stdio `FILE` not available in WASILibc?

[helje5]

export PATH=/Library/Developer/Toolchains/swift-wasm-5.3.1-RELEASE.xctoolchain/usr/bin:$PATH

echo 'import struct WASILibc.FILE; print("Hello, world!")' > hello.swift

swiftc -target wasm32-unknown-wasi hello.swift -o hello.wasm
hello.swift:1:15: error: struct 'FILE' does not exist in module 'WASILibc'
import struct WASILibc.FILE; print("Hello, world!")
              ^        ~~~~

[kateinoigakukun]

wasi-libc's FILE type is an incomplete struct, so swiftc can't import that from C header. Please use OpaquePointer instead of UnsafePointer<FILE>

#if canImport(Darwin)
    import Darwin
    typealias FilePointer = UnsafeMutablePointer<FILE>
#elseif canImport(WASILibc)
    import WASILibc
    typealias FilePointer = OpaquePointer
#elseif canImport(Glibc)
    import Glibc
    typealias FilePointer = UnsafeMutablePointer<FILE>
#endif

See also: #2313 (comment)

[helje5]

Do you plan to add a shim for this? A lot of code will expect a FILE type.

[kateinoigakukun]

Hmm, it requires

1. Change FILE type as complete type in wasi-libc
2. or new language feature to import incomplete type from C

to provide FILE type in Swift world.

[helje5]

My thinking was that maybe somehow you can use Swift/C mapping annotations to convert the type from Opaque to Mutable. FILE itself could be an (empty) Swift struct, C code doesn't need to peek into the structure, it just needs to have it available.

Not sure this is possible, but I'd guess "maybe".