate, Ty, TyCtxt};
 use rustc_span::{sym, Span};
@@ -356,6 +358,50 @@ fn param_env_reveal_all_normalized(tcx: TyCtxt<'_>, def_id: DefId) -> ty::ParamE
     tcx.param_env(def_id).with_reveal_all_normalized(tcx)
 }
 
+fn reveal_all_and_erase_trivial_caller_bounds<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+) -> ty::ParamEnv<'tcx> {
+    let mut old_caller_bounds = param_env.caller_bounds();
+    loop {
+        // The caller bounds that we must continue to include in our ParamEnv,
+        // because they cause errors without them...
+        let mut needed_caller_bounds = vec![];
+        for (idx, bound) in old_caller_bounds.iter().enumerate() {
+            let new_param_env = param_env.with_caller_bounds(tcx.mk_predicates(
+                needed_caller_bounds.iter().chain(old_caller_bounds[idx + 1..].iter()).copied(),
+            ));
+            // a bound is trivial if it does not have const projections
+            // (which might induce cycles), and if we can prove that bound
+            // given a copy of our param-env that has the bound removed.
+            let is_bound_trivial = tcx.infer_ctxt().enter(|infcx| {
+                let mut fulfillcx = traits::FulfillmentContext::new_in_snapshot();
+                fulfillcx.register_predicate_obligation(
+                    &infcx,
+                    traits::Obligation::new(traits::ObligationCause::dummy(), new_param_env, bound),
+                );
+                let errors = fulfillcx.select_all_or_error(&infcx);
+                if !errors.is_empty() {
+                    info!("{:?} is NOT trivial: {:?}", bound, errors);
+                } else {
+                    info!("{:?} is trivial", bound);
+                }
+                // is trivial iff there are no errors when fulfilling this bound
+                errors.is_empty()
+            });
+            if !is_bound_trivial {
+                needed_caller_bounds.push(bound);
+            }
+        }
+        let new_caller_bounds = tcx.mk_predicates(needed_caller_bounds.into_iter());
+        if new_caller_bounds == old_caller_bounds {
+            return param_env.with_caller_bounds(new_caller_bounds).with_reveal_all_normalized(tcx);
+        } else {
+            old_caller_bounds = new_caller_bounds;
+        }
+    }
+}
+
 fn instance_def_size_estimate<'tcx>(
     tcx: TyCtxt<'tcx>,
     instance_def: ty::InstanceDef<'tcx>,
@@ -495,6 +541,7 @@ pub fn provide(providers: &mut ty::query::Providers) {
         def_ident_span,
         param_env,
         param_env_reveal_all_normalized,
+        reveal_all_and_erase_trivial_caller_bounds,
         instance_def_size_estimate,
         issue33140_self_ty,
         impl_defaultness,

src/test/ui/mir/mir-inlining/caller-with-trivial-bound.rs

@@ -0,0 +1,21 @@
+// check-pass
+// compile-flags: -Zmir-opt-level=3 --crate-type=lib
+
+pub trait Factory<T> {
+    type Item;
+}
+
+pub struct IntFactory;
+
+impl<T> Factory<T> for IntFactory {
+    type Item = usize;
+}
+
+pub fn foo<T>() where IntFactory: Factory<T> {
+    let mut x: <IntFactory as Factory<T>>::Item = bar::<T>();
+}
+
+#[inline]
+pub fn bar<T>() -> <IntFactory as Factory<T>>::Item {
+    0usize
+}