1use proc_macro::TokenStream;
2use quote::quote;
3use syn::{parse_macro_input, Data, DeriveInput, Fields, Lit};
4
5#[proc_macro_derive(AssertOffsets, attributes(offset))]
6pub fn derive_assert_offsets(input: TokenStream) -> TokenStream {
7 let input = parse_macro_input!(input as DeriveInput);
8
9 let struct_name = &input.ident;
10 let fields = match &input.data {
11 Data::Struct(data_struct) => match &data_struct.fields {
12 Fields::Named(fields) => &fields.named,
13 _ => panic!("AssertOffsets only supports structs with named fields"),
14 },
15 _ => panic!("AssertOffsets can only be used with structs"),
16 };
17
18 let mut assertions = vec![];
19
20 for field in fields {
21 let field_name = field.ident.as_ref().unwrap();
22 let mut expected_offset = None;
23
24 for attr in &field.attrs {
25 if attr.path().is_ident("offset") {
26 if let Ok(Lit::Int(lit_int)) = attr.parse_args() {
27 expected_offset = Some(
28 lit_int
29 .base10_parse::<usize>()
30 .expect("Offset must be an integer"),
31 );
32 }
33 }
34 }
35
36 if let Some(expected_offset) = expected_offset {
37 let expected_offset_hex = format!("0x{:X}", expected_offset);
38 assertions.push(quote! {
39 const _: () = assert!(
40 std::mem::offset_of!(#struct_name, #field_name) == #expected_offset,
41 concat!(
42 "Field `",
43 stringify!(#struct_name),
44 "::",
45 stringify!(#field_name),
46 "` is not at expected offset ",
47 #expected_offset_hex
48 )
49 );
50 });
51 }
52 }
53
54 let expanded = quote! {
55 #(#assertions)*;
56 };
57
58 TokenStream::from(expanded)
59}