avoid instantiating infer vars with infer

Author: lcnr

compiler/rustc_infer/src/infer/combine.rs

@@ -34,6 +34,7 @@ use rustc_middle::infer::unify_key::{ConstVarValue, ConstVariableValue, EffectVa
 use rustc_middle::infer::unify_key::{ConstVariableOrigin, ConstVariableOriginKind};
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
 use rustc_middle::ty::relate::{RelateResult, TypeRelation};
+use rustc_middle::ty::TyVar;
 use rustc_middle::ty::{self, InferConst, ToPredicate, Ty, TyCtxt, TypeVisitableExt};
 use rustc_middle::ty::{IntType, UintType};
 use rustc_span::DUMMY_SP;
@@ -459,7 +460,12 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
             ambient_variance,
         )?;
 
-        self.infcx.inner.borrow_mut().type_variables().instantiate(b_vid, b_ty);
+        // Constrain `b_vid` to the generalized type `b_ty`.
+        if let &ty::Infer(TyVar(b_ty_vid)) = b_ty.kind() {
+            self.infcx.inner.borrow_mut().type_variables().equate(b_vid, b_ty_vid);
+        } else {
+            self.infcx.inner.borrow_mut().type_variables().instantiate(b_vid, b_ty);
+        }
 
         if needs_wf {
             self.obligations.push(Obligation::new(

compiler/rustc_infer/src/infer/type_variable.rs

@@ -229,12 +229,11 @@ impl<'tcx> TypeVariableTable<'_, 'tcx> {
     /// Precondition: `vid` must not have been previously instantiated.
     pub fn instantiate(&mut self, vid: ty::TyVid, ty: Ty<'tcx>) {
         let vid = self.root_var(vid);
+        debug_assert!(!ty.is_ty_var(), "instantiating ty var with var: {vid:?} {ty:?}");
         debug_assert!(self.probe(vid).is_unknown());
         debug_assert!(
             self.eq_relations().probe_value(vid).is_unknown(),
-            "instantiating type variable `{:?}` twice: new-value = {:?}, old-value={:?}",
-            vid,
-            ty,
+            "instantiating type variable `{vid:?}` twice: new-value = {ty:?}, old-value={:?}",
             self.eq_relations().probe_value(vid)
         );
         self.eq_relations().union_value(vid, TypeVariableValue::Known { value: ty });