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>

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/).