RFC: Recommend and enable using Wild rather than Mold on Linux for local builds (#37717)

# Summary 

Today, Zed uses Mold on Linux, but Wild can be significantly faster. 

On my machine, Wild is 14% faster at a whole-tree clean build, 20%
faster on an incremental build with a minimal change, and makes no
measurable effect on runtime performance of tests.

However, Wild's page says it's not yet ready for production, so it seems
to early to switch for production and CI builds.

This PR keeps using Mold in CI and lets developers choose in their own
config what linker to use. (The downside of this is that after landing
this change, developers will have to do some local config or it will
fall back to the default linker which may be slower.)

[Wild 0.6 is out, and their announcement has some
benchmarks](https://davidlattimore.github.io/posts/2025/09/23/wild-update-0.6.0.html).

cc @davidlattimore from Wild, just fyi

# Tasks

- [x] Measure Wild build, incremental build, and runtime performance in
different scenarios
- [x] Remove the Linux linker config from `.cargo/config.toml` in the
tree
- [x] Test rope benchmarks etc
- [x] Set the linker to Mold in CI 
- [x] Add instructions to use Wild or Mold into `linux.md`
- [x] Add a script to download Wild
- [x] Measure binary size
- [x] Recommend Wild from `scripts/linux`

# Benchmarks 

| | wild 0.6 (rust 1.89) | mold 2.37.1 (1.89) | lld (rust 1.90) | wild
advantage |
| -- | -- | -- | -- | -- |
| clean workspace build | 176s | 184s | 182s | 5% faster than mold |
| nextest run workspace after build | 137s | 142s | 137s | in the noise?
|
| incremental rebuild | 3.9s | 5.0s | 6.6s | 22% faster than mold | 

I didn't observe any apparent significant change in runtime performance
or binary size, or in the in-tree microbenchmarks.

Release Notes:

- N/A

---------

Co-authored-by: Mateusz Mikuła <oss@mateuszmikula.dev>

.cargo/ci-config.toml

@@ -10,3 +10,15 @@
 # Here, we opted to use `[target.'cfg(all())']` instead of `[build]` because `[target.'**']` is guaranteed to be cumulative.
 [target.'cfg(all())']
 rustflags = ["-D", "warnings"]
+
+# Use Mold on Linux, because it's faster than GNU ld and LLD.
+#
+# We no longer set this in the default `config.toml` so that developers can opt in to Wild, which
+# is faster than Mold, in their own ~/.cargo/config.toml.
+[target.x86_64-unknown-linux-gnu]
+linker = "clang"
+rustflags = ["-C", "link-arg=-fuse-ld=mold"]
+
+[target.aarch64-unknown-linux-gnu]
+linker = "clang"
+rustflags = ["-C", "link-arg=-fuse-ld=mold"]

.cargo/config.toml

@@ -7,14 +7,6 @@ xtask = "run --package xtask --"
 perf-test = ["test", "--profile", "release-fast", "--lib", "--bins", "--tests", "--all-features", "--config", "target.'cfg(true)'.runner='cargo run -p perf --release'", "--config", "target.'cfg(true)'.rustflags=[\"--cfg\", \"perf_enabled\"]"]
 perf-compare = ["run", "--release", "-p", "perf", "--", "compare"]
 
-[target.x86_64-unknown-linux-gnu]
-linker = "clang"
-rustflags = ["-C", "link-arg=-fuse-ld=mold"]
-
-[target.aarch64-unknown-linux-gnu]
-linker = "clang"
-rustflags = ["-C", "link-arg=-fuse-ld=mold"]
-
 [target.'cfg(target_os = "windows")']
 rustflags = [
     "--cfg",

docs/src/development/linux.md

@@ -20,6 +20,33 @@ Clone down the [Zed repository](https://github.com/zed-industries/zed).
 
 If you are looking to develop Zed collaboration features using a local collaboration server, please see: [Local Collaboration](./local-collaboration.md) docs.
 
+### Linkers {#linker}
+
+On Linux, Rust's default linker is [LLVM's `lld`](https://blog.rust-lang.org/2025/09/18/Rust-1.90.0/). Alternative linkers, especially [Wild](https://github.com/davidlattimore/wild) and [Mold](https://github.com/rui314/mold) can significantly improve clean and incremental build time.
+
+At present Zed uses Mold in CI because it's more mature. For local development Wild is recommended because it's 5-20% faster than Mold.
+
+These linkers can be installed with `script/install-mold` and `script/install-wild`.
+
+To use Wild as your default, add these lines to your `~/.cargo/config.toml`:
+
+```toml
+[target.x86_64-unknown-linux-gnu]
+linker = "clang"
+rustflags = ["-C", "link-arg=--ld-path=wild"]
+
+[target.aarch64-unknown-linux-gnu]
+linker = "clang"
+rustflags = ["-C", "link-arg=--ld-path=wild"]
+```
+
+To use Mold as your default:
+
+```toml
+[target.'cfg(target_os = "linux")']
+rustflags = ["-C", "link-arg=-fuse-ld=mold"]
+```
+
 ## Building from source
 
 Once the dependencies are installed, you can build Zed using [Cargo](https://doc.rust-lang.org/cargo/).

script/install-wild

@@ -0,0 +1,29 @@
+#!/usr/bin/env bash
+
+# Install wild-linker official binaries from GitHub Releases.
+
+set -euo pipefail
+
+WILD_VERSION="${WILD_VERSION:-${1:-0.6.0}}"
+if [ "$(uname -s)" != "Linux" ]; then
+    echo "Error: This script is intended for Linux systems only."
+    exit 1
+elif [ -z "$WILD_VERSION" ]; then
+    echo "Usage: $0 [version]"
+    exit 1
+elif which -s wild && wild --version | grep -Fq "$WILD_VERSION" ; then
+    echo "Warning: existing wild $WILD_VERSION found at $(which wild). Skipping installation."
+    exit 0
+fi
+
+if [ "$(whoami)" = root ]; then SUDO=; else SUDO="$(command -v sudo || command -v doas || true)"; fi
+
+ARCH="$(uname -m)"
+WILD_REPO="${WILD_REPO:-https://github.com/davidlattimore/wild}"
+WILD_PACKAGE="wild-linker-${ARCH}-unknown-linux-gnu"
+WILD_URL="${WILD_URL:-$WILD_REPO}/releases/download/$WILD_VERSION/${WILD_PACKAGE}.tar.gz"
+
+echo "Downloading from $WILD_URL"
+curl -fsSL --output - "$WILD_URL" \
+    | $SUDO tar -C /usr/local/bin --strip-components=1 --no-overwrite-dir -xJf - \
+    "wild-linker-${ARCH}-unknown-linux-gnu/wild"

script/linux

@@ -11,12 +11,23 @@ fi
 function finalize {
   # after packages install (curl, etc), get the rust toolchain
   which rustup > /dev/null 2>&1 || curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y
-  # verify the mold situation
-  if ! command -v mold >/dev/null 2>&1; then
-    echo "Warning: Mold binaries are unavailable on your system." >&2
-    echo "    Builds will be slower without mold. Try: script/install-mold" >&2
-  fi
-  echo "Finished installing Linux dependencies with script/linux"
+  cat <<EOF
+Note: It's recommended to install and configure Wild or Mold for faster builds.
+      Run script/install-wild or script/install-mold.
+      Then add these lines to your ~/.cargo/config.toml:
+
+[target.x86_64-unknown-linux-gnu]
+linker = "clang"
+rustflags = ["-C", "link-arg=--ld-path=wild"]
+
+[target.aarch64-unknown-linux-gnu]
+linker = "clang"
+rustflags = ["-C", "link-arg=--ld-path=wild"]
+
+
+
+Finished installing Linux dependencies with script/linux
+EOF
 }
 
 # Ubuntu, Debian, Mint, Kali, Pop!_OS, Raspbian, etc.