Add traits on empty structs

Author: lucic71

cpp2rust/converter/converter.cpp

@@ -76,6 +76,7 @@ std::string Converter::EmitMethodsOnPtr() {
 std::string Converter::EmitOpaqueRecords() {
   std::string out;
   record_decls_.ForEachUndefined([&](const std::string &name) {
+    out += "#[derive(Clone, Copy, Default, ByteRepr)]";
     out += "pub struct ";
     out += name;
     out += ";\n";

libcc2rs-macros/src/byte_repr.rs

@@ -0,0 +1,31 @@
+// Copyright (c) 2022-present INESC-ID.
+// Distributed under the MIT license that can be found in the LICENSE file.
+
+use proc_macro::TokenStream;
+use syn::{Data, DeriveInput, Fields, parse_macro_input};
+
+pub fn expand(input: TokenStream) -> TokenStream {
+    let input = parse_macro_input!(input as DeriveInput);
+    let name = &input.ident;
+    let empty = match &input.data {
+        Data::Struct(s) => match &s.fields {
+            Fields::Unit => true,
+            Fields::Named(f) => f.named.is_empty(),
+            Fields::Unnamed(f) => f.unnamed.is_empty(),
+        },
+        _ => false,
+    };
+    if !empty {
+        panic!("derive(ByteRepr) is only supported on empty structs, `{name}` is not empty");
+    }
+    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
+    quote::quote! {
+        impl #impl_generics ::libcc2rs::ByteRepr for #name #ty_generics #where_clause {
+            #[inline]
+            fn byte_size() -> usize {
+                1
+            }
+        }
+    }
+    .into()
+}

libcc2rs-macros/src/lib.rs

@@ -3,6 +3,7 @@
 
 use proc_macro::TokenStream;
 
+mod byte_repr;
 mod goto;
 mod state_machine;
 mod switch;
@@ -79,3 +80,14 @@ pub fn goto(_input: TokenStream) -> TokenStream {
     }
     .into()
 }
+
+//     #[derive(ByteRepr)]
+//     pub struct S;
+//
+// Adds ByteRepr implementation for S. Currently only empty structs are handled. Non-empty structs
+// panic.
+
+#[proc_macro_derive(ByteRepr)]
+pub fn derive_byte_repr(input: TokenStream) -> TokenStream {
+    byte_repr::expand(input)
+}

libcc2rs-macros/tests/ui/byte_repr_non_empty.rs

@@ -0,0 +1,8 @@
+use libcc2rs_macros::ByteRepr;
+
+#[derive(ByteRepr)]
+struct NonEmpty {
+    x: i32,
+}
+
+fn main() {}

libcc2rs-macros/tests/ui/byte_repr_non_empty.stderr

@@ -0,0 +1,7 @@
+error: proc-macro derive panicked
+ --> tests/ui/byte_repr_non_empty.rs:3:10
+  |
+3 | #[derive(ByteRepr)]
+  |          ^^^^^^^^
+  |
+  = help: message: derive(ByteRepr) is only supported on empty structs, `NonEmpty` is not empty

libcc2rs/src/lib.rs

@@ -51,4 +51,4 @@ pub use fd::*;
 mod format;
 pub use format::*;
 
-pub use libcc2rs_macros::{goto, goto_block, switch};
+pub use libcc2rs_macros::{ByteRepr, goto, goto_block, switch};

tests/multi-file/opaque_forward_decl/out/refcount/opaque_forward_decl.rs

@@ -51,4 +51,5 @@ pub fn touch_0(c: Ptr<container>) {
     let c: Value<Ptr<container>> = Rc::new(RefCell::new(c));
     &(*(*(*c.borrow()).upgrade().deref()).p.borrow());
 }
+#[derive(Clone, Copy, Default, ByteRepr)]
 pub struct opaque;

tests/multi-file/opaque_forward_decl/out/unsafe/opaque_forward_decl.rs

@@ -30,4 +30,5 @@ unsafe fn main_0() -> i32 {
 pub unsafe fn touch_0(mut c: *mut container) {
     &((*c).p);
 }
+#[derive(Clone, Copy, Default, ByteRepr)]
 pub struct opaque;

tests/unit/out/refcount/opaque_forward_decl.rs

@@ -45,4 +45,5 @@ fn main_0() -> i32 {
     &(*(*c.borrow()).p.borrow());
     return ((*(*c.borrow()).x.borrow()) - 42);
 }
+#[derive(Clone, Copy, Default, ByteRepr)]
 pub struct opaque;

tests/unit/out/unsafe/opaque_forward_decl.rs

@@ -25,4 +25,5 @@ unsafe fn main_0() -> i32 {
     &(c.p);
     return ((c.x) - (42));
 }
+#[derive(Clone, Copy, Default, ByteRepr)]
 pub struct opaque;