Do not use DerefMut on non-Copy union fields

Union fields not implementing `Copy` cannot implicitely use the `DerefMut`
trait.

Author: samueltardieu

clippy_lints/src/reference.rs

@@ -1,5 +1,6 @@
 use clippy_utils::diagnostics::span_lint_and_sugg;
 use clippy_utils::source::{SpanRangeExt, snippet_with_applicability};
+use clippy_utils::ty::is_copy;
 use rustc_errors::Applicability;
 use rustc_hir::{Expr, ExprKind, Mutability, UnOp};
 use rustc_lint::{LateContext, LateLintPass};
@@ -44,6 +45,7 @@ impl LateLintPass<'_> for DerefAddrOf {
             // NOTE(tesuji): `*&` forces rustc to const-promote the array to `.rodata` section.
             // See #12854 for details.
             && !matches!(addrof_target.kind, ExprKind::Array(_))
+            && !is_non_copy_union_field(cx, addrof_target)
             && deref_target.span.eq_ctxt(e.span)
             && !addrof_target.span.from_expansion()
         {
@@ -102,3 +104,11 @@ impl LateLintPass<'_> for DerefAddrOf {
         }
     }
 }
+
+fn is_non_copy_union_field(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
+    if let ExprKind::Field(parent, _) = expr.kind {
+        cx.typeck_results().expr_ty_adjusted(parent).is_union() && !is_copy(cx, cx.typeck_results().expr_ty(expr))
+    } else {
+        false
+    }
+}

tests/ui/deref_addrof.fixed

@@ -72,3 +72,37 @@ impl S {
         //~^ deref_addrof
     }
 }
+
+fn issue14386() {
+    use std::mem::ManuallyDrop;
+
+    struct Data {
+        num: u64,
+    }
+
+    union DataWithPadding {
+        data: ManuallyDrop<Data>,
+        x: ManuallyDrop<u64>,
+        with_padding: [u8; size_of::<Data>()],
+        y: (u32, u32),
+        z: (ManuallyDrop<Data>,),
+    }
+
+    let mut a = DataWithPadding {
+        with_padding: [0; size_of::<DataWithPadding>()],
+    };
+    unsafe {
+        // Do not lint as `DerefMut` will not work on non-`Copy` union
+        // fields.
+        (*(&raw mut a.data)).num = 42;
+        (*(&raw mut a.z)).0.num = 42;
+
+        // Lint, as `Copy` fields in unions are ok.
+        a.with_padding = [1; size_of::<DataWithPadding>()];
+        //~^ deref_addrof
+        *a.x = 0;
+        //~^ deref_addrof
+        a.y.0 = 0;
+        //~^ deref_addrof
+    }
+}

tests/ui/deref_addrof.rs

@@ -72,3 +72,37 @@ impl S {
         //~^ deref_addrof
     }
 }
+
+fn issue14386() {
+    use std::mem::ManuallyDrop;
+
+    struct Data {
+        num: u64,
+    }
+
+    union DataWithPadding {
+        data: ManuallyDrop<Data>,
+        x: ManuallyDrop<u64>,
+        with_padding: [u8; size_of::<Data>()],
+        y: (u32, u32),
+        z: (ManuallyDrop<Data>,),
+    }
+
+    let mut a = DataWithPadding {
+        with_padding: [0; size_of::<DataWithPadding>()],
+    };
+    unsafe {
+        // Do not lint as `DerefMut` will not work on non-`Copy` union
+        // fields.
+        (*(&raw mut a.data)).num = 42;
+        (*(&raw mut a.z)).0.num = 42;
+
+        // Lint, as `Copy` fields in unions are ok.
+        (*(&raw mut a.with_padding)) = [1; size_of::<DataWithPadding>()];
+        //~^ deref_addrof
+        *(*(&raw mut a.x)) = 0;
+        //~^ deref_addrof
+        (*(&raw mut a.y)).0 = 0;
+        //~^ deref_addrof
+    }
+}

tests/ui/deref_addrof.stderr

@@ -71,5 +71,23 @@ LL |         inline!(*&mut $(@expr self))
    |
    = note: this error originates in the macro `__inline_mac_impl` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: aborting due to 11 previous errors
+error: immediately dereferencing a reference
+  --> tests/ui/deref_addrof.rs:101:9
+   |
+LL |         (*(&raw mut a.with_padding)) = [1; size_of::<DataWithPadding>()];
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `a.with_padding`
+
+error: immediately dereferencing a reference
+  --> tests/ui/deref_addrof.rs:103:10
+   |
+LL |         *(*(&raw mut a.x)) = 0;
+   |          ^^^^^^^^^^^^^^^^^ help: try: `a.x`
+
+error: immediately dereferencing a reference
+  --> tests/ui/deref_addrof.rs:105:9
+   |
+LL |         (*(&raw mut a.y)).0 = 0;
+   |         ^^^^^^^^^^^^^^^^^ help: try: `a.y`
+
+error: aborting due to 14 previous errors