iOS build fails: fp16kernels feature check in build.rs doesn't work for unsupported targets

[durch]

Description

Building for iOS target (aarch64-apple-ios or aarch64-apple-ios-sim) fails with:

error: Unable to build f16 kernels on given target_arch. Please use x86_64 or aarch64 or remove the fp16kernels feature

This happens even when fp16kernels feature is not enabled.

Root Cause

In rust/lance-linalg/build.rs, there are two bugs:

Bug 1: Feature check uses wrong method

fn build_f16_with_flags(suffix: &str, flags: &[&str]) -> Result<(), cc::Error> {
    if cfg!(not(feature = "fp16kernels")) {  // ❌ This checks build.rs features, not the crate's
        return Ok(());
    }
    // ...
}

cfg!(feature = "fp16kernels") is a compile-time check on the build script itself, not the library being built. The correct check is:

if env::var("CARGO_FEATURE_FP16KERNELS").is_err() {
    return Ok(());
}

Bug 2: Error returned before feature check for unsupported targets

fn main() -> Result<(), String> {
    // ... arch/os checks ...
    
    if target_arch == "aarch64" && target_os == "macos" {
        build_f16_with_flags(...)?;  // Feature check happens inside
    } else if target_arch == "aarch64" && target_os == "linux" {
        build_f16_with_flags(...)?;
    } else if target_arch == "x86_64" {
        // ...
    } else {
        return Err("Unable to build f16 kernels...");  // ❌ Error BEFORE any feature check!
    }
}

For iOS (target_os = "ios"), the code falls through to the else branch and returns an error before build_f16_with_flags is ever called.

Proposed Fix

Add the feature check at the top of main(), before any architecture branching:

fn main() -> Result<(), String> {
    // ... nightly check, rerun-if-changed ...

    // Check if fp16kernels feature is enabled - if not, skip everything
    if env::var("CARGO_FEATURE_FP16KERNELS").is_err() {
        return Ok(());
    }

    // ... rest of the code ...
}

This way, when fp16kernels is disabled (which is the default), the build script exits cleanly on all platforms including iOS, Android, and any future targets.

Environment

• lancedb v0.23.1
• lance-linalg v1.0.1
• Target: aarch64-apple-ios-sim
• Using: lancedb = { version = "0.23", default-features = false }

Related

• Android build fails due to aarch64 in Rust #2411 - Similar issue for Android builds