Remove Binder::bind() and use Binder::dummy()

See [1] for motivation.

In one place, it manually uses the `BoundVarsCollector` for now because
we don't know what the bound variables are, and adding support for
tracking the bound variables will probably be a tricky change.

[1]: rust-lang#76814 (comment)

Author: camelid

compiler/rustc_middle/src/ty/sty.rs

@@ -5,7 +5,6 @@
 use self::TyKind::*;
 
 use crate::infer::canonical::Canonical;
-use crate::ty::fold::BoundVarsCollector;
 use crate::ty::fold::ValidateBoundVars;
 use crate::ty::subst::{GenericArg, InternalSubsts, Subst, SubstsRef};
 use crate::ty::InferTy::{self, *};
@@ -970,13 +969,6 @@ where
         Binder(value, ty::List::empty())
     }
 
-    /// Wraps `value` in a binder, binding higher-ranked vars (if any).
-    pub fn bind(value: T, tcx: TyCtxt<'tcx>) -> Binder<'tcx, T> {
-        let mut collector = BoundVarsCollector::new();
-        value.visit_with(&mut collector);
-        Binder(value, collector.into_vars(tcx))
-    }
-
     pub fn bind_with_vars(value: T, vars: &'tcx List<BoundVariableKind>) -> Binder<'tcx, T> {
         if cfg!(debug_assertions) {
             let mut validator = ValidateBoundVars::new(vars);

compiler/rustc_mir/src/transform/check_consts/validation.rs

@@ -822,12 +822,9 @@ impl Visitor<'tcx> for Validator<'mir, 'tcx> {
                     let obligation = Obligation::new(
                         ObligationCause::dummy(),
                         param_env,
-                        Binder::bind(
-                            TraitPredicate {
-                                trait_ref: TraitRef::from_method(tcx, trait_id, substs),
-                            },
-                            tcx,
-                        ),
+                        Binder::dummy(TraitPredicate {
+                            trait_ref: TraitRef::from_method(tcx, trait_id, substs),
+                        }),
                     );
 
                     let implsrc = tcx.infer_ctxt().enter(|infcx| {

compiler/rustc_trait_selection/src/traits/project.rs

@@ -1301,7 +1301,7 @@ fn confirm_pointee_candidate<'cx, 'tcx>(
         ty: self_ty.ptr_metadata_ty(tcx),
     };
 
-    confirm_param_env_candidate(selcx, obligation, ty::Binder::bind(predicate, tcx), false)
+    confirm_param_env_candidate(selcx, obligation, ty::Binder::dummy(predicate), false)
 }
 
 fn confirm_fn_pointer_candidate<'cx, 'tcx>(

compiler/rustc_ty_utils/src/instance.rs

@@ -1,6 +1,7 @@
 use rustc_errors::ErrorReported;
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_infer::infer::TyCtxtInferExt;
+use rustc_middle::ty::fold::BoundVarsCollector;
 use rustc_middle::ty::subst::SubstsRef;
 use rustc_middle::ty::{self, Instance, TyCtxt, TypeFoldable};
 use rustc_span::{sym, DUMMY_SP};
@@ -115,7 +116,12 @@ fn resolve_associated_item<'tcx>(
     );
 
     let trait_ref = ty::TraitRef::from_method(tcx, trait_id, rcvr_substs);
-    let vtbl = tcx.codegen_fulfill_obligation((param_env, ty::Binder::bind(trait_ref, tcx)))?;
+    // FIXME: we should instead track bound variables from their origin,
+    //        rather than using the `BoundVarsCollector` (cc #83825)
+    let mut bound_vars_collector = BoundVarsCollector::new();
+    trait_ref.visit_with(&mut bound_vars_collector);
+    let trait_binder = ty::Binder::bind_with_vars(trait_ref, bound_vars_collector.into_vars(tcx));
+    let vtbl = tcx.codegen_fulfill_obligation((param_env, trait_binder))?;
 
     // Now that we know which impl is being used, we can dispatch to
     // the actual function:

compiler/rustc_typeck/src/astconv/mod.rs

@@ -1694,7 +1694,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 };
 
                 self.one_bound_for_assoc_type(
-                    || traits::supertraits(tcx, ty::Binder::bind(trait_ref, tcx)),
+                    || traits::supertraits(tcx, ty::Binder::dummy(trait_ref)),
                     || "Self".to_string(),
                     assoc_ident,
                     span,

compiler/rustc_typeck/src/check/compare_method.rs

@@ -225,7 +225,7 @@ fn compare_predicate_entailment<'tcx>(
         let (impl_m_own_bounds, _) = infcx.replace_bound_vars_with_fresh_vars(
             impl_m_span,
             infer::HigherRankedType,
-            ty::Binder::bind(impl_m_own_bounds.predicates, tcx),
+            ty::Binder::dummy(impl_m_own_bounds.predicates),
         );
         for predicate in impl_m_own_bounds {
             let traits::Normalized { value: predicate, obligations } =
@@ -258,14 +258,14 @@ fn compare_predicate_entailment<'tcx>(
         );
         let impl_sig =
             inh.normalize_associated_types_in(impl_m_span, impl_m_hir_id, param_env, impl_sig);
-        let impl_fty = tcx.mk_fn_ptr(ty::Binder::bind(impl_sig, tcx));
+        let impl_fty = tcx.mk_fn_ptr(ty::Binder::dummy(impl_sig));
         debug!("compare_impl_method: impl_fty={:?}", impl_fty);
 
         let trait_sig = tcx.liberate_late_bound_regions(impl_m.def_id, tcx.fn_sig(trait_m.def_id));
         let trait_sig = trait_sig.subst(tcx, trait_to_placeholder_substs);
         let trait_sig =
             inh.normalize_associated_types_in(impl_m_span, impl_m_hir_id, param_env, trait_sig);
-        let trait_fty = tcx.mk_fn_ptr(ty::Binder::bind(trait_sig, tcx));
+        let trait_fty = tcx.mk_fn_ptr(ty::Binder::dummy(trait_sig));
 
         debug!("compare_impl_method: trait_fty={:?}", trait_fty);
 

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

@@ -119,7 +119,7 @@ impl<'a, 'tcx> ConfirmContext<'a, 'tcx> {
         // We won't add these if we encountered an illegal sized bound, so that we can use
         // a custom error in that case.
         if illegal_sized_bound.is_none() {
-            let method_ty = self.tcx.mk_fn_ptr(ty::Binder::bind(method_sig, self.tcx));
+            let method_ty = self.tcx.mk_fn_ptr(ty::Binder::dummy(method_sig));
             self.add_obligations(method_ty, all_substs, method_predicates);
         }
 

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

@@ -404,7 +404,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         obligations.extend(traits::predicates_for_generics(cause.clone(), self.param_env, bounds));
 
         // Also add an obligation for the method type being well-formed.
-        let method_ty = tcx.mk_fn_ptr(ty::Binder::bind(fn_sig, tcx));
+        let method_ty = tcx.mk_fn_ptr(ty::Binder::dummy(fn_sig));
         debug!(
             "lookup_in_trait_adjusted: matched method method_ty={:?} obligation={:?}",
             method_ty, obligation

compiler/rustc_typeck/src/check/wfcheck.rs

@@ -1087,14 +1087,13 @@ fn check_method_receiver<'fcx, 'tcx>(
     debug!("check_method_receiver: sig={:?}", sig);
 
     let self_ty = fcx.normalize_associated_types_in(span, self_ty);
-    let self_ty =
-        fcx.tcx.liberate_late_bound_regions(method.def_id, ty::Binder::bind(self_ty, fcx.tcx));
+    let self_ty = fcx.tcx.liberate_late_bound_regions(method.def_id, ty::Binder::dummy(self_ty));
 
     let receiver_ty = sig.inputs()[0];
 
     let receiver_ty = fcx.normalize_associated_types_in(span, receiver_ty);
     let receiver_ty =
-        fcx.tcx.liberate_late_bound_regions(method.def_id, ty::Binder::bind(receiver_ty, fcx.tcx));
+        fcx.tcx.liberate_late_bound_regions(method.def_id, ty::Binder::dummy(receiver_ty));
 
     if fcx.tcx.features().arbitrary_self_types {
         if !receiver_is_valid(fcx, span, receiver_ty, self_ty, true) {

compiler/rustc_typeck/src/collect.rs

@@ -1767,7 +1767,7 @@ fn fn_sig(tcx: TyCtxt<'_>, def_id: DefId) -> ty::PolyFnSig<'_> {
                     }
                     diag.emit();
 
-                    ty::Binder::bind(fn_sig, tcx)
+                    ty::Binder::dummy(fn_sig)
                 }
                 None => <dyn AstConv<'_>>::ty_of_fn(
                     &icx,
@@ -1811,10 +1811,13 @@ fn fn_sig(tcx: TyCtxt<'_>, def_id: DefId) -> ty::PolyFnSig<'_> {
             let ty = tcx.type_of(tcx.hir().get_parent_did(hir_id).to_def_id());
             let inputs =
                 data.fields().iter().map(|f| tcx.type_of(tcx.hir().local_def_id(f.hir_id)));
-            ty::Binder::bind(
-                tcx.mk_fn_sig(inputs, ty, false, hir::Unsafety::Normal, abi::Abi::Rust),
-                tcx,
-            )
+            ty::Binder::dummy(tcx.mk_fn_sig(
+                inputs,
+                ty,
+                false,
+                hir::Unsafety::Normal,
+                abi::Abi::Rust,
+            ))
         }
 
         Expr(&hir::Expr { kind: hir::ExprKind::Closure(..), .. }) => {
@@ -2098,7 +2101,7 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: DefId) -> ty::GenericP
                 param.bounds.iter().for_each(|bound| match bound {
                     hir::GenericBound::Outlives(lt) => {
                         let bound = <dyn AstConv<'_>>::ast_region_to_region(&icx, &lt, None);
-                        let outlives = ty::Binder::bind(ty::OutlivesPredicate(region, bound), tcx);
+                        let outlives = ty::Binder::dummy(ty::OutlivesPredicate(region, bound));
                         predicates.insert((outlives.to_predicate(tcx), lt.span));
                     }
                     _ => bug!(),