ffi struct produces warnings on Rust 1.79.0 or +nightly

[sax]

I have a struct that I use to convert wrapped errors to strings that can be sent to Swift:

#[swift_bridge::bridge]
mod ffi {
  #[swift_bridge(swift_repr = "struct")]
  pub struct BridgedError {
    pub inner: String
  }
}

This compiles without any errors in Rust 1.77.2, but when I compile it with nightly it produces the following warning:

> cargo +nightly build -Zbuild-std --package my-bridge --target aarch64-apple-darwin

error: field `0` is never read
  --> rust/my-bridge/src/lib.rs:17:14
   |
14 | #[swift_bridge::bridge]
   | ----------------------- field in this variant
...
17 |   pub struct BridgedError {
   |              ^^^^^^^^^^^^
   |
note: the lint level is defined here
  --> rust/my-bridge/src/lib.rs:2:38
   |
2  | #![cfg_attr(feature = "strict", deny(warnings))]
   |                                      ^^^^^^^^
   = note: `#[deny(dead_code)]` implied by `#[deny(warnings)]`
help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
   |
17 |   pub struct () {
   |              ~~

I'm starting to play around with cross-compiling to watchOS, tvOS, and eventually visionOS, but am finding these targets only seem to build on nightly.

[chinedufn]

Thanks for reporting this.

Looks like a similar to issue was opened for rustc 3 weeks ago https://github.com/rust-lang/rust/issues/123418

---

Mind sharing a simple bridge module that reproduces this warning?

I cannot reproduce this warning using the provided example.

[Bright-Shard]

Also on nightly some of the ui tests fail to run. It looks like very minor issues - unexpected warnings (maybe related to that issue?) and some of the spans seem to have changed.

Output

``` test tests/ui/args-into-argument-not-found.rs ... mismatch EXPECTED: ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈ error: Argument "arg_typo" was not found in "fn some_function(..)" --> tests/ui/args-into-argument-not-found.rs:7:42 | 7 | #[swift_bridge(args_into = (arg, arg_typo))] | ^^^^^^^^ error: Argument "bar" was not found in "fn some_method(..)" --> tests/ui/args-into-argument-not-found.rs:13:42 | 13 | #[swift_bridge(args_into = (foo, bar))] | ^^^ ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈ ACTUAL OUTPUT: ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈ error: Argument "arg_typo" was not found in "fn some_function(..)" --> tests/ui/args-into-argument-not-found.rs:7:42 | 7 | #[swift_bridge(args_into = (arg, arg_typo))] | ^^^^^^^^ error: Argument "bar" was not found in "fn some_method(..)" --> tests/ui/args-into-argument-not-found.rs:13:42 | 13 | #[swift_bridge(args_into = (foo, bar))] | ^^^ warning: unused variable: `arg` --> tests/ui/args-into-argument-not-found.rs:18:18 | 18 | fn some_function(arg: u8) {} | ^^^ help: if this is intentional, prefix it with an underscore: `_arg` | = note: `#[warn(unused_variables)]` on by default warning: unused variable: `foo` --> tests/ui/args-into-argument-not-found.rs:23:27 | 23 | fn some_method(&self, foo: u8) {} | ^^^ help: if this is intentional, prefix it with an underscore: `_foo` ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈ note: If the actual output is the correct output you can bless it by rerunning your test with the environment variable TRYBUILD=overwrite test tests/ui/incorrect-argument-type.rs ... mismatch EXPECTED: ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈ error[E0308]: mismatched types --> tests/ui/incorrect-argument-type.rs:15:16 | 10 | #[swift_bridge::bridge] | ----------------------- arguments to this function are incorrect ... 15 | fn fn1(arg: &str); | ^^^^^^ expected `u16`, found `&str` | note: function defined here --> tests/ui/incorrect-argument-type.rs:24:4 | 24 | fn some_function(_arg: u16) {} | ^^^^^^^^^^^^^ --------- ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈ ACTUAL OUTPUT: ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈ error[E0308]: mismatched types --> tests/ui/incorrect-argument-type.rs:15:16 | 10 | #[swift_bridge::bridge] | ----------------------- arguments to this function are incorrect ... 15 | fn fn1(arg: &str); | ^^^^^^^^^ expected `u16`, found `&str` | note: function defined here --> tests/ui/incorrect-argument-type.rs:24:4 | 24 | fn some_function(_arg: u16) {} | ^^^^^^^^^^^^^ --------- ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈ note: If the actual output is the correct output you can bless it by rerunning your test with the environment variable TRYBUILD=overwrite test tests/ui/incorrect-return-type.rs ... mismatch EXPECTED: ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈ error[E0308]: mismatched types --> tests/ui/incorrect-return-type.rs:6:1 | 6 | #[swift_bridge::bridge] | ^^^^^^^^^^^^^^^^^^^^^^^ expected `SomeType`, found `&SomeType` ... 9 | type SomeType; | ______________- 10 | | 11 | | #[swift_bridge(rust_name = "some_function")] | |_________- expected due to this | = note: this error originates in the attribute macro `swift_bridge::bridge` (in Nightly builds, run with -Z macro-backtrace for more info) error[E0308]: mismatched types --> tests/ui/incorrect-return-type.rs:6:1 | 6 | #[swift_bridge::bridge] | ^^^^^^^^^^^^^^^^^^^^^^^ expected `SomeType`, found `Option` ... 9 | type SomeType; | ______________- 10 | | 11 | | #[swift_bridge(rust_name = "some_function")] 12 | | fn fn1() -> SomeType; 13 | | #[swift_bridge(rust_name = "another_function")] | |_________- expected due to this | = note: expected struct `SomeType` found enum `Option` = note: this error originates in the attribute macro `swift_bridge::bridge` (in Nightly builds, run with -Z macro-backtrace for more info) help: consider using `Option::expect` to unwrap the `Option` value, panicking if the value is an `Option::None` | 6 | #[swift_bridge::bridge].expect("REASON") | +++++++++++++++++ ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈ ACTUAL OUTPUT: ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈ error[E0308]: mismatched types --> tests/ui/incorrect-return-type.rs:6:1 | 6 | #[swift_bridge::bridge] | ^^^^^^^^^^^^^^^^^^^^^^^ expected `SomeType`, found `&SomeType` ... 9 | type SomeType; | ______________- 10 | | 11 | | #[swift_bridge(rust_name = "some_function")] 12 | | fn fn1() -> SomeType; | |_____________________________- expected due to this | = note: this error originates in the attribute macro `swift_bridge::bridge` (in Nightly builds, run with -Z macro-backtrace for more info) error[E0308]: mismatched types --> tests/ui/incorrect-return-type.rs:6:1 | 6 | #[swift_bridge::bridge] | ^^^^^^^^^^^^^^^^^^^^^^^ expected `SomeType`, found `Option` ... 9 | type SomeType; | ______________- 10 | | 11 | | #[swift_bridge(rust_name = "some_function")] 12 | | fn fn1() -> SomeType; 13 | | #[swift_bridge(rust_name = "another_function")] 14 | | fn fn2() -> SomeType; | |_____________________________- expected due to this | = note: expected struct `SomeType` found enum `Option` = note: this error originates in the attribute macro `swift_bridge::bridge` (in Nightly builds, run with -Z macro-backtrace for more info) help: consider using `Option::expect` to unwrap the `Option` value, panicking if the value is an `Option::None` | 6 | #[swift_bridge::bridge].expect("REASON") | +++++++++++++++++ ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈ note: If the actual output is the correct output you can bless it by rerunning your test with the environment variable TRYBUILD=overwrite test tests/ui/invalid-associated-to-attribute.rs ... ok test tests/ui/invalid-copy-attribute.rs ... ok test tests/ui/invalid-module-item.rs ... ok test tests/ui/opaque-copy-type-mut-ref.rs ... ok test tests/ui/unrecognized-argument-attribute.rs ... ok test tests/ui/unrecognized-enum-attribute.rs ... ok test tests/ui/unrecognized-function-attribute.rs ... ok test tests/ui/unrecognized-opaque-type-attribute.rs ... ok test ui_tests::ui ... FAILED ```

[sax]

@chinedufn Took a bit longer than expected, but here's a gist you can clone and see the issue: https://gist.github.com/sax/9c4b3b841d15ed62fde135da28b783fa

[sax]

This error occurs on Rust 1.79.0, which was just released yesterday.

[sax]

Ok, I just verified that this problem happens when bridging a function or method returning a Result where the error variant is an ffi struct. When I instead change all of my code to return a Result<whatever, String>, then the compiler warning disappears.

I take advantage of the ? operator quite a bit, so am going to hold off on making the change. I've been going through the swift-bridge code as well as the expansion of my crate (using cargo-expand), but I really can't see where the problem is coming from.