Synthesize substitutions for bad auto traits in dyn types

compiler/rustc_middle/src/ty/generics.rs

@@ -101,6 +101,20 @@ impl GenericParamDef {
             _ => None,
         }
     }
+
+    pub fn to_error<'tcx>(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        preceding_substs: &[ty::GenericArg<'tcx>],
+    ) -> ty::GenericArg<'tcx> {
+        match &self.kind {
+            ty::GenericParamDefKind::Lifetime => tcx.lifetimes.re_static.into(),
+            ty::GenericParamDefKind::Type { .. } => tcx.ty_error().into(),
+            ty::GenericParamDefKind::Const { .. } => {
+                tcx.const_error(tcx.bound_type_of(self.def_id).subst(tcx, preceding_substs)).into()
+            }
+        }
+    }
 }
 
 #[derive(Default)]

compiler/rustc_middle/src/ty/sty.rs

@@ -722,8 +722,17 @@ impl<'tcx> PolyExistentialPredicate<'tcx> {
                 self.rebind(p.with_self_ty(tcx, self_ty)).to_predicate(tcx)
             }
             ExistentialPredicate::AutoTrait(did) => {
-                let trait_ref = self.rebind(tcx.mk_trait_ref(did, [self_ty]));
-                trait_ref.without_const().to_predicate(tcx)
+                let generics = tcx.generics_of(did);
+                let trait_ref = if generics.params.len() == 1 {
+                    tcx.mk_trait_ref(did, [self_ty])
+                } else {
+                    // If this is an ill-formed auto trait, then synthesize
+                    // new error substs for the missing generics.
+                    let err_substs =
+                        ty::InternalSubsts::extend_with_error(tcx, did, &[self_ty.into()]);
+                    tcx.mk_trait_ref(did, err_substs)
+                };
+                self.rebind(trait_ref).without_const().to_predicate(tcx)
             }
         }
     }

compiler/rustc_middle/src/ty/subst.rs

@@ -352,6 +352,22 @@ impl<'tcx> InternalSubsts<'tcx> {
         }
     }
 
+    // Extend an `original_substs` list to the full number of substs expected by `def_id`,
+    // filling in the missing parameters with error ty/ct or 'static regions.
+    pub fn extend_with_error(
+        tcx: TyCtxt<'tcx>,
+        def_id: DefId,
+        original_substs: &[GenericArg<'tcx>],
+    ) -> SubstsRef<'tcx> {
+        ty::InternalSubsts::for_item(tcx, def_id, |def, substs| {
+            if let Some(subst) = original_substs.get(def.index as usize) {
+                *subst
+            } else {
+                def.to_error(tcx, substs)
+            }
+        })
+    }
+
     #[inline]
     pub fn types(&'tcx self) -> impl DoubleEndedIterator<Item = Ty<'tcx>> + 'tcx {
         self.iter()

src/test/ui/auto-traits/bad-generics-on-dyn.rs

@@ -0,0 +1,11 @@
+#![feature(auto_traits)]
+
+auto trait Trait1<'a> {}
+//~^ ERROR auto traits cannot have generic parameters
+
+fn f<'a>(x: &dyn Trait1<'a>)
+{}
+
+fn main() {
+    f(&1);
+}

src/test/ui/auto-traits/bad-generics-on-dyn.stderr

@@ -0,0 +1,11 @@
+error[E0567]: auto traits cannot have generic parameters
+  --> $DIR/bad-generics-on-dyn.rs:3:18
+   |
+LL | auto trait Trait1<'a> {}
+   |            ------^^^^ help: remove the parameters
+   |            |
+   |            auto trait cannot have generic parameters
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0567`.