Call for Testing: Speeding up compilation with hint-mostly-unused
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cc @rust-lang/cargo |
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Kobzol
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Jul 13, 2025
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saethlin
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| ## Background | ||
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| Some crates provide comprehensive APIs with a very large surface area, yet many |
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Perhaps it will help intuition to say this is for APIs that are monomorphic? Not sure how to put that.
saethlin
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| Note that this option does not provide a universal performance improvement for | ||
| every crate. Using it for crates whose API surface is mostly used, and/or used | ||
| in multiple different crates or binaries (e.g. multiple test binaries that each |
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I think this explanation really undersells the downside. The problem is that when misused, the flag either completely removes CGU partitioning or causes more items to be compiled in multiple CGUs than before. The way this paragraph is written currently makes it seem like the downsides don't apply if I know have one bin target and no tests.
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This blog post serves as a call for testing on the
hint-mostly-unusedfeature: giving people guidance on how it can help and how to use it, providing
sample performance numbers, asking for specific testing, and pointing people to
a tracking issue for feedback.
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