## Keyboard shortcuts

Press `←` or `→` to navigate between chapters

Press `S` or `/` to search in the book

Press `?` to show this help

Press `Esc` to hide this help

# `hint-mostly-unused`

This flag hints to the compiler that most of the crate will probably go unused. The compiler can optimize its operation based on this assumption, in order to compile faster. This is a hint, and does not guarantee any particular behavior.

This option can substantially speed up compilation if applied to a large dependency where the majority of the dependency does not get used. This flag may slow down compilation in other cases.

Currently, this option makes the compiler defer as much code generation as possible from functions in the crate, until later crates invoke those functions. Functions that never get invoked will never have code generated for them. For instance, if a crate provides thousands of functions, but only a few of them will get called, this flag will result in the compiler only doing code generation for the called functions. (This uses the same mechanisms as cross-crate inlining of functions.) This does not affect `extern` functions, or functions marked as `#[inline(never)]`.

To try applying this flag to one dependency out of a dependency tree, use the `profile-rustflags` feature of nightly cargo:

    cargo-features = ["profile-rustflags"]

    # ...
    [dependencies]
    mostly-unused-dependency = "1.2.3"

    [profile.release.package.mostly-unused-dependency]
    rustflags = ["-Zhint-mostly-unused"]