Rust comes with a variety of things in its standard library. However, if you had to manually import every single thing that you used, it would be very verbose. But importing a lot of things that a program never uses isn’t good either. A balance needs to be struck.
The prelude is the list of things that Rust automatically imports into every Rust program. It’s kept as small as possible, and is focused on things, particularly traits, which are used in almost every single Rust program.
Preludes can be seen as a pattern to make using multiple types more
convenient. As such, you’ll find other preludes in the standard library,
std::io::prelude. Various libraries in the Rust ecosystem may
also define their own preludes.
The difference between ‘the prelude’ and these other preludes is that they
are not automatically
use’d, and must be imported manually. This is still
easier than importing all of their constituent components.
The current version of the prelude (version 1) lives in
std::prelude::v1, and re-exports the following:
Unpin}: marker traits that indicate fundamental properties of types.
FnOnce}: various operations for both destructors and overloading
drop: a convenience function for explicitly dropping a value.
Box: a way to allocate values on the heap.
ToOwned: the conversion trait that defines
to_owned, the generic method for creating an owned type from a borrowed type.
Clone: the ubiquitous trait that defines
clone, the method for producing a copy of a value.
Ord}: the comparison traits, which implement the comparison operators and are often seen in trait bounds.
From}: generic conversions, used by savvy API authors to create overloaded methods.
Default, types that have default values.
ExactSizeIterator}: iterators of various kinds.
None}, a type which expresses the presence or absence of a value. This type is so commonly used, its variants are also exported.
Err}: a type for functions that may succeed or fail. Like
Option, its variants are exported as well.
ToString}: heap-allocated strings.
Vec: a growable, heap-allocated vector.
The 2015 version of the prelude of The Rust Standard Library.
The 2018 version of the prelude of The Rust Standard Library.
The 2021 version of the prelude of The Rust Standard Library.
The first version of the prelude of The Rust Standard Library.