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The Rust Reference

    Preludes

    Aprelude is a collection of names that are automatically brought into scopeof every module in a crate.

    These prelude names are not part of the module itself: they are implicitlyqueried duringname resolution. For example, even though something likeBox is in scope in every module, you cannot refer to it asself::Boxbecause it is not a member of the current module.

    There are several different preludes:

    Standard library prelude

    Each crate has a standard library prelude, which consists of the names from a single standard library module.

    The module used depends on the crate’s edition, and on whether theno_std attribute is applied to the crate:

    Extern prelude

    External crates imported withextern crate in the root module or providedto the compiler (as with the--extern flag withrustc) are added to theextern prelude. If imported with an alias such asextern crate orig_name as new_name, then the symbolnew_name is instead added to the prelude.

    Thecore crate is always added to the extern prelude.

    Thestd crate is added as long as theno_std attribute is not specified in the crate root.

    2018 Edition differences

    In the 2015 edition, crates in the extern prelude cannot be referenced viause declarations, so it is generally standard practice to includeextern crate declarations to bring them into scope.

    Beginning in the 2018 edition,use declarations can reference crates in the extern prelude, so it is considered unidiomatic to useextern crate.

    Note

    Additional crates that ship withrustc, such asalloc, andtest, are not automatically included with the--extern flag when using Cargo. They must be brought into scope with anextern crate declaration, even in the 2018 edition.

    #![allow(unused)]fn main() {extern crate alloc;use alloc::rc::Rc;}

    Cargo does bring inproc_macro to the extern prelude for proc-macro crates only.

    Theno_std attribute

    By default, the standard library is automatically included in the crate rootmodule. Thestd crate is added to the root, along with an implicitmacro_use attribute pulling in all macros exported fromstd into themacro_use prelude. Bothcore andstd are added to theexternprelude.

    Theno_stdattribute may be applied at the crate level to prevent thestd crate from being automatically added into scope.

    It does three things:

    • Injects thecore crate into the crate root instead ofstd, and pullsin all macros exported fromcore in themacro_use prelude.

    Note

    Using the core prelude over the standard prelude is useful when either the crate is targeting a platform that does not support the standard library or is purposefully not using the capabilities of the standard library. Those capabilities are mainly dynamic memory allocation (e.g.Box andVec) and file and network capabilities (e.g.std::fs andstd::io).

    Warning

    Usingno_std does not prevent the standard library from being linked in. It is still valid to putextern crate std; into the crate and dependencies can also link it in.

    Language prelude

    The language prelude includes names of types and attributes that are built-into the language. The language prelude is always in scope.

    It includes the following:

    macro_use prelude

    Themacro_use prelude includes macros from external crates that wereimported by themacro_use attribute applied to anextern crate.

    Tool prelude

    The tool prelude includes tool names for external tools in thetypenamespace. See thetool attributes section for more details.

    Theno_implicit_prelude attribute

    Theno_implicit_preludeattribute may be applied at the crate level oron a module to indicate that it should not automatically bring thestandardlibrary prelude,extern prelude, ortool prelude into scope for thatmodule or any of its descendants.

    This attribute does not affect thelanguage prelude.

    2018 Edition differences

    In the 2015 edition, theno_implicit_prelude attribute does not affect themacro_use prelude, and all macros exported from the standard library are still included in themacro_use prelude. Starting in the 2018 edition, it will remove themacro_use prelude.

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