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The rustc `-Zhint-mostly-unused` flag tells rustc that most of a crate will go unused. This is useful for speeding up compilation of large dependencies from which you only use a few items. Plumb that option through as a profile option, to allow specifying it for specific dependencies: ```toml [profile.dev.package.huge-mostly-unused-dependency] hint-mostly-unused = true ``` To enable this feature, pass `-Zprofile-hint-mostly-unused`. However, since this option is a hint, using it without passing `-Zprofile-hint-mostly-unused` will only warn and ignore the profile option. Versions of Cargo prior to the introduction of this feature will give an "unused manifest key" warning, but will otherwise function without erroring. This allows using the hint in a crate's `Cargo.toml` without mandating the use of a newer Cargo to build it. Add a test verifying that the profile option gets ignored with a warning without passing `-Zprofile-hint-mostly-unused`, and another test verifying that it gets handled when passing `-Zprofile-hint-mostly-unused`.