preludes.md

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
• Extern prelude
• Language prelude
• macro_use prelude
• Tool prelude

Standard library prelude

The standard library prelude includes names from the std::prelude::v1 module. If the no_std attribute is used, then it instead uses the names from the core::prelude::v1 module.

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 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 use extern crate.

Note: Additional crates that ship with rustc, such as alloc, and test, are not automatically included with the --extern flag when using Cargo. They must be brought into scope with an extern crate declaration, even in the 2018 edition.

extern crate alloc;
use alloc::rc::Rc;

Cargo does bring in proc_macro to 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 standard library prelude includes everything from the std::prelude::v1 module.

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 std from being added to the extern prelude.
• Uses core::prelude::v1 in the standard library prelude instead of std::prelude::v1.
• Injects the core crate into the crate root instead of std, and pulls in all macros exported from core in the macro_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 and Vec) and file and network capabilities (e.g. std::fs and std::io).

Warning: Using no_std does not prevent the standard library from being linked in. It is still valid to put extern 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 — char and str
  • Integer types — i8, i16, i32, i64, i128, u8, u16, u32, u64, u128
  • Machine-dependent integer types — usize and isize
  • floating-point types — f32 and f64
• Macro namespace
  • Built-in attributes

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