encoding_rs 0.7.2

A Gecko-oriented implementation of the Encoding Standard


Build Status crates.io docs.rs Apache 2 / MIT dual-licensed

encoding_rs an implementation of the (non-JavaScript parts of) the Encoding Standard written in Rust and used in Gecko (starting with Firefox 56).

Additionally, the mem module provides various operations for dealing with in-RAM text (as opposed to data that's coming from or going to an IO boundary). The mem module is a module instead of a separate crate due to internal implementation detail efficiencies.


Due to the Gecko use case, encoding_rs supports decoding to and encoding from UTF-16 in addition to supporting the usual Rust use case of decoding to and encoding from UTF-8. Additionally, the API has been designed to be FFI-friendly to accommodate the C++ side of Gecko.

Specifically, encoding_rs does the following:

  • Decodes a stream of bytes in an Encoding Standard-defined character encoding into valid aligned native-endian in-RAM UTF-16 (units of u16 / char16_t).
  • Encodes a stream of potentially-invalid aligned native-endian in-RAM UTF-16 (units of u16 / char16_t) into a sequence of bytes in an Encoding Standard-defined character encoding as if the lone surrogates had been replaced with the REPLACEMENT CHARACTER before performing the encode. (Gecko's UTF-16 is potentially invalid.)
  • Decodes a stream of bytes in an Encoding Standard-defined character encoding into valid UTF-8.
  • Encodes a stream of valid UTF-8 into a sequence of bytes in an Encoding Standard-defined character encoding. (Rust's UTF-8 is guaranteed-valid.)
  • Does the above in streaming (input and output split across multiple buffers) and non-streaming (whole input in a single buffer and whole output in a single buffer) variants.
  • Avoids copying (borrows) when possible in the non-streaming cases when decoding to or encoding from UTF-8.
  • Resolves textual labels that identify character encodings in protocol text into type-safe objects representing the those encodings conceptually.
  • Maps the type-safe encoding objects onto strings suitable for returning from document.characterSet.
  • Validates UTF-8 (in common instruction set scenarios a bit faster for Web workloads than the standard library; hopefully will get upstreamed some day) and ASCII.

Additionally, encoding_rs::mem does the following:

  • Checks if a byte buffer contains only ASCII.
  • Checks if a potentially-invalid UTF-16 buffer contains only Basic Latin (ASCII).
  • Checks if a valid UTF-8, potentially-invalid UTF-8 or potentially-invalid UTF-16 buffer contains only Latin1 code points (below U+0100).
  • Checks if a valid UTF-8, potentially-invalid UTF-8 or potentially-invalid UTF-16 buffer or a code point or a UTF-16 code unit can trigger right-to-left behavior (suitable for checking if the Unicode Bidirectional Algorithm can be optimized out).
  • Combined versions of the above two checks.
  • Converts valid UTF-8, potentially-invalid UTF-8 and Latin1 to UTF-16.
  • Converts potentially-invalid UTF-16 and Latin1 to UTF-8.
  • Converts UTF-8 and UTF-16 to Latin1 (if in range).
  • Finds the first invalid code unit in a buffer of potentially-invalid UTF-16.
  • Makes a mutable buffer of potential-invalid UTF-16 contain valid UTF-16.
  • Copies ASCII from one buffer to another up to the first non-ASCII byte.
  • Converts ASCII to UTF-16 up to the first non-ASCII byte.
  • Converts UTF-16 to ASCII up to the first non-Basic Latin code unit.


Please see the file named COPYRIGHT.

API Documentation

Generated API documentation is available online.

C and C++ bindings

An FFI layer for encoding_rs is available as a separate crate. The crate comes with a demo C++ wrapper using the C++ standard library and GSL types.

For the Gecko context, there's a C++ wrapper using the MFBT/XPCOM types.

These bindings do not cover the mem module.

Sample programs

Optional features

There are currently three optional cargo features:


Enables SSE2 acceleration on x86 and x86_64 and NEON acceleration on Aarch64. Requires nightly Rust. Enabling this cargo feature is recommended when building for x86, x86_64 or Aarch64 on nightly Rust. The intention is for the functionality enabled by this feature to become the normal on-by-default behavior once explicit SIMD becames available on all Rust release channels.

Enabling this feature breaks the build unless the target is x86 with SSE2 (Rust's default 32-bit x86 target, i686, has SSE2, but Linux distros may use an x86 target without SSE2, i.e. i586 in rustup terms), x86_64 or Aarch64.


Enables support for serializing and deserializing &'static Encoding-typed struct fields using Serde.


Makes the binary size smaller at the expense of ideograph encode speed for Chinese and Japanese legacy encodings. (Does not affect decode speed.)

The speed resulting from enabling this feature is believed to be acceptable for Web browser-exposed encoder use cases. However, the result is likely unacceptable for other applications that need to produce output in Chinese or Japanese legacy encodings. (But applications really should always be using UTF-8 for output.)

Performance goals

For decoding to UTF-16, the goal is to perform at least as well as Gecko's old uconv. For decoding to UTF-8, the goal is to perform at least as well as rust-encoding.

Encoding to UTF-8 should be fast. (UTF-8 to UTF-8 encode should be equivalent to memcpy and UTF-16 to UTF-8 should be fast.)

Speed is a non-goal when encoding to legacy encodings. Encoding to legacy encodings should not be optimized for speed at the expense of code size as long as form submission and URL parsing in Gecko don't become noticeably too slow in real-world use.

Currently, by default, encoding_rs builds with limited encoder-specific accelation tables for GB2312 Level 1 Hanzi, Big5 Level 1 Hanzi and JIS X 0208 Level 1 Kanji. These tables use binary search and strike a balance between not having encoder-specific tables at all (doing linear search over the decode-optimized tables) and having larger directly-indexable encoder-side tables. It is not clear that anyone wants this in-between approach, and it may be changed in the future.

In the interest of binary size, Firefox builds with the no-static-ideograph-encoder-tables cargo feature, which omits the encoder-specific tables and performs linear search over the decode-optimized tables. With realistic work loads, this seemed fast enough not to be user-visibly slow on Raspberry Pi 3 (which stood in for a phone for testing) in the Web-exposed encoder use cases.

A framework for measuring performance is available separately.

Rust Version Compatibility

It is a goal to support the latest stable Rust, the latest nightly Rust and the version of Rust that's used for Firefox Nightly (currently 1.19.0). These are tested on Travis.

Additionally, beta and the oldest known to work Rust version (currently 1.15.0) are tested on Travis. The oldest Rust known to work is tested as a canary so that when the oldest known to work no longer works, the change can be documented here. At this time, there is no firm commitment to support a version older than what's required by Firefox, but there isn't an active plan to make changes that would make 1.15.0 no longer work, either.

Compatibility with rust-encoding

A compatibility layer that implements the rust-encoding API on top of encoding_rs is provided as a separate crate (cannot be uploaded to crates.io). The compatibility layer was originally written with the assuption that Firefox would need it, but it is not currently used in Firefox.


  • Design the low-level API.
  • Provide Rust-only convenience features.
  • Provide an stl/gsl-flavored C++ API.
  • Implement all decoders and encoders.
  • Add unit tests for all decoders and encoders.
  • Finish BOM sniffing variants in Rust-only convenience features.
  • Document the API.
  • Publish the crate on crates.io.
  • Create a solution for measuring performance.
  • Accelerate ASCII conversions using SSE2 on x86.
  • Accelerate ASCII conversions using ALU register-sized operations on non-x86 architectures (process an usize instead of u8 at a time).
  • Split FFI into a separate crate so that the FFI doesn't interfere with LTO in pure-Rust usage.
  • Compress CJK indices by making use of sequential code points as well as Unicode-ordered parts of indices.
  • Make lookups by label or name use binary search that searches from the end of the label/name to the start.
  • Make labels with non-ASCII bytes fail fast.
  • Parallelize UTF-8 validation using Rayon.
  • Provide an XPCOM/MFBT-flavored C++ API.
  • Investigate accelerating single-byte encode with a single fast-tracked range per encoding.
  • Replace uconv with encoding_rs in Gecko.
  • Implement the rust-encoding API in terms of encoding_rs.
  • Add SIMD acceleration for Aarch64.
  • Investigate the use of NEON on 32-bit ARM.
  • Investigate Björn Höhrmann's lookup table acceleration for UTF-8 as adapted to Rust in rust-encoding.

Release Notes


  • Add the mem module.
  • Refactor SIMD code which can affect performance outside the mem module.


  • When encoding from invalid UTF-16, correctly handle U+DC00 followed by another low surrogate.


  • Make replacement a label of the replacement encoding. (Spec change.)
  • Remove Encoding::for_name(). (Encoding::for_label(foo).unwrap() is now close enough after the above label change.)
  • Remove the parallel-utf8 cargo feature.
  • Add optional Serde support for &'static Encoding.
  • Performance tweaks for ASCII handling.
  • Performance tweaks for UTF-8 validation.
  • SIMD support on aarch64.


  • Make Encoder::has_pending_state() public.
  • Update the simd crate dependency to 0.2.0.


  • Reserve enough space for NCRs when encoding to ISO-2022-JP.
  • Correct max length calculations for multibyte decoders.
  • Correct max length calculations before BOM sniffing has been performed.
  • Correctly calculate max length when encoding from UTF-16 to GBK.



  • Correcly handle the case where the first buffer contains potentially partial BOM and the next buffer is the last buffer.
  • Decode byte 7F correctly in ISO-2022-JP.
  • Make UTF-16 to UTF-8 encode write closer to the end of the buffer.
  • Implement Hash for Encoding.



  • Correct max length calculation when a partial BOM prefix is part of the decoder's state.


  • Correct max length calculation in various encoders.
  • Correct max length calculation in the UTF-16 decoder.
  • Derive PartialEq and Eq for the CoderResult, DecoderResult and EncoderResult types.


  • Avoid panic when encoding with replacement and the destination buffer is too short to hold one numeric character reference.


  • Add support for 32-bit big-endian hosts. (For real this time.)


  • Fix a panic from subslicing with bad indices in Encoder::encode_from_utf16. (Due to an oversight, it lacked the fix that Encoder::encode_from_utf8 already had.)
  • Micro-optimize error status accumulation in non-streaming case.


  • Avoid panic near integer overflow in a case that's unlikely to actually happen.
  • Address Clippy lints.


  • Make the methods for computing worst-case buffer size requirements check for integer overflow.
  • Upgrade rayon to 0.7.0.


  • Reorder methods for better documentation readability.
  • Add support for big-endian hosts. (Only 64-bit case actually tested.)
  • Optimize the ALU (non-SIMD) case for 32-bit ARM instead of x86_64.


  • Avoid allocating an excessively long buffers in non-streaming decode.
  • Fix the behavior of ISO-2022-JP and replacement decoders near the end of the output buffer.
  • Annotate the result structs with #[must_use].


  • Split FFI into a separate crate.
  • Performance tweaks.
  • CJK binary size and encoding performance changes.
  • Parallelize UTF-8 validation in the case of long buffers (with optional feature parallel-utf8).
  • Borrow even with ISO-2022-JP when possible.


  • Fix moving pointers to alignment in ALU-based ASCII acceleration.
  • Fix errors in documentation and improve documentation.


  • Fix UTF-8 to UTF-16 decode for byte sequences beginning with 0xEE.
  • Make UTF-8 to UTF-8 decode SSE2-accelerated when feature simd-accel is used.
  • When decoding and encoding ASCII-only input from or to an ASCII-compatible encoding using the non-streaming API, return a borrow of the input.
  • Make encode from UTF-16 to UTF-8 faster.


  • Change the references to the instances of Encoding from const to static to make the referents unique across crates that use the refernces.
  • Introduce non-reference-typed FOO_INIT instances of Encoding to allow foreign crates to initialize static arrays with references to Encoding instances even under Rust's constraints that prohibit the initialization of &'static Encoding-typed array items with &'static Encoding-typed statics.
  • Document that the above two points will be reverted if Rust changes const to work so that cross-crate usage keeps the referents unique.
  • Return Cows from Rust-only non-streaming methods for encode and decode.
  • Encoding::for_bom() returns the length of the BOM.
  • ASCII-accelerated conversions for encodings other than UTF-16LE, UTF-16BE, ISO-2022-JP and x-user-defined.
  • Add SSE2 acceleration behind the simd-accel feature flag. (Requires nightly Rust.)
  • Fix panic with long bogus labels.
  • Map 0xCA to U+05BA in windows-1255. (Spec change.)
  • Correct the end of the Shift_JIS EUDC range. (Spec change.)


  • Polish FFI documentation.


  • Fix UTF-16 to UTF-8 encode.


  • Add Encoder.encode_from_utf8_to_vec_without_replacement().


  • Add Encoding.is_ascii_compatible().

  • Add Encoding::for_bom().

  • Make == for Encoding use name comparison instead of pointer comparison, because uses of the encoding constants in different crates result in different addresses and the constant cannot be turned into statics without breaking other things.


The initial release.