add support for generating JSG_STRUCT (#65)

Author: anonrig

.zed/settings.json

@@ -0,0 +1,33 @@
+// Folder-specific settings
+//
+// For a full list of overridable settings, and general information on folder-specific settings,
+// see the documentation: https://zed.dev/docs/configuring-zed#settings-files
+{
+  "file_types": {
+    "Cap'n Proto": ["*.wd-test"],
+    "Starlark": ["BUILD.*"]
+  },
+  "lsp": {
+    "clangd": {
+      "binary": {
+        "arguments": [
+          "--background-index",
+          "--header-insertion=never",
+          "--query-driver=**",
+          "--clang-tidy"
+        ]
+      }
+    },
+    "rust-analyzer": {
+      "initialization_options": {
+        "workspace": {
+          "discoverConfig": {
+            "command": ["just", "_rust-analyzer"],
+            "progressLabel": "generating rust analyzer config",
+            "filesToWatch": ["BUILD.bazel"]
+          }
+        }
+      }
+    }
+  }
+}

BUILD.bazel

@@ -89,6 +89,7 @@ rust_proc_macro(
     proc_macro_deps = [
         "@crates.io//:rustversion",
     ],
+    visibility = ["//tests:__pkg__"],
     deps = [
         ":gen",
         ":syntax",

MODULE.bazel

@@ -16,7 +16,10 @@ cc_configure = use_extension("@rules_cc//cc:extensions.bzl", "cc_configure_exten
 use_repo(cc_configure, "local_config_cc")
 
 rust = use_extension("@rules_rust//rust:extensions.bzl", "rust")
-rust.toolchain(versions = ["1.86.0", "nightly/2025-06-06"])
+rust.toolchain(versions = [
+    "1.86.0",
+    "nightly/2025-06-06",
+])
 use_repo(rust, "rust_toolchains")
 
 register_toolchains("@rust_toolchains//:all")
@@ -32,6 +35,6 @@ use_repo(capnp_cpp, "capnp-cpp")
 bazel_dep(name = "hedron_compile_commands", dev_dependency = True)
 git_override(
     module_name = "hedron_compile_commands",
-    remote = "https://github.com/hedronvision/bazel-compile-commands-extractor.git",
     commit = "4f28899228fb3ad0126897876f147ca15026151e",
+    remote = "https://github.com/hedronvision/bazel-compile-commands-extractor.git",
 )

gen/src/include.rs

@@ -44,6 +44,7 @@ pub struct Includes<'a> {
     pub sys_types: bool,
     pub content: Content<'a>,
     pub kj_rs: bool,
+    pub jsg: bool,
 }
 
 impl<'a> Includes<'a> {
@@ -107,6 +108,7 @@ pub fn write(out: &mut OutFile) {
         basetsd,
         sys_types,
         kj_rs,
+        jsg,
         content: _,
     } = *include;
 
@@ -191,6 +193,9 @@ pub fn write(out: &mut OutFile) {
     if kj_rs && !cxx_header {
         writeln!(out, "#include \"kj-rs/kj-rs.h\"");
     }
+    if jsg && !cxx_header {
+        writeln!(out, "#include \"jsg.h\"");
+    }
 }
 
 impl<'i, 'a> Extend<&'i Include> for Includes<'a> {

gen/src/write.rs

@@ -339,6 +339,19 @@ fn write_struct<'a>(out: &mut OutFile<'a>, strct: &'a Struct, methods: &[&Extern
     out.include.type_traits = true;
     writeln!(out, "  using IsRelocatable = ::std::true_type;");
 
+    if derive::contains(&strct.derives, Trait::JsgStruct) {
+        out.include.jsg = true;
+        writeln!(out);
+        write!(out, "  JSG_STRUCT(");
+        for (i, field) in strct.fields.iter().enumerate() {
+            if i > 0 {
+                write!(out, ", ");
+            }
+            write!(out, "{}", field.name.cxx);
+        }
+        writeln!(out, ");");
+    }
+
     writeln!(out, "}};");
     writeln!(out, "#endif // {}", guard);
 }

kj-rs/BUILD.bazel

@@ -10,6 +10,7 @@ cc_library(
         "@platforms//os:windows": True,
         "//conditions:default": False,
     }),
+    visibility = ["//tests:__pkg__"],
     deps = [
         ":bridge",
     ],
@@ -34,6 +35,7 @@ rust_cxx_bridge(
     src = "lib.rs",
     hdrs = glob(["*.h"]),
     include_prefix = "kj-rs",
+    visibility = ["//tests:__pkg__"],
     deps = [
         "@capnp-cpp//src/kj:kj",
         "@capnp-cpp//src/kj:kj-async",

kj-rs/lib.rs

@@ -31,6 +31,8 @@ pub mod repr {
 pub type Result<T> = std::io::Result<T>;
 pub type Error = std::io::Error;
 
+pub trait JsgStruct {}
+
 #[cxx::bridge(namespace = "kj_rs")]
 #[allow(clippy::needless_lifetimes)]
 mod ffi {

macro/src/derive.rs

@@ -26,6 +26,7 @@ pub(crate) fn expand_struct(
             Trait::PartialOrd => expanded.extend(struct_partial_ord(strct, span)),
             Trait::Serialize => traits.push(quote_spanned!(span=> ::serde::Serialize)),
             Trait::Deserialize => traits.push(quote_spanned!(span=> ::serde::Deserialize)),
+            Trait::JsgStruct => expanded.extend(expand_jsg_struct(strct, span)),
         }
     }
 
@@ -73,6 +74,7 @@ pub(crate) fn expand_enum(enm: &Enum, actual_derives: &mut Option<TokenStream>)
             Trait::PartialOrd => expanded.extend(enum_partial_ord(enm, span)),
             Trait::Serialize => traits.push(quote_spanned!(span=> ::serde::Serialize)),
             Trait::Deserialize => traits.push(quote_spanned!(span=> ::serde::Deserialize)),
+            Trait::JsgStruct => unreachable!(),
         }
     }
 
@@ -304,3 +306,13 @@ fn enum_partial_ord(enm: &Enum, span: Span) -> TokenStream {
         }
     }
 }
+
+fn expand_jsg_struct(strct: &Struct, span: Span) -> TokenStream {
+    let ident = &strct.name.rust;
+    let generics = &strct.generics;
+
+    quote_spanned! {span=>
+        #[automatically_derived]
+        impl ::kj_rs::JsgStruct for #ident #generics {}
+    }
+}

macro/src/lib.rs

@@ -71,6 +71,14 @@ pub fn bridge(args: TokenStream, input: TokenStream) -> TokenStream {
         .into()
 }
 
+/// JsgStruct derive macro for generating JSG_STRUCT in C++ output
+#[proc_macro_derive(JsgStruct)]
+pub fn derive_jsg_struct(_input: TokenStream) -> TokenStream {
+    // This derive macro doesn't generate any Rust code
+    // The actual JSG_STRUCT generation happens in the C++ code generation phase
+    TokenStream::new()
+}
+
 #[doc(hidden)]
 #[proc_macro]
 pub fn type_id(input: TokenStream) -> TokenStream {

src/lib.rs

@@ -446,6 +446,7 @@ pub use crate::string::CxxString;
 pub use crate::unique_ptr::UniquePtr;
 pub use crate::weak_ptr::WeakPtr;
 pub use cxxbridge_macro::bridge;
+pub use cxxbridge_macro::JsgStruct;
 
 /// Synonym for `CxxString`.
 ///