commit fae114ca2f4da560b6c85da47227d6b4769be8d6
author Parker Timmerman <parker@parkertimmerman.com> Mon Dec 16 11:54:53 2024 -0500
committer GitHub <noreply@github.com> Mon Dec 16 16:54:53 2024 +0000
tree a72e786fb493e02c26c18003fe4f19455a5b2920
parent 4652fb58ddf8d310aca62b1f83a9fa867ab11a40

feat: Add `//rust/settings:lto` (#3104)

Fixes https://github.com/bazelbuild/rules_rust/issues/3045

This PR adds a new build setting `//rust/settings:lto=(off|thin|fat)`
which changes how we specify the following flags:

*
[`lto`](https://doc.rust-lang.org/rustc/codegen-options/index.html#lto)
*
[`embed-bitcode`](https://doc.rust-lang.org/rustc/codegen-options/index.html#embed-bitcode)
*
[`linker-plugin-lto`](https://doc.rust-lang.org/rustc/codegen-options/index.html#linker-plugin-lto)

The way we invoke the flags was based on how Cargo does it today
([code](https://github.com/rust-lang/cargo/blob/769f622e12db0001431d8ae36d1093fb8727c5d9/src/cargo/core/compiler/lto.rs#L4))
and based on suggestions from the [Rust
docs](https://doc.rust-lang.org/rustc/codegen-options/index.html#embed-bitcode).

When LTO is not enabled, we will specify `-Cembed-bitcode=no` which
tells `rustc` to skip embedding LLVM bitcode and should speed up builds.
Similarly when LTO is enabled we specify `-Clinker-plugin-lto` which
will cause `rustc` to skip generating objects files entirely, and
instead replace them with LLVM bitcode*.

*only when building an `rlib`, when building other crate types we
continue generating object files.

I added unit tests to make sure we pass the flags correctly, as well as
some docs describing the new setting. Please let me know if I should add
more!