编译时间

虽然这本书主要关于提升Rust程序的性能，这一节是关于减少Rust程序的编译时间， 因为这也是大多人关注的重要话题。

Linking

编译时间，其实有很大一部分是链接时间。尤其是小更改以后重新构建程序时。 我们可以选择比默认更快的链接器。

这里推荐lld，它支持ELF，PE/COFF，Mach-O，wasm等等。

通过命令行指定使用 lld，你可以在你的构建命令前加上

RUSTFLAGS="-C link-arg=-fuse-ld=lld"

通过config.toml指定使用 lld（应用于一个或者多个项目)，加入这些行：

[build]
rustflags = ["-C", "link-arg=-fuse-ld=lld"]

lld并未完全支持让Rust使用，但大多情况下可以。有个Github Issue追踪lld的完整支持。

另外你也可以选择mold，目前只支持ELF。指定使用它和lld一样，把lld换成mold就可以了。

mold通常比lld更快。它也更新，有时无法工作。

增量编译

Rust编译器支持增量编译，避免在重编译的时候做重复的工作。它可以大大提升编译速度， 但有时会让生成的可执行程序运行的慢一些。因此，它只默认为调试构建启用。 如果你也想为发布构建启用，把这些加到Cargo.toml：

[profile.release]
incremental = true

incremental设置，以及为不同配置启用特定设置详见Cargo文档。

可视化

Cargo有个功能，可以可视化你的程序的编译过程。在Rust 1.60以上使用该命令构建：

cargo build --timings

或者（1.59以下）：

cargo +nightly build -Ztimings

On completion it will print the name of an HTML file. Open that file in a web browser. It contains a Gantt chart that shows the dependencies between the various crates in your program. This shows how much parallelism there is in your crate graph, which can indicate if any large crates that serialize compilation should be broken up. See the documentation for more details on how to read the graphs.

LLVM IR

The Rust compiler uses LLVM for its back-end. LLVM’s execution can be a large part of compile times, especially when the Rust compiler’s front end generates a lot of IR which takes LLVM a long time to optimize.

These problems can be diagnosed with cargo llvm-lines, which shows which Rust functions cause the most LLVM IR to be generated. Generic functions are often the most important ones, because they can be instantiated dozens or even hundreds of times in large programs.

If a generic function causes IR bloat, there are several ways to fix it. The simplest is to just make the function smaller. Example 1, Example 2.

Another way is to move the non-generic parts of the function into a separate, non-generic function, which will only be instantiated once. Whether or not this is possible will depend on the details of the generic function. The non-generic function can often be written as an inner function within the generic function, to minimize its exposure to the rest of the code. Example.

Sometimes common utility functions like Option::map and Result::map_err are instantiated many times. Replacing them with equivalent match expressions can help compile times.

The effects of these sorts of changes on compile times will usually be small, though occasionally they can be large. Example.