Call is_freeze less in unsafety-checking

This is to avoid cycles when calling `is_freeze` on an opaque type.

Author: matthewjasper

src/librustc_mir/transform/check_unsafety.rs

@@ -190,6 +190,16 @@ impl<'a, 'tcx> Visitor<'tcx> for UnsafetyChecker<'a, 'tcx> {
     }
 
     fn visit_place(&mut self, place: &Place<'tcx>, context: PlaceContext, _location: Location) {
+        // prevent
+        // * `&mut x.field`
+        // * `x.field = y;`
+        // * `&x.field` if `field`'s type has interior mutability
+        // because either of these would allow modifying the layout constrained field and
+        // insert values that violate the layout constraints.
+        if context.is_mutating_use() || context.is_borrow() {
+            self.check_mut_borrowing_layout_constrained_field(place, context.is_mutating_use());
+        }
+
         for (i, elem) in place.projection.iter().enumerate() {
             let proj_base = &place.projection[..i];
 
@@ -216,21 +226,6 @@ impl<'a, 'tcx> Visitor<'tcx> for UnsafetyChecker<'a, 'tcx> {
                     );
                 }
             }
-            let is_borrow_of_interior_mut = context.is_borrow()
-                && !Place::ty_from(&place.local, proj_base, self.body, self.tcx).ty.is_freeze(
-                    self.tcx,
-                    self.param_env,
-                    self.source_info.span,
-                );
-            // prevent
-            // * `&mut x.field`
-            // * `x.field = y;`
-            // * `&x.field` if `field`'s type has interior mutability
-            // because either of these would allow modifying the layout constrained field and
-            // insert values that violate the layout constraints.
-            if context.is_mutating_use() || is_borrow_of_interior_mut {
-                self.check_mut_borrowing_layout_constrained_field(place, context.is_mutating_use());
-            }
             let old_source_info = self.source_info;
             if let (local, []) = (&place.local, proj_base) {
                 let decl = &self.body.local_decls[*local];
@@ -412,6 +407,9 @@ impl<'a, 'tcx> UnsafetyChecker<'a, 'tcx> {
             cursor = proj_base;
 
             match elem {
+                // Modifications behind a dereference don't affect the value of
+                // the pointer.
+                ProjectionElem::Deref => return,
                 ProjectionElem::Field(..) => {
                     let ty =
                         Place::ty_from(&place.local, proj_base, &self.body.local_decls, self.tcx)
@@ -426,14 +424,23 @@ impl<'a, 'tcx> UnsafetyChecker<'a, 'tcx> {
                                         "mutating layout constrained fields cannot statically be \
                                         checked for valid values",
                                     )
-                                } else {
+
+                                // Check `is_freeze` as late as possible to avoid cycle errors
+                                // with opaque types.
+                                } else if !place.ty(self.body, self.tcx).ty.is_freeze(
+                                    self.tcx,
+                                    self.param_env,
+                                    self.source_info.span,
+                                ) {
                                     (
                                         "borrow of layout constrained field with interior \
                                         mutability",
                                         "references to fields of layout constrained fields \
                                         lose the constraints. Coupled with interior mutability, \
                                         the field can be changed to invalid values",
                                     )
+                                } else {
+                                    continue;
                                 };
                                 let source_info = self.source_info;
                                 self.register_violations(

src/test/ui/impl-trait/unsafety-checking-cycle.rs

@@ -0,0 +1,32 @@
+// Ensure that we don't get a cycle error from trying to determine whether an
+// opaque type implements `Freeze` in safety checking, when it doesn't matter.
+
+// check-pass
+
+#![feature(rustc_attrs)]
+
+struct AnyValue<T>(T);
+
+// No need to check for `Freeze` here, there's no
+// `rustc_layout_scalar_valid_range_start` involved.
+fn not_restricted(c: bool) -> impl Sized {
+    if c {
+        let x = AnyValue(not_restricted(false));
+        &x.0;
+    }
+    2u32
+}
+
+#[rustc_layout_scalar_valid_range_start(1)]
+struct NonZero<T>(T);
+
+// No need to check for `Freeze` here, we're not borrowing the field.
+fn not_field(c: bool) -> impl Sized {
+    if c {
+        let x = unsafe { NonZero(not_field(false)) };
+        &x;
+    }
+    5u32
+}
+
+fn main() {}