Issue a warning on packed record member access

src/aro/Diagnostics.zig

@@ -21,6 +21,10 @@ pub const Message = struct {
 
     pub const Extra = union {
         str: []const u8,
+        record_member: struct {
+            record: []const u8,
+            member: []const u8,
+        },
         tok_id: struct {
             expected: Tree.Token.Id,
             actual: Tree.Token.Id,
@@ -219,6 +223,7 @@ pub const Options = struct {
     @"shift-count-overflow": Kind = .default,
     @"constant-conversion": Kind = .default,
     @"sign-conversion": Kind = .default,
+    @"address-of-packed-member": Kind = .default,
     nonnull: Kind = .default,
 };
 
@@ -392,6 +397,7 @@ pub fn renderMessage(comp: *Compilation, m: anytype, msg: Message) void {
     const prop = msg.tag.property();
     switch (prop.extra) {
         .str => printRt(m, prop.msg, .{"{s}"}, .{msg.extra.str}),
+        .record_member => printRt(m, prop.msg, .{ "{s}", "{s}" }, .{ msg.extra.record_member.member, msg.extra.record_member.record }),
         .tok_id => printRt(m, prop.msg, .{ "{s}", "{s}" }, .{
             msg.extra.tok_id.expected.symbol(),
             msg.extra.tok_id.actual.symbol(),

src/aro/Diagnostics/messages.def

@@ -2560,3 +2560,9 @@ subtract_pointers_zero_elem_size
     .kind = .warning
     .opt = W("pointer-arith")
     .extra = .str
+
+packed_member_address
+    .msg = "taking address of packed member '{s}' of class or structure '{s}' may result in an unaligned pointer value"
+    .opt = W("address-of-packed-member")
+    .kind = .warning
+    .extra = .record_member

src/aro/Parser.zig

@@ -7211,6 +7211,7 @@ fn unExpr(p: *Parser) Error!Result {
                 try p.err(.invalid_preproc_operator);
                 return error.ParsingFailed;
             }
+            const orig_tok_i = p.tok_i;
             p.tok_i += 1;
             var operand = try p.castExpr();
             try operand.expect(p);
@@ -7221,6 +7222,14 @@ fn unExpr(p: *Parser) Error!Result {
                 p.getNode(operand.node, .member_access_ptr_expr)) |member_node|
             {
                 if (tree.isBitfield(member_node)) try p.errTok(.addr_of_bitfield, tok);
+                const data = p.nodes.items(.data)[@intFromEnum(member_node)];
+                const lhs_ty = p.nodes.items(.ty)[@intFromEnum(data.member.lhs)];
+                if (lhs_ty.getAttribute(.@"packed") != null) {
+                    const record = lhs_ty.getRecord().?;
+                    const mapper = p.comp.string_interner.getSlowTypeMapper();
+                    const extra = Diagnostics.Message.Extra{ .record_member = .{ .record = mapper.lookup(record.name), .member = mapper.lookup(record.fields[data.member.index].name) } };
+                    try p.errExtra(.packed_member_address, orig_tok_i, extra);
+                }
             }
             const operand_ty_valid = !operand.ty.is(.invalid);
             if (!tree.isLval(operand.node) and operand_ty_valid) {

test/cases/packed member address.c

@@ -0,0 +1,8 @@
+struct __attribute__((packed)) Foo {
+    int x;
+};
+struct Foo foo;
+int *p = &foo.x;
+
+
+#define EXPECTED_ERRORS "packed member address.c:5:10: warning: taking address of packed member 'x' of class or structure 'Foo' may result in an unaligned pointer value [-Waddress-of-packed-member]"

test/cases/relocations.c

@@ -62,6 +62,8 @@ void foo(void) {
 #define EXPECTED_ERRORS "relocations.c:24:1: error: static assertion failed" \
     "relocations.c:29:16: error: static_assert expression is not an integral constant expression" \
     "relocations.c:30:16: error: static_assert expression is not an integral constant expression" \
+    "relocations.c:39:16: warning: taking address of packed member 'x' of class or structure 'Packed' may result in an unaligned pointer value [-Waddress-of-packed-member]" \
+    "relocations.c:39:28: warning: taking address of packed member 'y' of class or structure 'Packed' may result in an unaligned pointer value [-Waddress-of-packed-member]" \
     "relocations.c:50:26: warning: subtraction of pointers to type 'union Empty' of zero size has undefined behavior [-Wpointer-arith]" \
     "relocations.c:50:16: error: static_assert expression is not an integral constant expression" \
     "relocations.c:60:20: error: static_assert expression is not an integral constant expression" \