Crate smallvec

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Small vectors in various sizes. These store a certain number of elements inline, and fall back to the heap for larger allocations. This can be a useful optimization for improving cache locality and reducing allocator traffic for workloads that fit within the inline buffer.

no_std support

By default, smallvec does not depend on std. However, the optional write feature implements the std::io::Write trait for vectors of u8. When this feature is enabled, smallvec depends on std.

Optional features


When this optional dependency is enabled, SmallVec implements the serde::Serialize and serde::Deserialize traits.


When this feature is enabled, SmallVec<[u8; _]> implements the std::io::Write trait. This feature is not compatible with #![no_std] programs.


This feature requires Rust 1.49.

When the union feature is enabled smallvec will track its state (inline or spilled) without the use of an enum tag, reducing the size of the smallvec by one machine word. This means that there is potentially no space overhead compared to Vec. Note that smallvec can still be larger than Vec if the inline buffer is larger than two machine words.

To use this feature add features = ["union"] in the smallvec section of Cargo.toml. Note that this feature requires Rust 1.49.

Tracking issue: rust-lang/rust#55149


This feature requires Rust 1.51.

When this feature is enabled, SmallVec works with any arrays of any size, not just a fixed list of sizes.


This feature requires Rust 1.51.

This feature exposes the functions [SmallVec::new_const], [SmallVec::from_const], and [smallvec_inline] which enables the SmallVec to be initialized from a const context. For details, see the Rust Reference.


This feature is unstable and requires a nightly build of the Rust toolchain.

When this feature is enabled, SmallVec::from(slice) has improved performance for slices of Copy types. (Without this feature, you can use SmallVec::from_slice to get optimal performance for Copy types.)

Tracking issue: rust-lang/rust#31844


This feature is unstable and requires a nightly build of the Rust toolchain.

This feature makes the Rust compiler less strict about use of vectors that contain borrowed references. For details, see the Rustonomicon.

Tracking issue: rust-lang/rust#34761



  • An iterator that removes the items from a SmallVec and yields them by value.
  • An iterator that consumes a SmallVec and yields its items by value.
  • A Vec-like container that can store a small number of elements inline.



  • Types that can be used as the backing store for a SmallVec
  • Convenience trait for constructing a SmallVec