Convert static lifetime to an nll var

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -1581,12 +1581,18 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                                     .unwrap();
                                 }
                                 (None, None) => {
+                                    // `struct_tail` returns regions which haven't been mapped
+                                    // to nll vars yet so we do it here as `outlives_constraints`
+                                    // expects nll vars.
+                                    let src_lt = self.universal_regions.to_region_vid(src_lt);
+                                    let dst_lt = self.universal_regions.to_region_vid(dst_lt);
+
                                     // The principalless (no non-auto traits) case:
                                     // You can only cast `dyn Send + 'long` to `dyn Send + 'short`.
                                     self.constraints.outlives_constraints.push(
                                         OutlivesConstraint {
-                                            sup: src_lt.as_var(),
-                                            sub: dst_lt.as_var(),
+                                            sup: src_lt,
+                                            sub: dst_lt,
                                             locations: location.to_locations(),
                                             span: location.to_locations().span(self.body),
                                             category: ConstraintCategory::Cast {

tests/ui/cast/ptr-to-ptr-unsized-adt-tail-with-static-region.rs

@@ -0,0 +1,15 @@
+//@ check-pass
+
+// During MIR typeck when casting `*mut dyn Sync + '?x` to
+// `*mut Wrap` we compute the tail of `Wrap` as `dyn Sync + 'static`.
+//
+// This test ensures that we first convert the `'static` lifetime to
+// the nll var `'?0` before introducing the region constraint `'?x: 'static`.
+
+struct Wrap(dyn Sync + 'static);
+
+fn cast(x: *mut (dyn Sync + 'static)) {
+    x as *mut Wrap;
+}
+
+fn main() {}