Preludes
A prelude is a collection of names that are automatically brought into scope of every module in a crate.
These prelude names are not part of the module itself: they are implicitly
queried during name resolution. For example, even though something like
Box is in scope in every module, you cannot refer to it as self::Box
because 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 the no_std attribute is applied to the crate:
| Edition | no_std not applied | no_std applied |
|---|---|---|
| 2015 | std::prelude::rust_2015 | core::prelude::rust_2015 |
| 2018 | std::prelude::rust_2018 | core::prelude::rust_2018 |
| 2021 | std::prelude::rust_2021 | core::prelude::rust_2021 |
| 2024 | std::prelude::rust_2024 | core::prelude::rust_2024 |
Note
std::prelude::rust_2015andstd::prelude::rust_2018have the same contents asstd::prelude::v1.
core::prelude::rust_2015andcore::prelude::rust_2018have the same contents ascore::prelude::v1.
Extern prelude
External crates imported with extern crate in the root module or provided
to the compiler (as with the --extern flag with rustc) are added to the
extern prelude. If imported with an alias such as extern crate orig_name as new_name, then the symbol new_name is instead added to the prelude.
The core crate is always added to the extern prelude.
The std crate is added as long as the no_std attribute is not specified in the crate root.
Edition differences: In the 2015 edition, crates in the extern prelude cannot be referenced via use declarations, so it is generally standard practice to include
extern cratedeclarations 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 use
extern crate.
Note
Additional crates that ship with
rustc, such asalloc, andtest, are not automatically included with the--externflag when using Cargo. They must be brought into scope with anextern cratedeclaration, even in the 2018 edition.#![allow(unused)] fn main() { extern crate alloc; use alloc::rc::Rc; }Cargo does bring in
proc_macroto the extern prelude for proc-macro crates only.
The no_std attribute
By default, the standard library is automatically included in the crate root
module. The std crate is added to the root, along with an implicit
macro_use attribute pulling in all macros exported from std into the
macro_use prelude. Both core and std are added to the extern
prelude.
The no_std attribute may be applied at the crate level to prevent the
std crate from being automatically added into scope.
It does three things:
- Prevents
stdfrom being added to the extern prelude.
- Affects which module is used to make up the standard library prelude (as described above).
- Injects the
corecrate into the crate root instead ofstd, and pulls in all macros exported fromcorein themacro_useprelude.
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.
BoxandVec) and file and network capabilities (e.g.std::fsandstd::io).
Warning
Using
no_stddoes 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-in to the language. The language prelude is always in scope.
It includes the following:
- Type namespace
- Boolean type —
bool - Textual types —
charandstr - Integer types —
i8,i16,i32,i64,i128,u8,u16,u32,u64,u128 - Machine-dependent integer types —
usizeandisize - floating-point types —
f32andf64
- Boolean type —
- Macro namespace
macro_use prelude
The macro_use prelude includes macros from external crates that were
imported by the macro_use attribute applied to an extern crate.
Tool prelude
The tool prelude includes tool names for external tools in the type namespace. See the tool attributes section for more details.
The no_implicit_prelude attribute
The no_implicit_prelude attribute may be applied at the crate level or
on a module to indicate that it should not automatically bring the standard
library prelude, extern prelude, or tool prelude into scope for that
module or any of its descendants.
This attribute does not affect the language prelude.
Edition differences: In the 2015 edition, the
no_implicit_preludeattribute does not affect themacro_useprelude, and all macros exported from the standard library are still included in themacro_useprelude. Starting in the 2018 edition, it will remove themacro_useprelude.