feat: Add //rust/settings:lto
#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!