A ld -lazy_framework for macOS. Designed to build Deno.

Usage

Build the static lib (make), then drive the final link through lzld and link liblzld_arm64.a:

[target.aarch64-apple-darwin]
rustflags = [
  "-C",
  "link-args=-fuse-ld=/path/to/lzld/lzld -L/path/to/lzld -llzld_arm64",
]

On Nix-based toolchains, set the linker driver to the system clang so the macOS SDK (with libiconv.tbd) is used:

CARGO_TARGET_AARCH64_APPLE_DARWIN_LINKER=/usr/bin/cc

Usage without lzld wrapper

1. rustc -Z link-native-libraries=no -L/path/to/lzld -llzld: Requires nightly but doesn't need a wrapper linker.

2. Manaully source modification: Remove #[link] attributes from all dependencies and link to liblzld.a.

Design

It's pretty simple. Drop in lzld as the linker. It strips -framework and -weak_framework arguments and links a static library (liblzld.a) that lazy loads each framework via dlopen on first use. Rest of the arguments are passed as-is to ld64.lld.

Each symbol Deno imports from a stripped framework is provided as a thin shim: on first call it dlopens the framework and caches the resolved function pointer. Data constants (which have no call to intercept) are handled per symbol — kCFAllocatorDefault is NULL (the documented default-allocator sentinel); kCFTypeArrayCallBacks / kCFRunLoopDefaultMode are substituted by the wrapper shims at the use site once their framework is loaded.

Measured effect (Deno, aarch64, warm)

Same tree and flags, lzld vs. stock link:

invocation	stock	lzld	delta
deno --version	6.29 ms	5.32 ms	-0.97 ms (-15%)
deno run hello	12.49 ms	12.14 ms	-0.35 ms

The minimal path never touches the frameworks, so deferral is a clean win. A full deno run pulls part of the framework graph back via dyld initializers, so the gain is smaller there.

Covered symbols

Generated from the live import set (dyld_info -imports <deno>): CoreFoundation (CFArray / CFString / CFURL / CFRunLoop + data constants), CoreServices (FSEvents), Security (Keychain), Foundation (NSKeyValueChangeNewKey), Metal (MTLCopyAllDevices). QuartzCore, CoreGraphics, MetalPerformanceShaders and AVFoundation are linked but import no symbols, so they are simply dropped.

Regenerate the shim set when Deno's imports change:

dyld_info -imports <deno> | grep -E '\(from (CoreFoundation|CoreServices|Security|Foundation|Metal)\)'

Set LZLD_TRACE=1 at runtime to print a backtrace the first time each framework is forced to load — useful for finding startup call sites that defeat laziness.

Contributions

Pull requests are disabled. Send a git format-patch attachment to me@littledivy.com.