Suppress dead_code warning when returning -> Result<*, TransparentType> (#278)

In Rust 1.79.0, dead code warnings are emitted from enums where the compiler does not infer that wrapped data is used, even when the enum has been annotated with `#[repr(C]`. This warning triggers in `swift-bridge` when defining transparent structs or enums that will be returned in error results.

This appears to be a regression in the `dead_code` rustc lint. Pending upstream fixes to rustc, this change to `swift-bridge` annotates generated result enums with `#[allow(unused)]`.

Fixes #270

```
error: field `0` is never read
   |
1  | #[swift_bridge::bridge]
   | ----------------------- field in this variant
...
3  |     enum ResultTransparentEnum {
   |          ^^^^^^^^^^^^^^^^^^^^^
   |
help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
   |
3  |     enum () {
   |          ~~
```

## Example bridge

The following bridge code is the minimal reproduction case:

``` rust
#[swift_bridge::bridge]
mod ffi {
    #[swift_bridge(swift_repr = "struct")]
    struct TransparentErrorStruct(pub String);

    extern "Rust" {
        fn rust_func_returns_result_transparent_struct(
            succeed: bool
        ) -> Result<(), TransparentErrorStruct>;
    }
}

fn rust_func_returns_result_transparent_struct(
    succeed: bool
) -> Result<(), ffi::ResultTestTransparentStruct> {
    if succeed {
        Ok(())
    } else {
        Err(ffi::ResultTestTransparentStruct("failed".to_string()))
    }
}

impl std::error::Error for ffi:: TransparentErrorStruct {}

impl std::fmt::Debug for ffi:: TransparentErrorStruct {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{}", self)
    }
}

impl std::fmt::Display for ffi:: TransparentErrorStruct {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{}", self.0)
    }
}
```

Author: sax

SwiftRustIntegrationTestRunner/SwiftRustIntegrationTestRunner/Result.swift

@@ -35,6 +35,9 @@ extension AsyncResultOpaqueRustType2: Error {}
 extension ResultTransparentEnum: @unchecked Sendable {}
 extension ResultTransparentEnum: Error {}
 
+extension ResultTransparentStruct: @unchecked Sendable {}
+extension ResultTransparentStruct: Error {}
+
 extension SameEnum: @unchecked Sendable {}
 extension SameEnum: Error {}
 

SwiftRustIntegrationTestRunner/SwiftRustIntegrationTestRunnerTests/ResultTests.swift

@@ -193,6 +193,46 @@ class ResultTests: XCTestCase {
         }
     }
 
+    /// Verify that we can receive a Result<(), TransparentStruct> from Rust
+    func testResultNullTransparentStruct() throws {
+        try! rust_func_return_result_null_transparent_struct(true)
+
+        do {
+            try rust_func_return_result_null_transparent_struct(false)
+            XCTFail("The function should have returned an error.")
+        } catch let error as ResultTransparentStruct {
+            XCTAssertEqual(error.inner.toString(), "failed")
+        }
+
+        XCTContext.runActivity(named: "Should return a Unit type") {
+            _ in
+            do {
+                let _ :() = try rust_func_return_result_unit_type_enum_opaque_rust(true)
+            } catch {
+                XCTFail()
+            }
+        }
+
+        XCTContext.runActivity(named: "Should throw an error") {
+            _ in
+            do {
+                let _ :() = try rust_func_return_result_unit_type_enum_opaque_rust(false)
+                XCTFail("The function should have returned an error.")
+            } catch let error as ResultTransparentEnum {
+                switch error {
+                case .NamedField(let data):
+                    XCTAssertEqual(data, 123)
+                case .UnnamedFields(_, _):
+                    XCTFail()
+                case .NoFields:
+                    XCTFail()
+                }
+            } catch {
+                XCTFail()
+            }
+        }
+    }
+
     /// Verify that we can receive a Result<Vec<>, OpaqueRust> from Rust
     func testSwiftCallRustResultVecUInt32Rust() throws {
         let vec = try! rust_func_return_result_of_vec_u32()

crates/swift-bridge-ir/src/bridged_type/bridgeable_result.rs

@@ -375,10 +375,14 @@ impl BuiltInResult {
             .err_ty
             .to_ffi_compatible_rust_type(swift_bridge_path, types);
         let mut custom_rust_ffi_types = vec![];
+        // TODO: remove `#[allow(unused)]` when rustc no longer issues dead code warnings for `#[repr(C)]`
+        //  structs or enums: https://github.com/rust-lang/rust/issues/126706
         custom_rust_ffi_types.push(quote! {
             #[repr(C)]
             pub enum #ty {
+                #[allow(unused)]
                 Ok #ok,
+                #[allow(unused)]
                 Err(#err),
             }
         });

crates/swift-bridge-ir/src/codegen/codegen_tests/result_codegen_tests.rs

@@ -407,11 +407,16 @@ mod extern_rust_fn_return_result_opaque_rust_type_and_transparent_enum_type {
         }
     }
 
+    // In Rust 1.79.0 dead_code warnings are issued for wrapped data in enums in spite of the enum
+    // having `#[repr(C)]`. `#[allow(unused)]` can be removed following resolution and release of this
+    // issue: https://github.com/rust-lang/rust/issues/126706
     fn expected_rust_tokens() -> ExpectedRustTokens {
         ExpectedRustTokens::Contains(quote! {
             #[repr(C)]
             pub enum ResultSomeOkTypeAndSomeErrEnum{
+                #[allow(unused)]
                 Ok(*mut super::SomeOkType),
+                #[allow(unused)]
                 Err(__swift_bridge__SomeErrEnum),
             }
 
@@ -484,11 +489,16 @@ mod extern_rust_fn_return_result_transparent_enum_type_and_opaque_rust_type {
         }
     }
 
+    // In Rust 1.79.0 dead_code warnings are issued for wrapped data in enums in spite of the enum
+    // having `#[repr(C)]`. `#[allow(unused)]` can be removed following resolution and release of this
+    // issue: https://github.com/rust-lang/rust/issues/126706
     fn expected_rust_tokens() -> ExpectedRustTokens {
         ExpectedRustTokens::Contains(quote! {
             #[repr(C)]
             pub enum ResultSomeOkEnumAndSomeErrType{
+                #[allow(unused)]
                 Ok(__swift_bridge__SomeOkEnum),
+                #[allow(unused)]
                 Err(*mut super::SomeErrType),
             }
 
@@ -558,11 +568,16 @@ mod extern_rust_fn_return_result_unit_type_and_transparent_enum_type {
         }
     }
 
+    // In Rust 1.79.0 dead_code warnings are issued for wrapped data in enums in spite of the enum
+    // having `#[repr(C)]`. `#[allow(unused)]` can be removed following resolution and release of this
+    // issue: https://github.com/rust-lang/rust/issues/126706
     fn expected_rust_tokens() -> ExpectedRustTokens {
         ExpectedRustTokens::Contains(quote! {
             #[repr(C)]
             pub enum ResultVoidAndSomeErrEnum{
+                #[allow(unused)]
                 Ok,
+                #[allow(unused)]
                 Err(__swift_bridge__SomeErrEnum),
             }
 
@@ -628,12 +643,17 @@ mod extern_rust_fn_return_result_tuple_type_and_transparent_enum_type {
         }
     }
 
+    // In Rust 1.79.0 dead_code warnings are issued for wrapped data in enums in spite of the enum
+    // having `#[repr(C)]`. `#[allow(unused)]` can be removed following resolution and release of this
+    // issue: https://github.com/rust-lang/rust/issues/126706
     fn expected_rust_tokens() -> ExpectedRustTokens {
         ExpectedRustTokens::ContainsMany(vec![
             quote! {
                 #[repr(C)]
                 pub enum ResultTupleI32U32AndSomeErrEnum{
+                    #[allow(unused)]
                     Ok(__swift_bridge__tuple_I32U32),
+                    #[allow(unused)]
                     Err(__swift_bridge__SomeErrEnum),
                 }
             },
@@ -687,3 +707,77 @@ typedef struct __swift_bridge__$ResultTupleI32U32AndSomeErrEnum{__swift_bridge__
         .test();
     }
 }
+
+/// Test code generation for Rust function that returns a Result<T, E> where T is () and
+/// E is a transparent struct type.
+mod extern_rust_fn_return_result_unit_type_and_transparent_struct_type {
+    use super::*;
+
+    fn bridge_module_tokens() -> TokenStream {
+        quote! {
+            mod ffi {
+                struct SomeErrStruct {
+                    inner: String,
+                }
+                extern "Rust" {
+                    fn some_function() -> Result<(), SomeErrStruct>;
+                }
+            }
+        }
+    }
+
+    // In Rust 1.79.0 dead_code warnings are issued for wrapped data in enums in spite of the enum
+    // having `#[repr(C)]`. `#[allow(unused)]` can be removed following resolution and release of this
+    // issue: https://github.com/rust-lang/rust/issues/126706
+    fn expected_rust_tokens() -> ExpectedRustTokens {
+        ExpectedRustTokens::Contains(quote! {
+            #[repr(C)]
+            pub enum ResultVoidAndSomeErrStruct{
+                #[allow(unused)]
+                Ok,
+                #[allow(unused)]
+                Err(__swift_bridge__SomeErrStruct),
+            }
+
+            #[export_name = "__swift_bridge__$some_function"]
+            pub extern "C" fn __swift_bridge__some_function() -> ResultVoidAndSomeErrStruct{
+                match super::some_function() {
+                    Ok(ok) => ResultVoidAndSomeErrStruct::Ok,
+                    Err(err) => ResultVoidAndSomeErrStruct::Err(err.into_ffi_repr()),
+                }
+            }
+        })
+    }
+
+    fn expected_swift_code() -> ExpectedSwiftCode {
+        ExpectedSwiftCode::ContainsAfterTrim(
+            r#"
+public func some_function() throws -> () {
+    try { let val = __swift_bridge__$some_function(); switch val.tag { case __swift_bridge__$ResultVoidAndSomeErrStruct$ResultOk: return case __swift_bridge__$ResultVoidAndSomeErrStruct$ResultErr: throw val.payload.err.intoSwiftRepr() default: fatalError() } }()
+}
+"#,
+        )
+    }
+
+    fn expected_c_header() -> ExpectedCHeader {
+        ExpectedCHeader::ContainsManyAfterTrim(vec![
+            r#"
+typedef enum __swift_bridge__$ResultVoidAndSomeErrStruct$Tag {__swift_bridge__$ResultVoidAndSomeErrStruct$ResultOk, __swift_bridge__$ResultVoidAndSomeErrStruct$ResultErr} __swift_bridge__$ResultVoidAndSomeErrStruct$Tag;
+union __swift_bridge__$ResultVoidAndSomeErrStruct$Fields {struct __swift_bridge__$SomeErrStruct err;};
+typedef struct __swift_bridge__$ResultVoidAndSomeErrStruct{__swift_bridge__$ResultVoidAndSomeErrStruct$Tag tag; union __swift_bridge__$ResultVoidAndSomeErrStruct$Fields payload;} __swift_bridge__$ResultVoidAndSomeErrStruct;
+"#,
+            r#"struct __swift_bridge__$ResultVoidAndSomeErrStruct __swift_bridge__$some_function(void)"#,
+        ])
+    }
+
+    #[test]
+    fn extern_rust_result_transparent_struct_type_and_opaque_rust_type() {
+        CodegenTest {
+            bridge_module: bridge_module_tokens().into(),
+            expected_rust_tokens: expected_rust_tokens(),
+            expected_swift_code: expected_swift_code(),
+            expected_c_header: expected_c_header(),
+        }
+        .test();
+    }
+}

crates/swift-integration-tests/src/result.rs

@@ -1,8 +1,4 @@
 //! See also: crates/swift-bridge-ir/src/codegen/codegen_tests/result_codegen_tests.rs
-// This is a temporary workaround until https://github.com/chinedufn/swift-bridge/issues/270
-// is closed. When tests are compiled they have `-D warnings` (deny warnings) enabled, so
-// tests won't even compile unless this warning is ignored.
-#![allow(dead_code)]
 
 #[swift_bridge::bridge]
 mod ffi {
@@ -41,6 +37,17 @@ mod ffi {
         fn val(&self) -> u32;
     }
 
+    #[swift_bridge(swift_repr = "struct")]
+    struct ResultTransparentStruct {
+        pub inner: String,
+    }
+
+    extern "Rust" {
+        fn rust_func_return_result_null_transparent_struct(
+            succeed: bool,
+        ) -> Result<(), ResultTransparentStruct>;
+    }
+
     enum ResultTransparentEnum {
         NamedField { data: i32 },
         UnnamedFields(u8, String),
@@ -141,6 +148,32 @@ fn rust_func_return_result_unit_struct_opaque_rust(
     }
 }
 
+fn rust_func_return_result_null_transparent_struct(
+    succeed: bool,
+) -> Result<(), ffi::ResultTransparentStruct> {
+    if succeed {
+        Ok(())
+    } else {
+        Err(ffi::ResultTransparentStruct {
+            inner: "failed".to_string(),
+        })
+    }
+}
+
+impl std::error::Error for ffi::ResultTransparentStruct {}
+
+impl std::fmt::Debug for ffi::ResultTransparentStruct {
+    fn fmt(&self, _f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        unreachable!("Debug impl was added to pass `Error: Debug + Display` type checking")
+    }
+}
+
+impl std::fmt::Display for ffi::ResultTransparentStruct {
+    fn fmt(&self, _f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        unreachable!("Display impl was added to pass `Error: Debug + Display` type checking")
+    }
+}
+
 pub struct ResultTestOpaqueRustType {
     val: u32,
 }