rust-witness fails to compile for iOS targets if using both Circom and Noir

[moven0831]

Description

When building a project that uses rust-witness v0.1.6 for iOS targets, the build fails during the w2c2 WASI compilation phase with an implicit function declaration error for getentropy in building C++ bindings, which is a part of Noir dependencies.

Environment

• rust-witness version: 0.1.6
• Selected proving system: Circom, Halo2, and Noir
• Rust toolchain: Stable 1.89.0
• Xcode: 16.0

Error Output

error: implicit declaration of function 'getentropy' [-Werror,-Wimplicit-function-declaration]
 3970 |     result = getentropy(
      |              ^

Full error context shows this occurs in w2c2/wasi/wasi.c during CMake build for iOS targets.

Root Cause

The issue appears to be in the w2c2 WASI implementation that rust-witness uses for transpiling WebAssembly to C. When cross-compiling for iOS:

1. The getentropy function is not properly declared/available for iOS targets
2. The CMake build uses macOS sysroot while targeting iOS, causing compatibility issues
3. The w2c2 transpiler doesn't properly handle iOS cross-compilation requirements

Acceptance Criteria

mopro build should have no problems if using all of the supported proving systems. By either

• Detect iOS cross-compilation and use appropriate build flags/sysroot
• Consider providing pre-transpiled witnesses or alternative witness generation methods for mobile targets

Related Issues

• Should be considered and fixed with the refactor of #536