windows-core: declarative interface_decl! macro_rules alternative to #[interface] by Copilot · Pull Request #4403 · microsoft/windows-rs

Copilot · 2026-05-11T18:08:53Z

Adds macro_rules!-based alternatives to #[interface] / #[implement] in windows-core, migrates the remaining hand-written generic implementers in windows-collections and windows-future over to them, and makes the proc-macros optional so consumers can drop the syn / quote / proc-macro2 build dependencies.

Changes Made

interface_decl! — declarative alternative to #[interface] for COM interfaces inheriting directly from IUnknown. The generated interface struct, vtable, _Impl trait, and safe-caller wrappers are always pub (matching define_interface!). Supported method shapes are -> Result<()> (vtable returns HRESULT) and void; value-returning and Result<T> shapes are intentionally not accepted.
implement_decl! — declarative alternative to #[implement]. Single macro with two top-level arms:
- non-generic: impl Name as Vis Name_Impl: [Iface, …]
- generic: impl<G,…> Name as Vis Name_Impl: [Iface<…>, …] where … (the where clause is mandatory and forwarded verbatim).
  All generic-form helpers live alongside the non-generic helpers in implement_macro.rs (no separate implement_generic_macro.rs). Vtable type for each interface entry is reached via <Iface as Interface>::Vtable, so callers don't spell out _Vtbl idents.
Callsite migrations — every hand-written generic implementer in windows-collections (iterable, map, vector, observable_map, observable_vector, vector_view, map_view, key_value_pair) and windows-future (async_ready, async_spawn) now uses implement_decl! instead of bespoke unsafe wiring.
Cargo features — windows-core gains a default-on proc-macros feature that gates the re-exports of #[implement] / #[interface]. windows-collections and windows-future disable default features so they no longer pull in the proc-macro toolchain. The top-level windows crate keeps windows-core/proc-macros enabled by default for backwards compatibility.
Test coverage — new integration test crates/tests/libs/interface_core/tests/macro_rules_decl.rs exercises both interface_decl! and implement_decl! end-to-end (multi-interface implementer, QueryInterface across the chain, refcount drop, IID matching).

Testing

✅ cargo check -p windows-core -p windows-collections -p windows-future passes
✅ cargo test -p test_interface_core -p test_collections passes (4/4 in macro_rules_decl)
✅ parallel_validation (Code Review + CodeQL) clean

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Copilot

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds macro_rules!-based alternatives to #[interface] / #[implement] in windows-core, migrates remaining hand-written implementers to the declarative macros, and adjusts feature flags so consumers can opt out of proc-macro dependencies.

Changes:

Introduce interface_decl!, implement_decl!, and implement_decl_generic! in windows-core.
Port several windows-collections / windows-future types from #[implement] to the new declarative macros.
Update Cargo feature wiring to make proc-macros optional and fix test/CI configuration.

Reviewed changes

Copilot reviewed 24 out of 24 changed files in this pull request and generated 10 comments.

Show a summary per file

File	Description
crates/tests/misc/no_std/Cargo.toml	Enables `windows-core/proc-macros` for the no-std test crate.
crates/tests/misc/linux/Cargo.toml	Enables `windows-core/proc-macros` for the linux misc test crate.
crates/tests/libs/interface_core/tests/macro_rules_decl.rs	New tests exercising `interface_decl!`/`implement_decl!` interoperability and ABI basics.
crates/tests/libs/interface_core/Cargo.toml	Enables `windows-core/proc-macros` feature for interface_core tests.
crates/tests/libs/implement_core/Cargo.toml	Enables `windows-core/proc-macros` feature for implement_core tests.
crates/tests/libs/collections/Cargo.toml	Enables `windows-core/proc-macros` feature for collections tests.
crates/libs/windows/Cargo.toml	Changes default features to include `windows-core/proc-macros`.
crates/libs/future/src/async_spawn.rs	Ports async implementers from `#[implement]` to `implement_decl!`/`implement_decl_generic!`.
crates/libs/future/src/async_ready.rs	Ports ready-state implementers from `#[implement]` to `implement_decl!`/`implement_decl_generic!`.
crates/libs/future/Cargo.toml	Disables `windows-core` default features for `windows-future`.
crates/libs/core/src/lib.rs	Adds new macro modules and gates proc-macro re-exports behind `proc-macros` feature.
crates/libs/core/src/interface_macro.rs	Adds `interface_decl!` + helpers to declare interfaces without proc macros.
crates/libs/core/src/implement_macro.rs	Adds `implement_decl!` + helpers to wire non-generic implementers without proc macros.
crates/libs/core/src/implement_generic_macro.rs	Adds `implement_decl_generic!` for generic implementers without proc macros.
crates/libs/core/Cargo.toml	Makes proc-macro deps optional and adds `proc-macros` feature (enabled by default).
crates/libs/collections/src/vector_view.rs	Ports `#[implement]` sites to `implement_decl_generic!`.
crates/libs/collections/src/vector.rs	Ports `#[implement]` sites to `implement_decl_generic!`.
crates/libs/collections/src/observable_vector.rs	Ports `#[implement]` sites to `implement_decl_generic!`/`implement_decl!`.
crates/libs/collections/src/observable_map.rs	Ports `#[implement]` sites to `implement_decl_generic!`.
crates/libs/collections/src/map_view.rs	Ports `#[implement]` sites to `implement_decl_generic!`.
crates/libs/collections/src/map.rs	Ports `#[implement]` sites to `implement_decl_generic!`.
crates/libs/collections/src/key_value_pair.rs	Ports `#[implement]` sites to `implement_decl_generic!`.
crates/libs/collections/src/iterable.rs	Ports `#[implement]` sites to `implement_decl_generic!`.
crates/libs/collections/Cargo.toml	Disables `windows-core` default features for `windows-collections`.

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+        $vis unsafe fn $mname(&self $(, $aname: $aty)*) -> $crate::Result<$res> {
+            unsafe {
+                ($crate::Interface::vtable(self).$mname)($crate::Interface::as_raw(self) $(, $aname)*).ok()
+            }
+        }


+                    let this_outer: &Identity = unsafe {
+                        &*((this as *const *const ()).offset(OFFSET) as *const Identity)
+                    };
+                    unsafe { <Identity as $impl_trait>::$mname(this_outer $(, $aname)*) }.into()



 [dependencies.windows-core]
 workspace = true
+features = ["proc-macros"]



 [features]
-default = ["std"]
+default = ["std", "windows-core/proc-macros"]


+macro_rules! __implement_decl_offset_negate {
+    (()) => {
+        -1isize
+    };


+        -16isize
+    }; // Hand-written implementers rarely declare more than a handful of interfaces; the
+       // hard cap is more than the practical maximum.  If you hit this, split your
+       // implementation across multiple objects or use the proc-macro.
+}


+macro_rules! __implement_decl_index_plus_two {
+    () => {
+        2usize
+    };


+    };
+    (() () () () () () () () () () () () () () ()) => {
+        17usize
+    };


+        // Each interface needs an internal field/const name on the generated wrapper. The
+        // names are never user-visible (no item generated by this macro lets external code
+        // refer to them), so we draw them from a fixed anonymous pool. The pool size
+        // matches the cap in `__implement_decl_offset_negate!`.


+// this macro accumulates four token lists (vtbl fields, vtbl initializers, thunk fn defs,
+// and matches-arms for parent chain traversal — currently unused but reserved for a
+// future extension) and emits the whole `struct $vtbl { ... } impl $vtbl { ... }` block
+// at the end.


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Copilot

Pull request overview

Copilot reviewed 23 out of 23 changed files in this pull request and generated 5 comments.

Kenny Kerr (kennykerr) · 2026-05-12T12:54:07Z

+macro_rules! interface_decl {
+    (
+        $(#[doc = $doc:expr])*
+        $vis:vis unsafe trait $name:ident ( $vtbl:ident, $impl_trait:ident ) : $parent:ty = $iid:literal {
+            $($methods:tt)*
+        }
+    ) => {
+        // Struct + Interface + Debug.
+        $crate::imp::define_interface!($name, $vtbl, $iid);
+        // CanInto + From conversions to ancestors.
+        $crate::imp::interface_hierarchy!($name, $parent);


Don't bother capturing docs at all.

Done in 2268a7a — the doc capture is gone from interface_decl! and both helper macros.

Kenny Kerr (kennykerr) · 2026-05-12T12:54:53Z

+    ($vis:vis,
+        $(#[doc = $mdoc:expr])*
+        unsafe fn $mname:ident (&self $(, $aname:ident : $aty:ty)* $(,)? ) -> $ret:ty ;
+        $($rest:tt)*
+    ) => {
+        $(#[doc = $mdoc])*
+        #[inline]
+        $vis unsafe fn $mname(&self $(, $aname: $aty)*) -> $ret {
+            unsafe {


Just remove these unused and untested arms.

Removed in 2268a7a — only Result<()> and void return-type arms remain in the wrapper, trait-method, and vtable helpers. Updated the test to drop the value-returning method.

Kenny Kerr (kennykerr) · 2026-05-12T12:55:43Z

+        #[repr(C)]
+        #[allow(non_camel_case_types, non_snake_case)]
+        $impl_vis struct $impl_name < $($gp),+ >
+        where $($wc)*
+        {


Let's just require the where clause and pass it through as is.

Done in 2268a7a — the where clause is now mandatory and forwarded verbatim. Every existing call site already supplies one.

+//! The leading ident of every interface entry must be unique within a single invocation —
+//! it doubles as the per-chain struct field name and as the name of the per-chain
+//! associated constant on `Foo_Impl`.


Kenny Kerr (kennykerr) · 2026-05-12T12:56:42Z

+        $vis:vis unsafe trait $name:ident ( $vtbl:ident, $impl_trait:ident ) : $parent:ty = $iid:literal {
+            $($methods:tt)*
+        }
+    ) => {
+        // Struct + Interface + Debug.
+        $crate::imp::define_interface!($name, $vtbl, $iid);
+        // CanInto + From conversions to ancestors.
+        $crate::imp::interface_hierarchy!($name, $parent);


Just assuming pub as define_interface does is sufficient.

Done in 2268a7a — $vis dropped from interface_decl!; the trait, struct, vtable, and wrapper fns are all pub unconditionally.

Kenny Kerr (kennykerr) · 2026-05-12T12:56:58Z

Copilot address new review comments.

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Copilot AI and others added 2 commits May 11, 2026 17:41

windows-core: make proc-macros optional via default feature

67ec195

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add interface_decl! macro_rules alternative + fix Phase 1 regression

c00405c

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add implement_decl! macro_rules alternative to #[implement]

f725285

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strip trailing blank line from macro_rules_decl test

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infer vtable type via Interface::Vtable so callers don't write _Vtbl

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Add implement_decl_generic! macro; port StockIterable/StockIterator

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Port all 22 generic-implementer sites in windows-collections and wind…

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Drop proc-macros feature from windows-collections/windows-future

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interface_decl!: restrict Result to Result<()>, drop stale comments

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implement_decl: merge generic form into single macro

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interface_decl!: support methods with non-Result return types #4405

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windows-core: declarative interface_decl! macro_rules alternative to #[interface] - #4403

windows-core: declarative interface_decl! macro_rules alternative to #[interface]#4403
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