Use BoundVarsCollector for now

compiler/rustc_middle/src/ty/util.rs

@@ -7,7 +7,7 @@
 use crate::ty::query::TyCtxtAt;
 use crate::ty::subst::{GenericArgKind, Subst, SubstsRef};
 use crate::ty::TyKind::*;
-use crate::ty::{self, DefIdTree, List, Ty, TyCtxt, TypeFoldable};
+use crate::ty::{self, DebruijnIndex, DefIdTree, List, Ty, TyCtxt, TypeFoldable};
 use rustc_apfloat::Float as _;
 use rustc_ast as ast;
 use rustc_attr::{self as attr, SignedInt, UnsignedInt};
@@ -905,6 +905,10 @@ pub fn peel_refs(&'tcx self) -> Ty<'tcx> {
         }
         ty
     }
+
+    pub fn outer_exclusive_binder(&'tcx self) -> DebruijnIndex {
+        self.outer_exclusive_binder
+    }
 }
 
 pub enum ExplicitSelf<'tcx> {

compiler/rustc_ty_utils/src/instance.rs

@@ -2,14 +2,98 @@
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::ty::subst::SubstsRef;
-use rustc_middle::ty::{self, Instance, TyCtxt, TypeFoldable};
+use rustc_middle::ty::{self, Binder, Instance, Ty, TyCtxt, TypeFoldable, TypeVisitor};
 use rustc_span::{sym, DUMMY_SP};
 use rustc_target::spec::abi::Abi;
 use rustc_trait_selection::traits;
 use traits::{translate_substs, Reveal};
 
+use rustc_data_structures::sso::SsoHashSet;
+use std::collections::BTreeMap;
+use std::ops::ControlFlow;
+
 use tracing::debug;
 
+// FIXME(#86795): `BoundVarsCollector` here should **NOT** be used
+// outside of `resolve_associated_item`. It's just to address #64494,
+// #83765, and #85848 which are creating bound types/regions that lose
+// their `Binder` *unintentionally*.
+// It's ideal to remove `BoundVarsCollector` and just use
+// `ty::Binder::*` methods but we use this stopgap until we figure out
+// the "real" fix.
+struct BoundVarsCollector<'tcx> {
+    binder_index: ty::DebruijnIndex,
+    vars: BTreeMap<u32, ty::BoundVariableKind>,
+    // We may encounter the same variable at different levels of binding, so
+    // this can't just be `Ty`
+    visited: SsoHashSet<(ty::DebruijnIndex, Ty<'tcx>)>,
+}
+
+impl<'tcx> BoundVarsCollector<'tcx> {
+    fn new() -> Self {
+        BoundVarsCollector {
+            binder_index: ty::INNERMOST,
+            vars: BTreeMap::new(),
+            visited: SsoHashSet::default(),
+        }
+    }
+
+    fn into_vars(self, tcx: TyCtxt<'tcx>) -> &'tcx ty::List<ty::BoundVariableKind> {
+        let max = self.vars.iter().map(|(k, _)| *k).max().unwrap_or_else(|| 0);
+        for i in 0..max {
+            if let None = self.vars.get(&i) {
+                panic!("Unknown variable: {:?}", i);
+            }
+        }
+
+        tcx.mk_bound_variable_kinds(self.vars.into_iter().map(|(_, v)| v))
+    }
+}
+
+impl<'tcx> TypeVisitor<'tcx> for BoundVarsCollector<'tcx> {
+    type BreakTy = ();
+
+    fn visit_binder<T: TypeFoldable<'tcx>>(
+        &mut self,
+        t: &Binder<'tcx, T>,
+    ) -> ControlFlow<Self::BreakTy> {
+        self.binder_index.shift_in(1);
+        let result = t.super_visit_with(self);
+        self.binder_index.shift_out(1);
+        result
+    }
+
+    fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
+        if t.outer_exclusive_binder() < self.binder_index
+            || !self.visited.insert((self.binder_index, t))
+        {
+            return ControlFlow::CONTINUE;
+        }
+        use std::collections::btree_map::Entry;
+        match *t.kind() {
+            ty::Bound(debruijn, bound_ty) if debruijn == self.binder_index => {
+                match self.vars.entry(bound_ty.var.as_u32()) {
+                    Entry::Vacant(entry) => {
+                        entry.insert(ty::BoundVariableKind::Ty(bound_ty.kind));
+                    }
+                    Entry::Occupied(entry) => match entry.get() {
+                        ty::BoundVariableKind::Ty(_) => {}
+                        _ => bug!("Conflicting bound vars"),
+                    },
+                }
+            }
+
+            _ => (),
+        };
+
+        t.super_visit_with(self)
+    }
+
+    fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
+        r.super_visit_with(self)
+    }
+}
+
 #[instrument(level = "debug", skip(tcx))]
 fn resolve_instance<'tcx>(
     tcx: TyCtxt<'tcx>,
@@ -115,14 +199,12 @@ fn resolve_associated_item<'tcx>(
     );
 
     let trait_ref = ty::TraitRef::from_method(tcx, trait_id, rcvr_substs);
-    let vtbl = if trait_item.kind == ty::AssocKind::Const {
-        let bound_vars = tcx
-            .mk_bound_variable_kinds(std::iter::once(ty::BoundVariableKind::Region(ty::BrAnon(0))));
-        let bind = ty::Binder::bind_with_vars(trait_ref, bound_vars);
-        tcx.codegen_fulfill_obligation((param_env, bind))?
-    } else {
-        tcx.codegen_fulfill_obligation((param_env, ty::Binder::bind(trait_ref, tcx)))?
-    };
+
+    // See FIXME on `BoundVarsCollector`.
+    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_ty_utils/src/lib.rs

@@ -5,6 +5,7 @@
 //! This API is completely unstable and subject to change.
 
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
+#![feature(control_flow_enum)]
 #![feature(half_open_range_patterns)]
 #![feature(exclusive_range_pattern)]
 #![feature(nll)]