Swift 6.2: Generated code collides with new Data.bytes property (SE-0456)

[llbartekll]

Problem

Swift 6.2 introduced a new Data.bytes property that returns RawSpan (via SE-0456). This collides with UniFFI's generated Swift code which uses bytes as a parameter name.

Generated code that causes the issue

fileprivate extension RustBuffer {
    // Allocate a new buffer, copying the contents of a `UInt8` array.
    init(bytes: [UInt8]) {
        let rbuf = bytes.withUnsafeBufferPointer { ptr in
            RustBuffer.from(ptr)
        }
        self.init(capacity: rbuf.capacity, len: rbuf.len, data: rbuf.data)
    }
    // ...
}

Compilation errors

When building with Xcode 26.x / Swift 6.2.x:

1. Cannot convert value of type 'RawSpan' to expected argument type 'UnsafePointer<UInt8>'
2. Ambiguous use of 'bytes'

The Swift compiler gets confused between:

• The local parameter bytes: [UInt8]
• The new Data.bytes property returning RawSpan

Suggested Fix

Rename the bytes parameter to something that won't collide, like byteArray:

fileprivate extension RustBuffer {
    init(byteArray: [UInt8]) {
        let rbuf = byteArray.withUnsafeBufferPointer { ptr in
            RustBuffer.from(ptr)
        }
        self.init(capacity: rbuf.capacity, len: rbuf.len, data: rbuf.data)
    }
}

And update the call site:

// From:
return RustBuffer(bytes: writer)
// To:
return RustBuffer(byteArray: writer)

Workaround

We've implemented a post-processing sed script that patches the generated Swift code:

sed -i '' 's/init(bytes: \[UInt8\])/init(byteArray: [UInt8])/g' "$swift_file"
sed -i '' 's/let rbuf = bytes\.withUnsafeBufferPointer/let rbuf = byteArray.withUnsafeBufferPointer/g' "$swift_file"
sed -i '' 's/RustBuffer(bytes: writer)/RustBuffer(byteArray: writer)/g' "$swift_file"

This is backwards compatible and works on both Swift 5.x and Swift 6.x.

Environment

• UniFFI version: 0.31.0
• Swift version: 6.2.3 (Xcode 26.2)
• Platform: iOS/macOS

Related

• Swift Evolution SE-0456: https://github.com/swiftlang/swift-evolution/blob/main/proposals/0456-stdlib-span-properties.md

[pronebird]

Could you reg-replace it in here https://github.com/mozilla/uniffi-rs/blob/main/uniffi_bindgen/src/bindings/swift/templates/RustBufferTemplate.swift#L3 perhaps

[bendk]

Yeah, a PR for this would be great. Maybe you could use uniffiBytes as the argument name, we use the uniffi prefix in a bunch of other cases to avoid name conflicts.