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Add alternative linker to the build performance guide #15991

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a7aa52d
Clarify build time improvements
Kobzol Oct 12, 2025
2d87627
Move Markdown links near their correct section
Kobzol Oct 12, 2025
1c18e7e
Add section about using an alternative linker
Kobzol Oct 12, 2025
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32 changes: 26 additions & 6 deletions src/doc/src/guide/build-performance.md
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This reminds maybe we should consider a cargo native lld option for build.linker: #11378.

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That might be nice indeed, but the situation with linkers is a bit complicated. You can use the self-contained LLD, or an external LLD, on Linux you invoke LLD through cc and -fuse-ld=lld, while elsewhere through -Clinker, if the linker is not lld, but something else unsupported by the CC driver, it cannot be specified by name, but has to be passed through some hacks with an absolute path.. it's quite messy.

Original file line number Diff line number Diff line change
Expand Up @@ -38,7 +38,7 @@ This will:
- Provide an opt-in for when debugging via [`--profile debugging`](../reference/profiles.md#custom-profiles)

Trade-offs:
- ✅ Faster build times
- ✅ Faster code generation (`cargo build`)
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imo that is too jargony without the surrounding context. When I see code generation, I think of the type of thing build scripts usually do

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Should I create some basic terminology at the start of the page, where we'll say that compilation is split into three categories (frontend, backend, codegen), and then we'll say in each mechanism which of these categories it helps?

- ✅ Faster link times
- ✅ Smaller disk usage of the `target` directory
- ❌ Requires a full rebuild to have a high-quality debugger experience
Expand Down Expand Up @@ -83,9 +83,33 @@ rustflags = "-Zthreads=8"
This [`rustflags`][build.rustflags] will enable the [parallel frontend][parallel-frontend-blog] of the Rust compiler, and tell it to use `n` threads. The value of `n` should be chosen according to the number of cores available on your system, although there are diminishing returns. We recommend using at most `8` threads.

Trade-offs:
- ✅ Faster build times
- ✅ Faster build times (both `cargo check` and `cargo build`)
- ❌ **Requires using nightly Rust and an [unstable Rust feature][parallel-frontend-issue]**

[parallel-frontend-blog]: https://blog.rust-lang.org/2023/11/09/parallel-rustc/
[parallel-frontend-issue]: https://github.com/rust-lang/rust/issues/113349
[build.rustflags]: ../reference/config.md#buildrustflags

### Use an alternative linker

Consider: installing and configuring an alternative linker, like [LLD](https://lld.llvm.org/), [mold](https://github.com/rui314/mold) or [wild](https://github.com/davidlattimore/wild). For example, to configure mold on Linux, you can add to your `.cargo/config.toml`:

```toml
[target.'cfg(target_os = "linux")']
# mold, if you have GCC 12+
rustflags = ["-C", "link-arg=-fuse-ld=mold"]

# mold, otherwise
linker = "clang"
rustflags = ["-C", "link-arg=-fuse-ld=/path/to/mold"]
```

While dependencies may be built in parallel, linking all of your dependencies happens at once at the end of your build, which can make linking dominate your build times, especially for incremental rebuilds. Often, the linker Rust uses is already fairly fast and the gains from switching may not be worth it, but it is not always the case. For example, Linux targets besides `x86_64-unknown-linux-gnu` still use the Linux system linker which is quite slow (see [rust#39915](https://github.com/rust-lang/rust/issues/39915) for more details).

Trade-offs:
- ✅ Faster link times
- ❌ Might not support all use-cases, in particular if you depend on C or C++ dependencies

## Reducing built code

### Removing unused dependencies
Expand All @@ -103,7 +127,3 @@ it can be easy to miss that a dependency is no longer used and can be removed.
Trade-offs:
- ✅ Faster full build and link times
- ❌ May incorrectly flag dependencies as unused or miss some

[parallel-frontend-blog]: https://blog.rust-lang.org/2023/11/09/parallel-rustc/
[parallel-frontend-issue]: https://github.com/rust-lang/rust/issues/113349
[build.rustflags]: ../reference/config.md#buildrustflags