review

Author: lcnr

compiler/rustc_infer/src/infer/combine.rs

@@ -326,18 +326,16 @@ impl<'tcx> InferCtxt<'tcx> {
     ) -> RelateResult<'tcx, ty::Const<'tcx>> {
         let span =
             self.inner.borrow_mut().const_unification_table().probe_value(target_vid).origin.span;
-        let Generalization { value, needs_wf: _ } = generalize::generalize(
+        // FIXME(generic_const_exprs): Occurs check failures for unevaluated
+        // constants and generic expressions are not yet handled correctly.
+        let Generalization { value_may_be_infer: value, needs_wf: _ } = generalize::generalize(
             self,
             &mut CombineDelegate { infcx: self, span, param_env },
             ct,
             target_vid,
             ty::Variance::Invariant,
         )?;
 
-        // FIXME(generic_const_exprs): Occurs check failures for unevaluated
-        // constants and generic expressions are not yet handled correctly.
-        let value = value.may_be_infer();
-
         self.inner.borrow_mut().const_unification_table().union_value(
             target_vid,
             ConstVarValue {
@@ -449,7 +447,7 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
         // `'?2` and `?3` are fresh region/type inference
         // variables. (Down below, we will relate `a_ty <: b_ty`,
         // adding constraints like `'x: '?2` and `?1 <: ?3`.)
-        let Generalization { value, needs_wf } = generalize::generalize(
+        let Generalization { value_may_be_infer: b_ty, needs_wf } = generalize::generalize(
             self.infcx,
             &mut CombineDelegate {
                 infcx: self.infcx,
@@ -461,7 +459,6 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
             ambient_variance,
         )?;
 
-        let b_ty = value.may_be_infer(); // we handle this further down.
         self.infcx.inner.borrow_mut().type_variables().instantiate(b_vid, b_ty);
 
         if needs_wf {
@@ -501,6 +498,11 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
                     }
                 }
             }
+
+            // FIXME: This does not handle subtyping correctly, we should switch to
+            // alias-relate in the new solver and could instead create a new inference
+            // variable for `a_ty`, emitting `Projection(a_ty, a_infer)` and
+            // `a_infer <: b_ty`.
             self.obligations.push(Obligation::new(
                 self.tcx(),
                 self.trace.cause.clone(),

compiler/rustc_infer/src/infer/generalize.rs

@@ -47,9 +47,9 @@ pub(super) fn generalize<'tcx, D: GeneralizerDelegate<'tcx>, T: Into<Term<'tcx>>
     };
 
     assert!(!term.has_escaping_bound_vars());
-    let value = generalizer.relate(term, term)?;
+    let value_may_be_infer = generalizer.relate(term, term)?;
     let needs_wf = generalizer.needs_wf;
-    Ok(Generalization { value: HandleProjection(value), needs_wf })
+    Ok(Generalization { value_may_be_infer, needs_wf })
 }
 
 /// Abstracts the handling of region vars between HIR and MIR/NLL typechecking
@@ -500,20 +500,20 @@ where
     }
 }
 
-#[derive(Debug)]
-pub(super) struct HandleProjection<T>(T);
-impl<T> HandleProjection<T> {
-    pub(super) fn may_be_infer(self) -> T {
-        self.0
-    }
-}
-
 /// Result from a generalization operation. This includes
 /// not only the generalized type, but also a bool flag
 /// indicating whether further WF checks are needed.
 #[derive(Debug)]
 pub(super) struct Generalization<T> {
-    pub(super) value: HandleProjection<T>,
+    /// When generalizing `<?0 as Trait>::Assoc` or
+    /// `<T as Bar<<?0 as Foo>::Assoc>>::Assoc`
+    /// for `?0` generalization returns an inference
+    /// variable.
+    ///
+    /// This has to be handled wotj care as it can
+    /// otherwise very easily result in infinite
+    /// recursion.
+    pub(super) value_may_be_infer: T,
 
     /// If true, then the generalized type may not be well-formed,
     /// even if the source type is well-formed, so we should add an

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

@@ -217,15 +217,14 @@ where
     }
 
     fn generalize(&mut self, ty: Ty<'tcx>, for_vid: ty::TyVid) -> RelateResult<'tcx, Ty<'tcx>> {
-        let Generalization { value, needs_wf: _ } = generalize::generalize(
+        let Generalization { value_may_be_infer: ty, needs_wf: _ } = generalize::generalize(
             self.infcx,
             &mut self.delegate,
             ty,
             for_vid,
             self.ambient_variance,
         )?;
 
-        let ty = value.may_be_infer();
         if ty.is_ty_var() {
             span_bug!(self.delegate.span(), "occurs check failure in MIR typeck");
         }