Allow &raw [mut | const] for union field in safe code

compiler/rustc_mir_build/src/check_unsafety.rs

@@ -554,6 +554,21 @@ impl<'a, 'tcx> Visitor<'a, 'tcx> for UnsafetyVisitor<'a, 'tcx> {
                     visit::walk_expr(self, &self.thir[arg]);
                     return;
                 }
+
+                // Secondly, we allow raw borrows of union field accesses. Peel
+                // any of those off, and recurse normally on the LHS, which should
+                // reject any unsafe operations within.
+                let mut peeled = arg;
+                while let ExprKind::Scope { value: arg, .. } = self.thir[peeled].kind
+                    && let ExprKind::Field { lhs, name: _, variant_index: _ } = self.thir[arg].kind
+                    && let ty::Adt(def, _) = &self.thir[lhs].ty.kind()
+                    && def.is_union()
+                {
+                    peeled = lhs;
+                }
+                visit::walk_expr(self, &self.thir[peeled]);
+                // And return so we don't recurse directly onto the union field access(es).
+                return;
             }
             ExprKind::Deref { arg } => {
                 if let ExprKind::StaticRef { def_id, .. } | ExprKind::ThreadLocalRef(def_id) =

src/tools/miri/tests/pass/both_borrows/smallvec.rs

@@ -25,7 +25,7 @@ impl<T, const N: usize> RawSmallVec<T, N> {
     }
 
     const fn as_mut_ptr_inline(&mut self) -> *mut T {
-        (unsafe { &raw mut self.inline }) as *mut T
+        &raw mut self.inline as *mut T
     }
 
     const unsafe fn as_mut_ptr_heap(&mut self) -> *mut T {

tests/ui/union/union-unsafe.rs

@@ -1,5 +1,6 @@
 use std::cell::RefCell;
 use std::mem::ManuallyDrop;
+use std::ops::Deref;
 
 union U1 {
     a: u8,
@@ -17,6 +18,10 @@ union U4<T: Copy> {
     a: T,
 }
 
+union U5 {
+    a: usize,
+}
+
 union URef {
     p: &'static mut i32,
 }
@@ -31,6 +36,20 @@ fn deref_union_field(mut u: URef) {
     *(u.p) = 13; //~ ERROR access to union field is unsafe
 }
 
+union A {
+    a: usize,
+    b: &'static &'static B,
+}
+
+union B {
+    c: usize,
+}
+
+fn raw_deref_union_field(mut u: URef) {
+    // This is unsafe because we first dereference u.p (reading uninitialized memory)
+    let _p = &raw const *(u.p); //~ ERROR access to union field is unsafe
+}
+
 fn assign_noncopy_union_field(mut u: URefCell) {
     u.a = (ManuallyDrop::new(RefCell::new(0)), 1); // OK (assignment does not drop)
     u.a.0 = ManuallyDrop::new(RefCell::new(0)); // OK (assignment does not drop)
@@ -57,6 +76,20 @@ fn main() {
     let a = u1.a; //~ ERROR access to union field is unsafe
     u1.a = 11; // OK
 
+    let mut u2 = U1 { a: 10 };
+    let a = &raw mut u2.a; // OK
+    unsafe { *a = 3 };
+
+    let mut u3 = U1 { a: 10 };
+    let a = std::ptr::addr_of_mut!(u3.a); // OK
+    unsafe { *a = 14 };
+
+    let u4 = U5 { a: 2 };
+    let vec = vec![1, 2, 3];
+    // This is unsafe because we read u4.a (potentially uninitialized memory)
+    // to use as an array index
+    let _a = &raw const vec[u4.a]; //~ ERROR access to union field is unsafe
+
     let U1 { a } = u1; //~ ERROR access to union field is unsafe
     if let U1 { a: 12 } = u1 {} //~ ERROR access to union field is unsafe
     if let Some(U1 { a: 13 }) = Some(u1) {} //~ ERROR access to union field is unsafe
@@ -73,4 +106,44 @@ fn main() {
     let mut u3 = U3 { a: ManuallyDrop::new(String::from("old")) }; // OK
     u3.a = ManuallyDrop::new(String::from("new")); // OK (assignment does not drop)
     *u3.a = String::from("new"); //~ ERROR access to union field is unsafe
+
+    let mut unions = [U1 { a: 1 }, U1 { a: 2 }];
+
+    // Array indexing + union field raw borrow - should be OK
+    let ptr = &raw mut unions[0].a; // OK
+    let ptr2 = &raw const unions[1].a; // OK
+
+    let a = A { a: 0 };
+    let _p = &raw const (**a.b).c; //~ ERROR access to union field is unsafe
+
+    arbitrary_deref();
+}
+
+// regression test for https://github.com/rust-lang/rust/pull/141469#discussion_r2312546218
+fn arbitrary_deref() {
+    use std::ops::Deref;
+
+    union A {
+        a: usize,
+        b: B,
+    }
+
+    #[derive(Copy, Clone)]
+    struct B(&'static str);
+
+    impl Deref for B {
+        type Target = C;
+
+        fn deref(&self) -> &C {
+            println!("{:?}", self.0);
+            &C { c: 0 }
+        }
+    }
+
+    union C {
+        c: usize,
+    }
+
+    let a = A { a: 0 };
+    let _p = &raw const (*a.b).c; //~ ERROR access to union field is unsafe
 }

tests/ui/union/union-unsafe.stderr

@@ -1,83 +1,115 @@
 error[E0133]: access to union field is unsafe and requires unsafe function or block
-  --> $DIR/union-unsafe.rs:31:6
+  --> $DIR/union-unsafe.rs:36:6
    |
 LL |     *(u.p) = 13;
    |      ^^^^^ access to union field
    |
    = note: the field may not be properly initialized: using uninitialized data will cause undefined behavior
 
 error[E0133]: access to union field is unsafe and requires unsafe function or block
-  --> $DIR/union-unsafe.rs:43:6
+  --> $DIR/union-unsafe.rs:50:26
+   |
+LL |     let _p = &raw const *(u.p);
+   |                          ^^^^^ access to union field
+   |
+   = note: the field may not be properly initialized: using uninitialized data will cause undefined behavior
+
+error[E0133]: access to union field is unsafe and requires unsafe function or block
+  --> $DIR/union-unsafe.rs:62:6
    |
 LL |     *u3.a = T::default();
    |      ^^^^ access to union field
    |
    = note: the field may not be properly initialized: using uninitialized data will cause undefined behavior
 
 error[E0133]: access to union field is unsafe and requires unsafe function or block
-  --> $DIR/union-unsafe.rs:49:6
+  --> $DIR/union-unsafe.rs:68:6
    |
 LL |     *u3.a = T::default();
    |      ^^^^ access to union field
    |
    = note: the field may not be properly initialized: using uninitialized data will cause undefined behavior
 
 error[E0133]: access to union field is unsafe and requires unsafe function or block
-  --> $DIR/union-unsafe.rs:57:13
+  --> $DIR/union-unsafe.rs:76:13
    |
 LL |     let a = u1.a;
    |             ^^^^ access to union field
    |
    = note: the field may not be properly initialized: using uninitialized data will cause undefined behavior
 
 error[E0133]: access to union field is unsafe and requires unsafe function or block
-  --> $DIR/union-unsafe.rs:60:14
+  --> $DIR/union-unsafe.rs:91:29
+   |
+LL |     let _a = &raw const vec[u4.a];
+   |                             ^^^^ access to union field
+   |
+   = note: the field may not be properly initialized: using uninitialized data will cause undefined behavior
+
+error[E0133]: access to union field is unsafe and requires unsafe function or block
+  --> $DIR/union-unsafe.rs:93:14
    |
 LL |     let U1 { a } = u1;
    |              ^ access to union field
    |
    = note: the field may not be properly initialized: using uninitialized data will cause undefined behavior
 
 error[E0133]: access to union field is unsafe and requires unsafe function or block
-  --> $DIR/union-unsafe.rs:61:20
+  --> $DIR/union-unsafe.rs:94:20
    |
 LL |     if let U1 { a: 12 } = u1 {}
    |                    ^^ access to union field
    |
    = note: the field may not be properly initialized: using uninitialized data will cause undefined behavior
 
 error[E0133]: access to union field is unsafe and requires unsafe function or block
-  --> $DIR/union-unsafe.rs:62:25
+  --> $DIR/union-unsafe.rs:95:25
    |
 LL |     if let Some(U1 { a: 13 }) = Some(u1) {}
    |                         ^^ access to union field
    |
    = note: the field may not be properly initialized: using uninitialized data will cause undefined behavior
 
 error[E0133]: access to union field is unsafe and requires unsafe function or block
-  --> $DIR/union-unsafe.rs:67:6
+  --> $DIR/union-unsafe.rs:100:6
    |
 LL |     *u2.a = String::from("new");
    |      ^^^^ access to union field
    |
    = note: the field may not be properly initialized: using uninitialized data will cause undefined behavior
 
 error[E0133]: access to union field is unsafe and requires unsafe function or block
-  --> $DIR/union-unsafe.rs:71:6
+  --> $DIR/union-unsafe.rs:104:6
    |
 LL |     *u3.a = 1;
    |      ^^^^ access to union field
    |
    = note: the field may not be properly initialized: using uninitialized data will cause undefined behavior
 
 error[E0133]: access to union field is unsafe and requires unsafe function or block
-  --> $DIR/union-unsafe.rs:75:6
+  --> $DIR/union-unsafe.rs:108:6
    |
 LL |     *u3.a = String::from("new");
    |      ^^^^ access to union field
    |
    = note: the field may not be properly initialized: using uninitialized data will cause undefined behavior
 
-error: aborting due to 10 previous errors
+error[E0133]: access to union field is unsafe and requires unsafe function or block
+  --> $DIR/union-unsafe.rs:117:28
+   |
+LL |     let _p = &raw const (**a.b).c;
+   |                            ^^^ access to union field
+   |
+   = note: the field may not be properly initialized: using uninitialized data will cause undefined behavior
+
+error[E0133]: access to union field is unsafe and requires unsafe function or block
+  --> $DIR/union-unsafe.rs:148:27
+   |
+LL |     let _p = &raw const (*a.b).c;
+   |                           ^^^ access to union field
+   |
+   = note: the field may not be properly initialized: using uninitialized data will cause undefined behavior
+
+error: aborting due to 14 previous errors
 
 For more information about this error, try `rustc --explain E0133`.