Switch from ld (the default linker) to using lld for GNU Linux targets

**What's wrong with `ld`?** It's very slow and uses a lot of memory.

**Why `lld`?** It's fast, mature, and well-supported. Meta and Google use it for all their linking workloads. We're already using it for macos and x86-64 Windows. There is [ongoing work to make it the default for rustc](rust-lang/rust#71515), and it already is default on a few platforms.

**Why not `mold`?** Mold is generally faster, but the margin is slim enough for our workloads that it doesn't really matter. Mold only recently got support for LTO, doesn't support v0 rust symbol demanging, doesn't support BOLT (though we don't use that yet), etc. Mold is maturing quickly, but `lld` still seems like the "safer" choice.

Author: bgw

.cargo/config.toml

@@ -15,11 +15,18 @@ rustdocflags = ["-Znormalize-docs"]
 [target.x86_64-pc-windows-msvc]
 linker = "rust-lld"
 
-[target.aarch64-apple-darwin]
+[target.'cfg(target_os = "macos")']
 linker = "rust-lld"
 
-[target.x86_64-apple-darwin]
-linker = "rust-lld"
+[target.'cfg(all(target_os = "linux", target_env = "gnu"))']
+rustflags = [
+  "--cfg",
+  "tokio_unstable",
+  "-Zshare-generics=y",
+  "-Zthreads=8",
+  "-Csymbol-mangling-version=v0",
+  "-Clink-arg=-fuse-ld=lld",
+]
 
 [alias]
 xtask = "run --package xtask --"

CONTRIBUTING.md

@@ -29,6 +29,10 @@ Thanks for your interest in contributing to Turbo!
 - [Rust](https://www.rust-lang.org/tools/install)
 - [cargo-groups](https://github.com/nicholaslyang/cargo-groups) (used to group crates into Turborepo-specific ones and Turbopack-specific ones)
 
+### Linux Dependencies
+
+- LLD (LLVM Linker), as it's not installed by default on many Linux distributions (e.g. `apt install lld`).
+
 ## Contributing to Turborepo
 
 ### Building Turborepo