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TLDR: Profile guided optimization was not supported on windows. That has been enabled now. So this only helps you if you are compiling on windows and want to go the extra mile of running PGO builds.

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They're shipping PGO builds of the Rust compiler, so for faster compilation you don't have to do anything ("Windows builds now use profile-guided optimization, providing 10-20% improvements to compiler performance", per Rust's release notes: https://github.com/rust-lang/rust/blob/master/RELEASES.md).

After enabling PGO, that was used to compile `rustc` itself.

So compiling Rust code on Windows is faster with 1.64 than 1.63.

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