Handle field projections like slice indexing in invalid_reference_casting

(cherry picked from commit 0ca1a94)

Author: saethlin

compiler/rustc_lint/src/reference_casting.rs

@@ -201,7 +201,8 @@ fn is_cast_to_bigger_memory_layout<'tcx>(
 
     // if the current expr looks like this `&mut expr[index]` then just looking
     // at `expr[index]` won't give us the underlying allocation, so we just skip it
-    if let ExprKind::Index(..) = e_alloc.kind {
+    // the same logic applies field access like `&mut expr.field`
+    if let ExprKind::Index(..) | ExprKind::Field(..) = e_alloc.kind {
         return None;
     }
 

tests/ui/lint/reference_casting.rs

@@ -255,6 +255,12 @@ unsafe fn bigger_layout() {
         let a3 = a2 as *mut u64;
         unsafe { *a3 = 3 };
     }
+
+    unsafe fn field_access(v: &mut Vec3<i32>) {
+        let r = &mut v.0;
+        let ptr = r as *mut i32 as *mut Vec3<i32>;
+        unsafe { *ptr = Vec3(0, 0, 0) }
+    }
 }
 
 const RAW_PTR: *mut u8 = 1 as *mut u8;