Fix an issue with const inference variables sticking around under Chalk + NLL

src/librustc/infer/combine.rs

@@ -602,19 +602,15 @@ impl TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
     ) -> RelateResult<'tcx, &'tcx ty::Const<'tcx>> {
         assert_eq!(c, c2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
 
-        match c {
-            ty::Const { val: ConstValue::Infer(InferConst::Var(vid)), .. } => {
+        match c.val {
+            ConstValue::Infer(InferConst::Var(vid)) => {
                 let mut variable_table = self.infcx.const_unification_table.borrow_mut();
-                match variable_table.probe_value(*vid).val.known() {
-                    Some(u) => {
-                        self.relate(&u, &u)
-                    }
+                match variable_table.probe_value(vid).val.known() {
+                    Some(u) => self.relate(&u, &u),
                     None => Ok(c),
                 }
             }
-            _ => {
-                relate::super_relate_consts(self, c, c)
-            }
+            _ => relate::super_relate_consts(self, c, c),
         }
     }
 }

src/librustc/infer/nll_relate/mod.rs

@@ -27,7 +27,7 @@ use crate::ty::error::TypeError;
 use crate::ty::fold::{TypeFoldable, TypeVisitor};
 use crate::ty::relate::{self, Relate, RelateResult, TypeRelation};
 use crate::ty::subst::GenericArg;
-use crate::ty::{self, Ty, TyCtxt};
+use crate::ty::{self, Ty, TyCtxt, InferConst};
 use crate::mir::interpret::ConstValue;
 use rustc_data_structures::fx::FxHashMap;
 use std::fmt::Debug;
@@ -616,15 +616,20 @@ where
     fn consts(
         &mut self,
         a: &'tcx ty::Const<'tcx>,
-        b: &'tcx ty::Const<'tcx>,
+        mut b: &'tcx ty::Const<'tcx>,
     ) -> RelateResult<'tcx, &'tcx ty::Const<'tcx>> {
-        if let ty::Const { val: ConstValue::Bound(..), .. } = a {
-            // FIXME(const_generics): I'm unsure how this branch should actually be handled,
-            // so this is probably not correct.
-            self.infcx.super_combine_consts(self, a, b)
-        } else {
-            debug!("consts(a={:?}, b={:?}, variance={:?})", a, b, self.ambient_variance);
-            relate::super_relate_consts(self, a, b)
+        let a = self.infcx.shallow_resolve(a);
+
+        if !D::forbid_inference_vars() {
+            b = self.infcx.shallow_resolve(b);
+        }
+
+        match b.val {
+            ConstValue::Infer(InferConst::Var(_)) if D::forbid_inference_vars() => {
+                // Forbid inference variables in the RHS.
+                bug!("unexpected inference var {:?}", b)
+            }
+            _ => self.infcx.super_combine_consts(self, a, b)
         }
     }
 
@@ -991,15 +996,15 @@ where
         a: &'tcx ty::Const<'tcx>,
         _: &'tcx ty::Const<'tcx>,
     ) -> RelateResult<'tcx, &'tcx ty::Const<'tcx>> {
-        debug!("TypeGeneralizer::consts(a={:?})", a);
-
-        if let ty::Const { val: ConstValue::Bound(..), .. } = a {
-            bug!(
-                "unexpected inference variable encountered in NLL generalization: {:?}",
-                a
-            );
-        } else {
-            relate::super_relate_consts(self, a, a)
+        match a.val {
+            ConstValue::Infer(InferConst::Var(vid)) => {
+                let mut variable_table = self.infcx.const_unification_table.borrow_mut();
+                match variable_table.probe_value(vid).val.known() {
+                    Some(u) => self.relate(&u, &u),
+                    None => Ok(a),
+                }
+            }
+            _ => relate::super_relate_consts(self, a, a),
         }
     }
 

src/test/ui/const-generics/issues/issue-65675.rs

@@ -0,0 +1,10 @@
+// run-pass
+// compile-flags: -Z chalk
+
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
+
+pub struct Foo<T, const N: usize>([T; N]);
+impl<T, const N: usize> Foo<T, {N}> {}
+
+fn main() {}

src/test/ui/const-generics/issues/issue-65675.stderr

@@ -0,0 +1,8 @@
+warning: the feature `const_generics` is incomplete and may cause the compiler to crash
+  --> $DIR/issue-65675.rs:4:12
+   |
+LL | #![feature(const_generics)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+