`borrow_as_ptr` and `ptr_as_ptr` false positives in `windows::core::implement` macro invocation

[PaulDance]

Summary

These lints trigger on the invocation of the external macro. As suggested in microsoft/windows-rs#3690 (comment), I'm also opening an issue here just in case. I haven't investigated any of windows-rs's or Clippy's side of this yet, so there still remains to see where the responsibility should lie.

Lint Name

borrow_as_ptr and ptr_as_ptr

Reproducer

Using the following crate files:

# `Cargo.toml`
[package]
name = "tmp"
version = "0.1.0"
edition = "2024"

[dependencies]
windows = { version = "0.61.3", features = [
    "std",
    "Win32",
    "Win32_System",
    "Win32_System_Com",
]}
windows-core = "0.61.2"

[lints]
clippy.borrow_as_ptr = "warn"
clippy.ptr_as_ptr = "warn"

and

// `src/lib.rs`
use std::ffi::c_void;
use windows::Win32::System::Com::{IClassFactory, IClassFactory_Impl};
use windows::core::{BOOL, GUID, IUnknown, Ref, Result, implement};

#[implement(IClassFactory)]
struct A;

impl IClassFactory_Impl for A_Impl {
    fn CreateInstance(
        &self,
        _outer: Ref<'_, IUnknown>,
        _iid: *const GUID,
        _object: *mut *mut c_void,
    ) -> Result<()> {
        Ok(())
    }

    fn LockServer(&self, _lock: BOOL) -> Result<()> {
        Ok(())
    }
}

then

$ cargo clippy --target x86_64-pc-windows-msvc
warning: `as` casting between raw pointers without changing their constness
 --> src/lib.rs:5:13
  |
5 | #[implement(IClassFactory)]
  |             ^^^^^^^^^^^^^ help: try `pointer::cast`, a safer alternative: `(IClassFactory as IClassFactory).cast::<IClassFactory>()`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_as_ptr
  = note: requested on the command line with `-W clippy::ptr-as-ptr`

warning: borrow as raw pointer
 --> src/lib.rs:5:13
  |
5 | #[implement(IClassFactory)]
  |             ^^^^^^^^^^^^^ help: try: `&raw const IClassFactory`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#borrow_as_ptr
  = note: requested on the command line with `-W clippy::borrow-as-ptr`

warning: `as` casting between raw pointers without changing their constness
 --> src/lib.rs:5:13
  |
5 | #[implement(IClassFactory)]
  |             ^^^^^^^^^^^^^ help: try `pointer::cast`, a safer alternative: `IClassFactory.cast::<IClassFactory>()`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_as_ptr

warning: `tmp` (lib) generated 3 warnings (run `cargo clippy --fix --lib -p tmp` to apply 3 suggestions)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.05s

The lints are triggered inside the code generated by the implement macro, while they shouldn't.

Version

rustc 1.88.0 (6b00bc388 2025-06-23)
binary: rustc
commit-hash: 6b00bc3880198600130e1cf62b8f8a93494488cc
commit-date: 2025-06-23
host: x86_64-unknown-linux-gnu
release: 1.88.0
LLVM version: 20.1.5

Additional Labels

No response

[ada4a]

This is indeed a false-positive in Clippy -- we generally avoid linting in proc-macros

[PaulDance]

What about microsoft/windows-rs#3690 (comment), though?

[ada4a]

That is the reason this issue happened. We usually check if a piece of code comes from an external macro, and if so avoid linting (since the user usually can't do anything about external code, as is your case). But since implement uses quote_spanned!, Clippy thinks that the code actually comes from the crate-internal code, and therefore lints. For those cases, we have the heavier artillery of is_from_proc_macro, which is more expensive but also recognizes more proc-macros. For a bit more context, see https://doc.rust-lang.org/clippy/development/macro_expansions.html#the-is_from_proc_macro-function

Does that answer your question?..

[PaulDance]

Yes, it definitely answers my question, thanks! My comment was more about whether the fix in Clippy was really necessary, since the fix in windows-rs will be attempted nevertheless.

[ada4a]

Ah, I see. I'm pretty sure the fix won't stop the lint from firing inside the macro completely, but only when Clippy runs outside the windows crate (in your case, when you use the implement macro). Inside the windows crate though, the devs should be able to run Clippy and get the lint, as before

[PaulDance]

Got it, thanks!

[PaulDance]

👍

[PaulDance]

Just tested under clippy 0.1.91 (425a9c0a0e 2025-08-17) and it still triggers with the reproducer:

$ cargo +nightly clippy --target x86_64-pc-windows-msvc
warning: `as` casting between raw pointers without changing their constness
 --> src/lib.rs:6:13
  |
6 | #[implement(IClassFactory)]
  |             ^^^^^^^^^^^^^ help: try `pointer::cast`, a safer alternative: `(IClassFactory as IClassFactory).cast::<IClassFactory>()`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_as_ptr
  = note: requested on the command line with `-W clippy::ptr-as-ptr`

warning: borrow as raw pointer
 --> src/lib.rs:6:13
  |
6 | #[implement(IClassFactory)]
  |             ^^^^^^^^^^^^^ help: try: `&raw const IClassFactory`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#borrow_as_ptr
  = note: requested on the command line with `-W clippy::borrow-as-ptr`

warning: `as` casting between raw pointers without changing their constness
 --> src/lib.rs:6:13
  |
6 | #[implement(IClassFactory)]
  |             ^^^^^^^^^^^^^ help: try `pointer::cast`, a safer alternative: `IClassFactory.cast::<IClassFactory>()`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_as_ptr

What does it mean with regards to the fix?

[ada4a]

Huh. I guess is_from_proc_macro isn't able to recognize this case for some reason? @Jarcho maybe you have any ideas?