feat: Add //rust/settings:lto
#3104
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github-merge-queue bot
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Related to #3119 Adds an option named "manual" to `//rust/settings/lto` which was introduced in #3104. When set to "manual" we will not automatically set any flags so users with customized setups can configure the relevant flags on their own. FWIW this was an oversight and IMO should have been included in the original PR. Apologies to anyone who stumbled over this or encountered build issues!
ParkMyCar
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Fixes bazelbuild#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!
gibfahn
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Oct 21, 2025
The -Clto flag can conflict with parent module's Rust toolchain configuration, particularly when embed-bitcode=no is set. This change removes the hardcoded LTO flag to allow proper inheritance of toolchain settings. Related: bazelbuild/rules_rust#3104
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Fixes #3045
This PR adds a new build setting
//rust/settings:lto=(off|thin|fat)which changes how we specify the following flags:ltoembed-bitcodelinker-plugin-ltoThe way we invoke the flags was based on how Cargo does it today (code) and based on suggestions from the Rust docs.
When LTO is not enabled, we will specify
-Cembed-bitcode=nowhich tellsrustcto skip embedding LLVM bitcode and should speed up builds. Similarly when LTO is enabled we specify-Clinker-plugin-ltowhich will causerustcto 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!