Use rebind instead of Binder::bind when possible

compiler/rustc_codegen_llvm/src/intrinsic.rs

@@ -673,17 +673,9 @@ fn codegen_emcc_try(
 fn gen_fn<'ll, 'tcx>(
     cx: &CodegenCx<'ll, 'tcx>,
     name: &str,
-    inputs: Vec<Ty<'tcx>>,
-    output: Ty<'tcx>,
+    rust_fn_sig: ty::PolyFnSig<'tcx>,
     codegen: &mut dyn FnMut(Builder<'_, 'll, 'tcx>),
 ) -> &'ll Value {
-    let rust_fn_sig = ty::Binder::bind(cx.tcx.mk_fn_sig(
-        inputs.into_iter(),
-        output,
-        false,
-        hir::Unsafety::Unsafe,
-        Abi::Rust,
-    ));
     let fn_abi = FnAbi::of_fn_ptr(cx, rust_fn_sig, &[]);
     let llfn = cx.declare_fn(name, &fn_abi);
     cx.set_frame_pointer_elimination(llfn);
@@ -710,22 +702,31 @@ fn get_rust_try_fn<'ll, 'tcx>(
     // Define the type up front for the signature of the rust_try function.
     let tcx = cx.tcx;
     let i8p = tcx.mk_mut_ptr(tcx.types.i8);
-    let try_fn_ty = tcx.mk_fn_ptr(ty::Binder::bind(tcx.mk_fn_sig(
+    // `unsafe fn(*mut i8) -> ()`
+    let try_fn_ty = tcx.mk_fn_ptr(ty::Binder::dummy(tcx.mk_fn_sig(
         iter::once(i8p),
         tcx.mk_unit(),
         false,
         hir::Unsafety::Unsafe,
         Abi::Rust,
     )));
-    let catch_fn_ty = tcx.mk_fn_ptr(ty::Binder::bind(tcx.mk_fn_sig(
+    // `unsafe fn(*mut i8, *mut i8) -> ()`
+    let catch_fn_ty = tcx.mk_fn_ptr(ty::Binder::dummy(tcx.mk_fn_sig(
         [i8p, i8p].iter().cloned(),
         tcx.mk_unit(),
         false,
         hir::Unsafety::Unsafe,
         Abi::Rust,
     )));
-    let output = tcx.types.i32;
-    let rust_try = gen_fn(cx, "__rust_try", vec![try_fn_ty, i8p, catch_fn_ty], output, codegen);
+    // `unsafe fn(unsafe fn(*mut i8) -> (), *mut i8, unsafe fn(*mut i8, *mut i8) -> ()) -> i32`
+    let rust_fn_sig = ty::Binder::dummy(cx.tcx.mk_fn_sig(
+        vec![try_fn_ty, i8p, catch_fn_ty].into_iter(),
+        tcx.types.i32,
+        false,
+        hir::Unsafety::Unsafe,
+        Abi::Rust,
+    ));
+    let rust_try = gen_fn(cx, "__rust_try", rust_fn_sig, codegen);
     cx.rust_try_fn.set(Some(rust_try));
     rust_try
 }

compiler/rustc_infer/src/infer/nll_relate/mod.rs

@@ -636,7 +636,7 @@ where
         if let (Some(a), Some(b)) = (a.no_bound_vars(), b.no_bound_vars()) {
             // Fast path for the common case.
             self.relate(a, b)?;
-            return Ok(ty::Binder::bind(a));
+            return Ok(ty::Binder::dummy(a));
         }
 
         if self.ambient_covariance() {

compiler/rustc_infer/src/traits/util.rs

@@ -126,14 +126,15 @@ impl Elaborator<'tcx> {
     fn elaborate(&mut self, obligation: &PredicateObligation<'tcx>) {
         let tcx = self.visited.tcx;
 
-        match obligation.predicate.skip_binders() {
+        let bound_predicate = obligation.predicate.bound_atom();
+        match bound_predicate.skip_binder() {
             ty::PredicateAtom::Trait(data, _) => {
                 // Get predicates declared on the trait.
                 let predicates = tcx.super_predicates_of(data.def_id());
 
                 let obligations = predicates.predicates.iter().map(|&(pred, _)| {
                     predicate_obligation(
-                        pred.subst_supertrait(tcx, &ty::Binder::bind(data.trait_ref)),
+                        pred.subst_supertrait(tcx, &bound_predicate.rebind(data.trait_ref)),
                         obligation.param_env,
                         obligation.cause.clone(),
                     )

compiler/rustc_middle/src/ty/mod.rs

@@ -1056,9 +1056,21 @@ impl<'tcx> Predicate<'tcx> {
         }
     }
 
+    /// Converts this to a `Binder<PredicateAtom<'tcx>>`. If the value was an
+    /// `Atom`, then it is not allowed to contain escaping bound vars.
+    pub fn bound_atom(self) -> Binder<PredicateAtom<'tcx>> {
+        match self.kind() {
+            &PredicateKind::ForAll(binder) => binder,
+            &PredicateKind::Atom(atom) => {
+                debug_assert!(!atom.has_escaping_bound_vars());
+                Binder::dummy(atom)
+            }
+        }
+    }
+
     /// Allows using a `Binder<PredicateAtom<'tcx>>` even if the given predicate previously
     /// contained unbound variables by shifting these variables outwards.
-    pub fn bound_atom(self, tcx: TyCtxt<'tcx>) -> Binder<PredicateAtom<'tcx>> {
+    pub fn bound_atom_with_opt_escaping(self, tcx: TyCtxt<'tcx>) -> Binder<PredicateAtom<'tcx>> {
         match self.kind() {
             &PredicateKind::ForAll(binder) => binder,
             &PredicateKind::Atom(atom) => Binder::wrap_nonbinding(tcx, atom),

compiler/rustc_middle/src/ty/print/pretty.rs

@@ -618,10 +618,9 @@ pub trait PrettyPrinter<'tcx>:
                         // may contain unbound variables. We therefore do this manually.
                         //
                         // FIXME(lcnr): Find out why exactly this is the case :)
-                        if let ty::PredicateAtom::Trait(pred, _) =
-                            predicate.bound_atom(self.tcx()).skip_binder()
-                        {
-                            let trait_ref = ty::Binder::bind(pred.trait_ref);
+                        let bound_predicate = predicate.bound_atom_with_opt_escaping(self.tcx());
+                        if let ty::PredicateAtom::Trait(pred, _) = bound_predicate.skip_binder() {
+                            let trait_ref = bound_predicate.rebind(pred.trait_ref);
                             // Don't print +Sized, but rather +?Sized if absent.
                             if Some(trait_ref.def_id()) == self.tcx().lang_items().sized_trait() {
                                 is_sized = true;

compiler/rustc_middle/src/ty/structural_impls.rs

@@ -549,7 +549,7 @@ impl<'a, 'tcx> Lift<'tcx> for ty::PredicateAtom<'a> {
 impl<'tcx, T: Lift<'tcx>> Lift<'tcx> for ty::Binder<T> {
     type Lifted = ty::Binder<T::Lifted>;
     fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
-        tcx.lift(self.as_ref().skip_binder()).map(ty::Binder::bind)
+        tcx.lift(self.as_ref().skip_binder()).map(|v| self.rebind(v))
     }
 }
 

compiler/rustc_middle/src/ty/sty.rs

@@ -703,14 +703,16 @@ impl<'tcx> Binder<ExistentialPredicate<'tcx>> {
         use crate::ty::ToPredicate;
         match self.skip_binder() {
             ExistentialPredicate::Trait(tr) => {
-                Binder(tr).with_self_ty(tcx, self_ty).without_const().to_predicate(tcx)
+                self.rebind(tr).with_self_ty(tcx, self_ty).without_const().to_predicate(tcx)
             }
             ExistentialPredicate::Projection(p) => {
-                Binder(p.with_self_ty(tcx, self_ty)).to_predicate(tcx)
+                self.rebind(p.with_self_ty(tcx, self_ty)).to_predicate(tcx)
             }
             ExistentialPredicate::AutoTrait(did) => {
-                let trait_ref =
-                    Binder(ty::TraitRef { def_id: did, substs: tcx.mk_substs_trait(self_ty, &[]) });
+                let trait_ref = self.rebind(ty::TraitRef {
+                    def_id: did,
+                    substs: tcx.mk_substs_trait(self_ty, &[]),
+                });
                 trait_ref.without_const().to_predicate(tcx)
             }
         }
@@ -775,7 +777,7 @@ impl<'tcx> List<ExistentialPredicate<'tcx>> {
 
 impl<'tcx> Binder<&'tcx List<ExistentialPredicate<'tcx>>> {
     pub fn principal(&self) -> Option<ty::Binder<ExistentialTraitRef<'tcx>>> {
-        self.skip_binder().principal().map(Binder::bind)
+        self.map_bound(|b| b.principal()).transpose()
     }
 
     pub fn principal_def_id(&self) -> Option<DefId> {
@@ -858,8 +860,7 @@ impl<'tcx> PolyTraitRef<'tcx> {
     }
 
     pub fn to_poly_trait_predicate(&self) -> ty::PolyTraitPredicate<'tcx> {
-        // Note that we preserve binding levels
-        Binder(ty::TraitPredicate { trait_ref: self.skip_binder() })
+        self.map_bound(|trait_ref| ty::TraitPredicate { trait_ref })
     }
 }
 
@@ -1001,6 +1002,19 @@ impl<T> Binder<T> {
         Binder(f(self.0))
     }
 
+    /// Wraps a `value` in a binder, using the same bound variables as the
+    /// current `Binder`. This should not be used if the new value *changes*
+    /// the bound variables. Note: the (old or new) value itself does not
+    /// necessarily need to *name* all the bound variables.
+    ///
+    /// This currently doesn't do anything different than `bind`, because we
+    /// don't actually track bound vars. However, semantically, it is different
+    /// because bound vars aren't allowed to change here, whereas they are
+    /// in `bind`. This may be (debug) asserted in the future.
+    pub fn rebind<U>(&self, value: U) -> Binder<U> {
+        Binder(value)
+    }
+
     /// Unwraps and returns the value within, but only if it contains
     /// no bound vars at all. (In other words, if this binder --
     /// and indeed any enclosing binder -- doesn't bind anything at

compiler/rustc_trait_selection/src/traits/auto_trait.rs

@@ -642,18 +642,19 @@ impl AutoTraitFinder<'tcx> {
             // We check this by calling is_of_param on the relevant types
             // from the various possible predicates
 
-            match predicate.skip_binders() {
+            let bound_predicate = predicate.bound_atom();
+            match bound_predicate.skip_binder() {
                 ty::PredicateAtom::Trait(p, _) => {
                     if self.is_param_no_infer(p.trait_ref.substs)
                         && !only_projections
                         && is_new_pred
                     {
                         self.add_user_pred(computed_preds, predicate);
                     }
-                    predicates.push_back(ty::Binder::bind(p));
+                    predicates.push_back(bound_predicate.rebind(p));
                 }
                 ty::PredicateAtom::Projection(p) => {
-                    let p = ty::Binder::bind(p);
+                    let p = bound_predicate.rebind(p);
                     debug!(
                         "evaluate_nested_obligations: examining projection predicate {:?}",
                         predicate
@@ -783,13 +784,13 @@ impl AutoTraitFinder<'tcx> {
                     }
                 }
                 ty::PredicateAtom::RegionOutlives(binder) => {
-                    let binder = ty::Binder::bind(binder);
+                    let binder = bound_predicate.rebind(binder);
                     if select.infcx().region_outlives_predicate(&dummy_cause, binder).is_err() {
                         return false;
                     }
                 }
                 ty::PredicateAtom::TypeOutlives(binder) => {
-                    let binder = ty::Binder::bind(binder);
+                    let binder = bound_predicate.rebind(binder);
                     match (
                         binder.no_bound_vars(),
                         binder.map_bound_ref(|pred| pred.0).no_bound_vars(),

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

@@ -255,9 +255,10 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                     return;
                 }
 
-                match obligation.predicate.skip_binders() {
+                let bound_predicate = obligation.predicate.bound_atom();
+                match bound_predicate.skip_binder() {
                     ty::PredicateAtom::Trait(trait_predicate, _) => {
-                        let trait_predicate = ty::Binder::bind(trait_predicate);
+                        let trait_predicate = bound_predicate.rebind(trait_predicate);
                         let trait_predicate = self.resolve_vars_if_possible(&trait_predicate);
 
                         if self.tcx.sess.has_errors() && trait_predicate.references_error() {
@@ -531,7 +532,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                     }
 
                     ty::PredicateAtom::RegionOutlives(predicate) => {
-                        let predicate = ty::Binder::bind(predicate);
+                        let predicate = bound_predicate.rebind(predicate);
                         let predicate = self.resolve_vars_if_possible(&predicate);
                         let err = self
                             .region_outlives_predicate(&obligation.cause, predicate)
@@ -1078,9 +1079,10 @@ impl<'a, 'tcx> InferCtxtPrivExt<'tcx> for InferCtxt<'a, 'tcx> {
         }
 
         // FIXME: It should be possible to deal with `ForAll` in a cleaner way.
-        let (cond, error) = match (cond.skip_binders(), error.skip_binders()) {
+        let bound_error = error.bound_atom();
+        let (cond, error) = match (cond.skip_binders(), bound_error.skip_binder()) {
             (ty::PredicateAtom::Trait(..), ty::PredicateAtom::Trait(error, _)) => {
-                (cond, ty::Binder::bind(error))
+                (cond, bound_error.rebind(error))
             }
             _ => {
                 // FIXME: make this work in other cases too.
@@ -1089,9 +1091,10 @@ impl<'a, 'tcx> InferCtxtPrivExt<'tcx> for InferCtxt<'a, 'tcx> {
         };
 
         for obligation in super::elaborate_predicates(self.tcx, std::iter::once(cond)) {
-            if let ty::PredicateAtom::Trait(implication, _) = obligation.predicate.skip_binders() {
+            let bound_predicate = obligation.predicate.bound_atom();
+            if let ty::PredicateAtom::Trait(implication, _) = bound_predicate.skip_binder() {
                 let error = error.to_poly_trait_ref();
-                let implication = ty::Binder::bind(implication.trait_ref);
+                let implication = bound_predicate.rebind(implication.trait_ref);
                 // FIXME: I'm just not taking associated types at all here.
                 // Eventually I'll need to implement param-env-aware
                 // `Γ₁ ⊦ φ₁ => Γ₂ ⊦ φ₂` logic.
@@ -1169,12 +1172,13 @@ impl<'a, 'tcx> InferCtxtPrivExt<'tcx> for InferCtxt<'a, 'tcx> {
             //
             // this can fail if the problem was higher-ranked, in which
             // cause I have no idea for a good error message.
-            if let ty::PredicateAtom::Projection(data) = predicate.skip_binders() {
+            let bound_predicate = predicate.bound_atom();
+            if let ty::PredicateAtom::Projection(data) = bound_predicate.skip_binder() {
                 let mut selcx = SelectionContext::new(self);
                 let (data, _) = self.replace_bound_vars_with_fresh_vars(
                     obligation.cause.span,
                     infer::LateBoundRegionConversionTime::HigherRankedType,
-                    &ty::Binder::bind(data),
+                    &bound_predicate.rebind(data),
                 );
                 let mut obligations = vec![];
                 let normalized_ty = super::normalize_projection_type(
@@ -1455,10 +1459,11 @@ impl<'a, 'tcx> InferCtxtPrivExt<'tcx> for InferCtxt<'a, 'tcx> {
             return;
         }
 
-        let mut err = match predicate.skip_binders() {
+        let bound_predicate = predicate.bound_atom();
+        let mut err = match bound_predicate.skip_binder() {
             ty::PredicateAtom::Trait(data, _) => {
-                let trait_ref = ty::Binder::bind(data.trait_ref);
-                let self_ty = trait_ref.skip_binder().self_ty();
+                let self_ty = data.trait_ref.self_ty();
+                let trait_ref = bound_predicate.rebind(data.trait_ref);
                 debug!("self_ty {:?} {:?} trait_ref {:?}", self_ty, self_ty.kind(), trait_ref);
 
                 if predicate.references_error() {
@@ -1582,7 +1587,7 @@ impl<'a, 'tcx> InferCtxtPrivExt<'tcx> for InferCtxt<'a, 'tcx> {
                 self.emit_inference_failure_err(body_id, span, a.into(), ErrorCode::E0282)
             }
             ty::PredicateAtom::Projection(data) => {
-                let trait_ref = ty::Binder::bind(data).to_poly_trait_ref(self.tcx);
+                let trait_ref = bound_predicate.rebind(data).to_poly_trait_ref(self.tcx);
                 let self_ty = trait_ref.skip_binder().self_ty();
                 let ty = data.ty;
                 if predicate.references_error() {

compiler/rustc_trait_selection/src/traits/fulfill.rs

@@ -353,7 +353,7 @@ impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
                 // This means we need to pass it the bound version of our
                 // predicate.
                 ty::PredicateAtom::Trait(trait_ref, _constness) => {
-                    let trait_obligation = obligation.with(Binder::bind(trait_ref));
+                    let trait_obligation = obligation.with(binder.rebind(trait_ref));
 
                     self.process_trait_obligation(
                         obligation,
@@ -362,7 +362,7 @@ impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
                     )
                 }
                 ty::PredicateAtom::Projection(data) => {
-                    let project_obligation = obligation.with(Binder::bind(data));
+                    let project_obligation = obligation.with(binder.rebind(data));
 
                     self.process_projection_obligation(
                         project_obligation,

compiler/rustc_trait_selection/src/traits/project.rs

@@ -623,7 +623,8 @@ fn prune_cache_value_obligations<'a, 'tcx>(
         .obligations
         .iter()
         .filter(|obligation| {
-            match obligation.predicate.skip_binders() {
+            let bound_predicate = obligation.predicate.bound_atom();
+            match bound_predicate.skip_binder() {
                 // We found a `T: Foo<X = U>` predicate, let's check
                 // if `U` references any unresolved type
                 // variables. In principle, we only care if this
@@ -634,7 +635,7 @@ fn prune_cache_value_obligations<'a, 'tcx>(
                 // but we have `T: Foo<X = ?1>` and `?1: Bar<X =
                 // ?0>`).
                 ty::PredicateAtom::Projection(data) => {
-                    infcx.unresolved_type_vars(&ty::Binder::bind(data.ty)).is_some()
+                    infcx.unresolved_type_vars(&bound_predicate.rebind(data.ty)).is_some()
                 }
 
                 // We are only interested in `T: Foo<X = U>` predicates, whre
@@ -907,8 +908,9 @@ fn assemble_candidates_from_predicates<'cx, 'tcx>(
     let infcx = selcx.infcx();
     for predicate in env_predicates {
         debug!(?predicate);
+        let bound_predicate = predicate.bound_atom();
         if let ty::PredicateAtom::Projection(data) = predicate.skip_binders() {
-            let data = ty::Binder::bind(data);
+            let data = bound_predicate.rebind(data);
             let same_def_id = data.projection_def_id() == obligation.predicate.item_def_id;
 
             let is_match = same_def_id