1# Preludes
2
3A *prelude* is a collection of names that are automatically brought into scope
4of every module in a crate.
5
6These prelude names are not part of the module itself: they are implicitly
7queried during [name resolution]. For example, even though something like
8[`Box`] is in scope in every module, you cannot refer to it as `self::Box`
9because it is not a member of the current module.
10
11There are several different preludes:
12
13- [Standard library prelude]
14- [Extern prelude]
15- [Language prelude]
16- [`macro_use` prelude]
17- [Tool prelude]
18
19## Standard library prelude
20
21The standard library prelude includes names from the [`std::prelude::v1`]
22module. If the [`no_std` attribute] is used, then it instead uses the names
23from the [`core::prelude::v1`] module.
24
25## Extern prelude
26
27External crates imported with [`extern crate`] in the root module or provided
28to the compiler (as with the `--extern` flag with `rustc`) are added to the
29*extern prelude*. If imported with an alias such as `extern crate orig_name as
30new_name`, then the symbol `new_name` is instead added to the prelude.
31
32The [`core`] crate is always added to the extern prelude. The [`std`] crate is
33added as long as the [`no_std` attribute] is not specified in the crate root.
34
35> **Edition Differences**: In the 2015 edition, crates in the extern prelude
36> cannot be referenced via [use declarations], so it is generally standard
37> practice to include `extern crate` declarations to bring them into scope.
38>
39> Beginning in the 2018 edition, [use declarations] can reference crates in
40> the extern prelude, so it is considered unidiomatic to use `extern crate`.
41
42> **Note**: Additional crates that ship with `rustc`, such as [`alloc`], and
43> [`test`], are not automatically included with the `--extern` flag when using
44> Cargo. They must be brought into scope with an `extern crate` declaration,
45> even in the 2018 edition.
46>
47> ```rust
48> extern crate alloc;
49> use alloc::rc::Rc;
50> ```
51>
52> Cargo does bring in `proc_macro` to the extern prelude for proc-macro crates
53> only.
54
55<!--
56See https://github.com/rust-lang/rust/issues/57288 for more about the
57alloc/test limitation.
58-->
59
60### The `no_std` attribute
61
62By default, the standard library is automatically included in the crate root
63module. The [`std`] crate is added to the root, along with an implicit
64[`macro_use` attribute] pulling in all macros exported from `std` into the
65[`macro_use` prelude]. Both [`core`] and [`std`] are added to the [extern
66prelude]. The [standard library prelude] includes everything from the
67[`std::prelude::v1`] module.
68
69The *`no_std` [attribute]* may be applied at the crate level to prevent the
70[`std`] crate from being automatically added into scope. It does three things:
71
72* Prevents `std` from being added to the [extern prelude](#extern-prelude).
73* Uses [`core::prelude::v1`] in the [standard library prelude] instead of
74  [`std::prelude::v1`].
75* Injects the [`core`] crate into the crate root instead of [`std`], and pulls
76  in all macros exported from `core` in the [`macro_use` prelude].
77
78> **Note**: Using the core prelude over the standard prelude is useful when
79> either the crate is targeting a platform that does not support the standard
80> library or is purposefully not using the capabilities of the standard
81> library. Those capabilities are mainly dynamic memory allocation (e.g. `Box`
82> and `Vec`) and file and network capabilities (e.g. `std::fs` and `std::io`).
83
84<div class="warning">
85
86Warning: Using `no_std` does not prevent the standard library from being
87linked in. It is still valid to put `extern crate std;` into the crate and
88dependencies can also link it in.
89
90</div>
91
92## Language prelude
93
94The language prelude includes names of types and attributes that are built-in
95to the language. The language prelude is always in scope. It includes the following:
96
97* [Type namespace]
98    * [Boolean type] — `bool`
99    * [Textual types] — `char` and `str`
100    * [Integer types] — `i8`, `i16`, `i32`, `i64`, `i128`, `u8`, `u16`, `u32`, `u64`, `u128`
101    * [Machine-dependent integer types] — `usize` and `isize`
102    * [floating-point types] — `f32` and `f64`
103* [Macro namespace]
104    * [Built-in attributes]
105
106## `macro_use` prelude
107
108The `macro_use` prelude includes macros from external crates that were
109imported by the [`macro_use` attribute] applied to an [`extern crate`].
110
111## Tool prelude
112
113The tool prelude includes tool names for external tools in the [type
114namespace]. See the [tool attributes] section for more details.
115
116## The `no_implicit_prelude` attribute
117
118The *`no_implicit_prelude` [attribute]* may be applied at the crate level or
119on a module to indicate that it should not automatically bring the [standard
120library prelude], [extern prelude], or [tool prelude] into scope for that
121module or any of its descendants.
122
123This attribute does not affect the [language prelude].
124
125> **Edition Differences**: In the 2015 edition, the `no_implicit_prelude`
126> attribute does not affect the [`macro_use` prelude], and all macros exported
127> from the standard library are still included in the `macro_use` prelude.
128> Starting in the 2018 edition, it will remove the `macro_use` prelude.
129
130[`alloc`]: ../../alloc/index.html
131[`Box`]: ../../std/boxed/struct.Box.html
132[`core::prelude::v1`]: ../../core/prelude/index.html
133[`core`]: ../../core/index.html
134[`extern crate`]: ../items/extern-crates.md
135[`macro_use` attribute]: ../macros-by-example.md#the-macro_use-attribute
136[`macro_use` prelude]: #macro_use-prelude
137[`no_std` attribute]: #the-no_std-attribute
138[`no_std` attribute]: #the-no_std-attribute
139[`std::prelude::v1`]: ../../std/prelude/index.html
140[`std`]: ../../std/index.html
141[`test`]: ../../test/index.html
142[attribute]: ../attributes.md
143[Boolean type]: ../types/boolean.md
144[Built-in attributes]: ../attributes.md#built-in-attributes-index
145[extern prelude]: #extern-prelude
146[floating-point types]: ../types/numeric.md#floating-point-types
147[Integer types]: ../types/numeric.md#integer-types
148[Language prelude]: #language-prelude
149[Machine-dependent integer types]: ../types/numeric.md#machine-dependent-integer-types
150[Macro namespace]: namespaces.md
151[name resolution]: name-resolution.md
152[Standard library prelude]: #standard-library-prelude
153[Textual types]: ../types/textual.md
154[tool attributes]: ../attributes.md#tool-attributes
155[Tool prelude]: #tool-prelude
156[Type namespace]: namespaces.md
157[use declarations]: ../items/use-declarations.md
158