Erase regions even if normalization fails in writeback

compiler/rustc_hir_typeck/src/writeback.rs

@@ -136,7 +136,10 @@ fn tcx(&self) -> TyCtxt<'tcx> {
 
     fn write_ty_to_typeck_results(&mut self, hir_id: hir::HirId, ty: Ty<'tcx>) {
         debug!("write_ty_to_typeck_results({:?}, {:?})", hir_id, ty);
-        assert!(!ty.has_infer() && !ty.has_placeholders() && !ty.has_free_regions());
+        assert!(
+            !ty.has_infer() && !ty.has_placeholders() && !ty.has_free_regions(),
+            "{ty} can't be put into typeck results"
+        );
         self.typeck_results.node_types_mut().insert(hir_id, ty);
     }
 
@@ -803,7 +806,11 @@ fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
                 // We must normalize erasing regions here, since later lints
                 // expect that types that show up in the typeck are fully
                 // normalized.
-                self.fcx.tcx.try_normalize_erasing_regions(self.fcx.param_env, t).unwrap_or(t)
+                if let Ok(t) = self.fcx.tcx.try_normalize_erasing_regions(self.fcx.param_env, t) {
+                    t
+                } else {
+                    EraseEarlyRegions { tcx: self.fcx.tcx }.fold_ty(t)
+                }
             }
             Ok(t) => {
                 // Do not anonymize late-bound regions

tests/ui/issues/issue-20605.current.stderr

@@ -1,5 +1,5 @@
 error[E0277]: the size for values of type `dyn Iterator<Item = &'a mut u8>` cannot be known at compilation time
-  --> $DIR/issue-20605.rs:2:17
+  --> $DIR/issue-20605.rs:5:17
    |
 LL |     for item in *things { *item = 0 }
    |                 ^^^^^^^ the trait `IntoIterator` is not implemented for `dyn Iterator<Item = &'a mut u8>`

tests/ui/issues/issue-20605.next.stderr

@@ -0,0 +1,25 @@
+error[E0277]: the trait bound `dyn Iterator<Item = &'a mut u8>: IntoIterator` is not satisfied
+  --> $DIR/issue-20605.rs:5:17
+   |
+LL |     for item in *things { *item = 0 }
+   |                 ^^^^^^^ the trait `IntoIterator` is not implemented for `dyn Iterator<Item = &'a mut u8>`
+
+error[E0277]: the size for values of type `<dyn Iterator<Item = &'a mut u8> as IntoIterator>::IntoIter` cannot be known at compilation time
+  --> $DIR/issue-20605.rs:5:17
+   |
+LL |     for item in *things { *item = 0 }
+   |                 ^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `<dyn Iterator<Item = &'a mut u8> as IntoIterator>::IntoIter`
+   = note: all local variables must have a statically known size
+   = help: unsized locals are gated as an unstable feature
+
+error: the type `<_ as IntoIterator>::IntoIter` is not well-formed
+  --> $DIR/issue-20605.rs:5:17
+   |
+LL |     for item in *things { *item = 0 }
+   |                 ^^^^^^^
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/issues/issue-20605.rs

@@ -1,6 +1,11 @@
+// revisions: current next
+//[next] compile-flags: -Ztrait-solver=next
+
 fn changer<'a>(mut things: Box<dyn Iterator<Item=&'a mut u8>>) {
     for item in *things { *item = 0 }
 //~^ ERROR the size for values of type
+//[next]~^^ ERROR the type `<_ as IntoIterator>::IntoIter` is not well-formed
+//[next]~| ERROR the trait bound `dyn Iterator<Item = &'a mut u8>: IntoIterator` is not satisfied
 }
 
 fn main() {}