Do not use DerefMut on ManuallyDrop<_> reached through unions

Author: samueltardieu

clippy_lints/src/reference.rs

@@ -1,7 +1,8 @@
 use clippy_utils::diagnostics::span_lint_and_sugg;
 use clippy_utils::source::{SpanRangeExt, snippet_with_applicability};
+use clippy_utils::ty::adjust_derefs_manually_drop;
 use rustc_errors::Applicability;
-use rustc_hir::{Expr, ExprKind, Mutability, UnOp};
+use rustc_hir::{Expr, ExprKind, Mutability, Node, UnOp};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::declare_lint_pass;
 use rustc_span::{BytePos, Span};
@@ -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_manually_drop_through_union(cx, addrof_target)
             && deref_target.span.eq_ctxt(e.span)
             && !addrof_target.span.from_expansion()
         {
@@ -102,3 +104,31 @@ impl LateLintPass<'_> for DerefAddrOf {
         }
     }
 }
+
+/// Check if `expr` is part of an access to a `ManuallyDrop` entity reached through a union
+fn is_manually_drop_through_union(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
+    let typeck = cx.typeck_results();
+    if is_reached_through_union(cx, expr) {
+        for (_, node) in cx.tcx.hir_parent_iter(expr.hir_id) {
+            if let Node::Expr(expr) = node {
+                if adjust_derefs_manually_drop(typeck.expr_adjustments(expr), typeck.expr_ty(expr)) {
+                    return true;
+                }
+            } else {
+                break;
+            }
+        }
+    }
+    false
+}
+
+/// Checks whether `expr` denotes an object reached through a union
+fn is_reached_through_union(cx: &LateContext<'_>, mut expr: &Expr<'_>) -> bool {
+    while let ExprKind::Field(parent, _) | ExprKind::Index(parent, _, _) = expr.kind {
+        if cx.typeck_results().expr_ty_adjusted(parent).is_union() {
+            return true;
+        }
+        expr = parent;
+    }
+    false
+}

tests/ui/deref_addrof.fixed

@@ -1,6 +1,6 @@
 //@aux-build:proc_macros.rs
 
-#![allow(clippy::return_self_not_must_use, clippy::useless_vec)]
+#![allow(dangerous_implicit_autorefs, clippy::return_self_not_must_use, clippy::useless_vec)]
 #![warn(clippy::deref_addrof)]
 
 extern crate proc_macros;
@@ -72,3 +72,45 @@ impl S {
         //~^ deref_addrof
     }
 }
+
+fn issue14386() {
+    use std::mem::ManuallyDrop;
+
+    #[derive(Copy, Clone)]
+    struct Data {
+        num: u64,
+    }
+
+    #[derive(Clone, Copy)]
+    struct M {
+        md: ManuallyDrop<[u8; 4]>,
+    }
+
+    union DataWithPadding<'lt> {
+        data: ManuallyDrop<Data>,
+        prim: ManuallyDrop<u64>,
+        padding: [u8; size_of::<Data>()],
+        tup: (ManuallyDrop<Data>, ()),
+        indirect: M,
+        indirect_arr: [M; 2],
+        indirect_ref: &'lt mut M,
+    }
+
+    let mut a = DataWithPadding {
+        padding: [0; size_of::<DataWithPadding>()],
+    };
+    unsafe {
+        a.padding = [1; size_of::<DataWithPadding>()];
+        //~^ deref_addrof
+        a.tup.1 = ();
+        //~^ deref_addrof
+        *a.prim = 0;
+        //~^ deref_addrof
+
+        (*(&raw mut a.data)).num = 42;
+        (*(&raw mut a.tup)).0.num = 42;
+        (*&mut a.indirect.md)[3] = 1;
+        (*&mut a.indirect_arr[1].md)[3] = 1;
+        (*&mut a.indirect_ref.md)[3] = 1;
+    }
+}

tests/ui/deref_addrof.rs

@@ -1,6 +1,6 @@
 //@aux-build:proc_macros.rs
 
-#![allow(clippy::return_self_not_must_use, clippy::useless_vec)]
+#![allow(dangerous_implicit_autorefs, clippy::return_self_not_must_use, clippy::useless_vec)]
 #![warn(clippy::deref_addrof)]
 
 extern crate proc_macros;
@@ -72,3 +72,45 @@ impl S {
         //~^ deref_addrof
     }
 }
+
+fn issue14386() {
+    use std::mem::ManuallyDrop;
+
+    #[derive(Copy, Clone)]
+    struct Data {
+        num: u64,
+    }
+
+    #[derive(Clone, Copy)]
+    struct M {
+        md: ManuallyDrop<[u8; 4]>,
+    }
+
+    union DataWithPadding<'lt> {
+        data: ManuallyDrop<Data>,
+        prim: ManuallyDrop<u64>,
+        padding: [u8; size_of::<Data>()],
+        tup: (ManuallyDrop<Data>, ()),
+        indirect: M,
+        indirect_arr: [M; 2],
+        indirect_ref: &'lt mut M,
+    }
+
+    let mut a = DataWithPadding {
+        padding: [0; size_of::<DataWithPadding>()],
+    };
+    unsafe {
+        (*(&raw mut a.padding)) = [1; size_of::<DataWithPadding>()];
+        //~^ deref_addrof
+        (*(&raw mut a.tup)).1 = ();
+        //~^ deref_addrof
+        *(*(&raw mut a.prim)) = 0;
+        //~^ deref_addrof
+
+        (*(&raw mut a.data)).num = 42;
+        (*(&raw mut a.tup)).0.num = 42;
+        (*&mut a.indirect.md)[3] = 1;
+        (*&mut a.indirect_arr[1].md)[3] = 1;
+        (*&mut a.indirect_ref.md)[3] = 1;
+    }
+}

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:103:9
+   |
+LL |         (*(&raw mut a.padding)) = [1; size_of::<DataWithPadding>()];
+   |         ^^^^^^^^^^^^^^^^^^^^^^^ help: try: `a.padding`
+
+error: immediately dereferencing a reference
+  --> tests/ui/deref_addrof.rs:105:9
+   |
+LL |         (*(&raw mut a.tup)).1 = ();
+   |         ^^^^^^^^^^^^^^^^^^^ help: try: `a.tup`
+
+error: immediately dereferencing a reference
+  --> tests/ui/deref_addrof.rs:107:10
+   |
+LL |         *(*(&raw mut a.prim)) = 0;
+   |          ^^^^^^^^^^^^^^^^^^^^ help: try: `a.prim`
+
+error: aborting due to 14 previous errors