When David Lattimore took the stage at RustForge 2025 in Wellington, he didn't just share linker benchmarks—he unveiled a masterclass in squeezing every drop of performance from Rust's type system. As the creator of the Wild linker, Lattimore demonstrated how clever zero-cost abstractions can revolutionize parallelism and memory management in systems programming. Here’s how his team achieves nanosecond-level optimizations for projects like Chromium.

🔄 Mutable Slicing for Lock-Free Parallelism

Wild's symbol resolution relies on a dense Vec<SymbolId> (where SymbolId wraps a u32). To enable parallel writes without locks, Lattimore leverages contiguous memory allocation per object and Rayon's par_bridge:

fn parallel_process_resolutions(mut resolutions: &mut [SymbolId], objects: &[Object]) {
   objects
       .iter()
       .map(|obj| (obj, resolutions.split_off_mut(..obj.num_symbols).unwrap()))
       .par_bridge()
       .for_each(|(obj, object_resolutions)| {
           obj.process_resolutions(object_resolutions);
       });
}

Key Insight: split_off_mut creates non-overlapping mutable slices, enabling threads to write to adjacent memory regions simultaneously. Cache locality is preserved by grouping symbols per object.

⚡️ Parallel Initialization with Sharded Vec Writer

Initializing a giant Vec serially is inefficient. Wild uses the sharded-vec-writer crate to populate memory in parallel:

let mut writer = VecWriter::new(&mut resolutions);
let mut shards = writer.take_shards(objects.iter().map(|o| o.num_symbols));

objects
   .par_iter()
   .zip_eq(&mut shards)
   .for_each(|(obj, shard)| {
      for symbol in obj.symbols() {
         shard.push(...);
      }
   });

writer.return_shards(shards);

Why it matters: This bypasses single-threaded initialization bottlenecks—critical for binaries with millions of symbols.

⚛️ Atomic Conversions: Zero-Cost Type Punning

When Chromium’s C++ headers caused symbol collisions, Wild needed atomic writes to SymbolId. But atomics add overhead. Solution? Type-safe in-place conversion:

fn into_atomic(symbols: Vec<SymbolId>) -> Vec<AtomicSymbolId> {
   symbols
       .into_iter()
       .map(|s| AtomicSymbolId(AtomicU32::new(s.0)))
       .collect()
}

Compiler Magic: Rust reuses the heap allocation, and the loop evaporates in assembly. Lattimore’s proof:

movups xmm0, xmmword ptr [rsi]
...
ret  // No loops, no branches!

">The representation of AtomicSymbolId is identical to SymbolId. We exploit this to make the optimizer do the heavy lifting," Lattimore noted.

♻️ Buffer Reuse Across Lifetimes

Recycling heap allocations often clashes with lifetimes. Wild’s solution? reuse_vec:

fn reuse_vec<T, U>(mut v: Vec<T>) -> Vec<U> {
   const {
       assert!(size_of::<T>() == size_of::<U>());
       assert!(align_of::<T>() == align_of::<U>());
   }
   v.clear();
   v.into_iter().map(|_| unreachable!()).collect()
}

Use Case: Convert Vec<&'a str> to Vec<&'b str> without reallocating. The compiler elides the loop, leaving only a length reset.

🧵 Offloading Deallocation

Freeing huge buffers blocks threads. Wild spawns Rayon tasks to drop them asynchronously:

rayon::spawn(|| drop(buffer));

Caveat: Only beneficial for massive allocations (verified via profiling). Combine with reuse_vec to sidestep lifetime issues.

💣 Bonus: Stripping Lifetimes with Non-Trivial Drop

For structs like Foo<'a> { owned: String, borrowed: &'a str }, Wild uses MaybeUninit to erase lifetimes pre-deallocation:

struct StaticFoo {
    owned: String,
    borrowed: MaybeUninit<&'static str>,
}

fn without_lifetime(foos: Vec<Foo>) -> Vec<StaticFoo> {
    foos.into_iter()
        .map(|f| StaticFoo { 
            owned: f.owned, 
            borrowed: MaybeUninit::uninit() 
        })
        .collect()
}

Again—zero runtime cost. The assembly is identical to a memcpy.

The Philosophy of Zero-Cost Fearlessness

Lattimore’s tricks reveal a deeper truth: Rust’s type system isn’t a barrier—it’s a toolkit for safe radical optimization. By leaning into representation guarantees and optimizer behavior, Wild achieves C-like speed without unsafe spaghetti. These patterns extend far beyond linkers; imagine applying atomic conversion to GPU buffers or reuse_vec to database caches. As Lattimore concluded: "When the compiler understands your intent, it becomes your most powerful ally in the quest for performance."

Source: David Lattimore's talk at RustForge 2025. Wild linker source code.