Compact inlined strings.
String type that's source compatible with
std::string::String, uses exactly the same amount of
space, doesn't heap allocate for short strings (up to 23 bytes on 64-bit archs) by storing them in
the space a
String would have taken up on the stack, making strings go faster overall.
This crate provides a wrapper for Rust's standard
String which uses the space a
on the stack to store inline string data, automatically promoting it to a
String when it grows
beyond the inline capacity. This has the advantage of avoiding heap allocations for short strings as
well as improving performance thanks to keeping the strings on the stack.
This is all accomplished without the need for an external discriminant, so a
exactly the same size as a
String on the stack, regardless of whether it's inlined or not, and
when not inlined it's pointer compatible with
String, meaning that you can safely coerce a
SmartString to a
pointer::cast() and go on using it as
if it had never been a
SmartString. (But please don't do that, there's an
implementation that's much safer.)
smartstring currently doesn't run on 32-bit big endian architectures like
powerpc, so its use
in any crates that intend to run on those architectures should ideally be gated behind a
platform specific dependency
Cargo.toml, like so:
 = "0.2"
This will ensure that
cargo does not try to build
smartstring on these unsupported
architectures, which will otherwise always fail.
smartstring gets by without a discriminant is dependent on the memory layout of the
std::string::String struct, which isn't something the Rust compiler and standard library make any
smartstring makes an assumption about how it's been laid out, which has held
basically since rustc came into existence, but is nonetheless not a safe assumption to make, and if
the layout ever changes,
smartstring will stop working properly (at least on little-endian
architectures, the assumptions made on big-endian archs will hold regardless of the actual memory
layout). Its test suite does comprehensive validation of these assumptions, and as long as the
CI build is passing for any given rustc version,
you can be sure it will do its job properly on all tested architectures. You can also check out the
smartstring source tree yourself and run
cargo test to validate it for your particular
As an extra precaution, some runtime checks are made as well, so that if the memory layout
assumption no longer holds,
smartstring will not work correctly, but there should be no security
implications and it should crash early.
Copyright 2020 Bodil Stokke
This software is subject to the terms of the Mozilla Public License, v. 2.0. If a copy of the MPL was not distributed with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
Code of Conduct
Please note that this project is released with a Contributor Code of Conduct. By participating in this project you agree to abide by its terms.