ty: add ty::ConstKind::Error to replace tcx.consts.err.

src/librustc_infer/infer/freshen.rs

@@ -251,7 +251,10 @@ impl<'a, 'tcx> TypeFolder<'tcx> for TypeFreshener<'a, 'tcx> {
                 bug!("unexpected const {:?}", ct)
             }
 
-            ty::ConstKind::Param(_) | ty::ConstKind::Value(_) | ty::ConstKind::Unevaluated(..) => {}
+            ty::ConstKind::Param(_)
+            | ty::ConstKind::Value(_)
+            | ty::ConstKind::Unevaluated(..)
+            | ty::ConstKind::Error => {}
         }
 
         ct.super_fold_with(self)

src/librustc_infer/infer/resolve.rs

@@ -227,7 +227,7 @@ impl<'a, 'tcx> TypeFolder<'tcx> for FullTypeResolver<'a, 'tcx> {
             match c.val {
                 ty::ConstKind::Infer(InferConst::Var(vid)) => {
                     self.err = Some(FixupError::UnresolvedConst(vid));
-                    return self.tcx().consts.err;
+                    return self.tcx().mk_const(ty::Const { val: ty::ConstKind::Error, ty: c.ty });
                 }
                 ty::ConstKind::Infer(InferConst::Fresh(_)) => {
                     bug!("Unexpected const in full const resolver: {:?}", c);

src/librustc_middle/ty/context.rs

@@ -182,7 +182,7 @@ pub struct CommonLifetimes<'tcx> {
 }
 
 pub struct CommonConsts<'tcx> {
-    pub err: &'tcx Const<'tcx>,
+    pub unit: &'tcx Const<'tcx>,
 }
 
 pub struct LocalTableInContext<'a, V> {
@@ -858,9 +858,9 @@ impl<'tcx> CommonConsts<'tcx> {
         let mk_const = |c| interners.const_.intern(c, |c| Interned(interners.arena.alloc(c))).0;
 
         CommonConsts {
-            err: mk_const(ty::Const {
+            unit: mk_const(ty::Const {
                 val: ty::ConstKind::Value(ConstValue::Scalar(Scalar::zst())),
-                ty: types.err,
+                ty: types.unit,
             }),
         }
     }

src/librustc_middle/ty/flags.rs

@@ -70,14 +70,7 @@ impl FlagComputation {
             | &ty::Str
             | &ty::Foreign(..) => {}
 
-            // You might think that we could just return Error for
-            // any type containing Error as a component, and get
-            // rid of the TypeFlags::HAS_TY_ERR flag -- likewise for ty_bot (with
-            // the exception of function types that return bot).
-            // But doing so caused sporadic memory corruption, and
-            // neither I (tjc) nor nmatsakis could figure out why,
-            // so we're doing it this way.
-            &ty::Error => self.add_flags(TypeFlags::HAS_TY_ERR),
+            &ty::Error => self.add_flags(TypeFlags::HAS_ERROR),
 
             &ty::Param(_) => {
                 self.add_flags(TypeFlags::HAS_TY_PARAM);
@@ -239,6 +232,7 @@ impl FlagComputation {
                 self.add_flags(TypeFlags::STILL_FURTHER_SPECIALIZABLE);
             }
             ty::ConstKind::Value(_) => {}
+            ty::ConstKind::Error => self.add_flags(TypeFlags::HAS_ERROR),
         }
     }
 

src/librustc_middle/ty/fold.rs

@@ -82,7 +82,7 @@ pub trait TypeFoldable<'tcx>: fmt::Debug + Clone {
         self.has_type_flags(TypeFlags::HAS_TY_OPAQUE)
     }
     fn references_error(&self) -> bool {
-        self.has_type_flags(TypeFlags::HAS_TY_ERR)
+        self.has_type_flags(TypeFlags::HAS_ERROR)
     }
     fn has_param_types_or_consts(&self) -> bool {
         self.has_type_flags(TypeFlags::HAS_TY_PARAM | TypeFlags::HAS_CT_PARAM)

src/librustc_middle/ty/mod.rs

@@ -567,8 +567,8 @@ bitflags! {
                                           | TypeFlags::HAS_TY_OPAQUE.bits
                                           | TypeFlags::HAS_CT_PROJECTION.bits;
 
-        /// Is an error type reachable?
-        const HAS_TY_ERR                  = 1 << 13;
+        /// Is an error type/const reachable?
+        const HAS_ERROR                   = 1 << 13;
 
         /// Does this have any region that "appears free" in the type?
         /// Basically anything but [ReLateBound] and [ReErased].

src/librustc_middle/ty/print/pretty.rs

@@ -938,6 +938,7 @@ pub trait PrettyPrinter<'tcx>:
                 self.pretty_print_bound_var(debruijn, bound_var)?
             }
             ty::ConstKind::Placeholder(placeholder) => p!(write("Placeholder({:?})", placeholder)),
+            ty::ConstKind::Error => p!(write("[const error]")),
         };
         Ok(self)
     }

src/librustc_middle/ty/relate.rs

@@ -510,12 +510,21 @@ pub fn super_relate_consts<R: TypeRelation<'tcx>>(
     let tcx = relation.tcx();
 
     let eagerly_eval = |x: &'tcx ty::Const<'tcx>| {
+        // FIXME(eddyb) this doesn't account for lifetime inference variables
+        // being erased by `eval`, *nor* for the polymorphic aspect of `eval`.
+        // That is, we could always use `eval` and it will just return the
+        // old value back if it doesn't succeed.
         if !x.val.needs_infer() {
             return x.eval(tcx, relation.param_env()).val;
         }
         x.val
     };
 
+    // FIXME(eddyb) doesn't look like everything below checks that `a.ty == b.ty`.
+    // We could probably always assert it early, as `const` generic parameters
+    // are not allowed to depend on other generic parameters, i.e. are concrete.
+    // (although there could be normalization differences)
+
     // Currently, the values that can be unified are primitive types,
     // and those that derive both `PartialEq` and `Eq`, corresponding
     // to structural-match types.
@@ -524,6 +533,9 @@ pub fn super_relate_consts<R: TypeRelation<'tcx>>(
             // The caller should handle these cases!
             bug!("var types encountered in super_relate_consts: {:?} {:?}", a, b)
         }
+
+        (ty::ConstKind::Error, _) | (_, ty::ConstKind::Error) => Ok(ty::ConstKind::Error),
+
         (ty::ConstKind::Param(a_p), ty::ConstKind::Param(b_p)) if a_p.index == b_p.index => {
             return Ok(a);
         }

src/librustc_middle/ty/structural_impls.rs

@@ -1022,9 +1022,10 @@ impl<'tcx> TypeFoldable<'tcx> for ty::ConstKind<'tcx> {
             ty::ConstKind::Unevaluated(did, substs, promoted) => {
                 ty::ConstKind::Unevaluated(did, substs.fold_with(folder), promoted)
             }
-            ty::ConstKind::Value(_) | ty::ConstKind::Bound(..) | ty::ConstKind::Placeholder(..) => {
-                *self
-            }
+            ty::ConstKind::Value(_)
+            | ty::ConstKind::Bound(..)
+            | ty::ConstKind::Placeholder(..)
+            | ty::ConstKind::Error => *self,
         }
     }
 
@@ -1033,9 +1034,10 @@ impl<'tcx> TypeFoldable<'tcx> for ty::ConstKind<'tcx> {
             ty::ConstKind::Infer(ic) => ic.visit_with(visitor),
             ty::ConstKind::Param(p) => p.visit_with(visitor),
             ty::ConstKind::Unevaluated(_, substs, _) => substs.visit_with(visitor),
-            ty::ConstKind::Value(_) | ty::ConstKind::Bound(..) | ty::ConstKind::Placeholder(_) => {
-                false
-            }
+            ty::ConstKind::Value(_)
+            | ty::ConstKind::Bound(..)
+            | ty::ConstKind::Placeholder(_)
+            | ty::ConstKind::Error => false,
         }
     }
 }

src/librustc_middle/ty/sty.rs

@@ -2429,6 +2429,10 @@ pub enum ConstKind<'tcx> {
 
     /// Used to hold computed value.
     Value(ConstValue<'tcx>),
+
+    /// A placeholder for a const which could not be computed; this is
+    /// propagated to avoid useless error messages.
+    Error,
 }
 
 #[cfg(target_arch = "x86_64")]

src/librustc_middle/ty/walk.rs

@@ -170,7 +170,8 @@ fn push_inner<'tcx>(stack: &mut TypeWalkerStack<'tcx>, parent: GenericArg<'tcx>)
                 | ty::ConstKind::Param(_)
                 | ty::ConstKind::Placeholder(_)
                 | ty::ConstKind::Bound(..)
-                | ty::ConstKind::Value(_) => {}
+                | ty::ConstKind::Value(_)
+                | ty::ConstKind::Error => {}
 
                 ty::ConstKind::Unevaluated(_, substs, _) => {
                     stack.extend(substs.iter().copied().rev());

src/librustc_mir/interpret/operand.rs

@@ -518,6 +518,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         // Early-return cases.
         let val_val = match val.val {
             ty::ConstKind::Param(_) => throw_inval!(TooGeneric),
+            ty::ConstKind::Error => throw_inval!(TypeckError),
             ty::ConstKind::Unevaluated(def_id, substs, promoted) => {
                 let instance = self.resolve(def_id, substs)?;
                 // We use `const_eval` here and `const_eval_raw` elsewhere in mir interpretation.

src/librustc_trait_selection/opaque_types.rs

@@ -972,7 +972,7 @@ impl TypeFolder<'tcx> for ReverseMapper<'tcx> {
                             )
                             .emit();
 
-                        self.tcx().consts.err
+                        self.tcx().mk_const(ty::Const { val: ty::ConstKind::Error, ty: ct.ty })
                     }
                 }
             }

src/librustc_typeck/astconv.rs

@@ -826,14 +826,14 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                         }
                     }
                     GenericParamDefKind::Const => {
+                        let ty = tcx.at(span).type_of(param.def_id);
                         // FIXME(const_generics:defaults)
                         if infer_args {
                             // No const parameters were provided, we can infer all.
-                            let ty = tcx.at(span).type_of(param.def_id);
                             self.ct_infer(ty, Some(param), span).into()
                         } else {
                             // We've already errored above about the mismatch.
-                            tcx.consts.err.into()
+                            tcx.mk_const(ty::Const { val: ty::ConstKind::Error, ty }).into()
                         }
                     }
                 }

src/librustc_typeck/check/writeback.rs

@@ -673,7 +673,7 @@ impl<'cx, 'tcx> TypeFolder<'tcx> for Resolver<'cx, 'tcx> {
                 // FIXME: we'd like to use `self.report_error`, but it doesn't yet
                 // accept a &'tcx ty::Const.
                 self.replaced_with_error = true;
-                self.tcx().consts.err
+                self.tcx().mk_const(ty::Const { val: ty::ConstKind::Error, ty: ct.ty })
             }
         }
     }

src/librustc_typeck/collect.rs

@@ -316,13 +316,13 @@ impl AstConv<'tcx> for ItemCtxt<'tcx> {
 
     fn ct_infer(
         &self,
-        _: Ty<'tcx>,
+        ty: Ty<'tcx>,
         _: Option<&ty::GenericParamDef>,
         span: Span,
     ) -> &'tcx Const<'tcx> {
         bad_placeholder_type(self.tcx(), vec![span]).emit();
 
-        self.tcx().consts.err
+        self.tcx().mk_const(ty::Const { val: ty::ConstKind::Error, ty })
     }
 
     fn projected_ty_from_poly_trait_ref(
@@ -2037,7 +2037,8 @@ fn associated_item_predicates(
             }
             ty::GenericParamDefKind::Const => {
                 unimplemented_error("const");
-                tcx.consts.err.into()
+                tcx.mk_const(ty::Const { val: ty::ConstKind::Error, ty: tcx.type_of(param.def_id) })
+                    .into()
             }
         }
     };