Rust Update Policy

We document here the decision making and planning around when we update the Rust toolchains used and required to build Firefox.

This allows contributors to know when new features will be usable, and downstream packagers to know what toolchain will be required for each Firefox release. Both benefit from the predictability of a schedule.

Policy

Official builds

We ship official stable Firefox with a stable Rust.

As a general rule, we update the Rust version used to build Firefox Nightly soon after its release, unless it’s less than 7 days away from a soft-freeze, in which case we wait for the next Nightly train.

We don’t upgrade the Rust version in the beta or release branches of Firefox.

The following exceptions apply:

  • We may use a Rust version from the Rust beta or nightly channels for new platforms (e.g. we did so for Android, arm64 Windows and arm64 macOS), and later upgrade when that Rust version becomes stable (we may even do so on the Firefox beta branch).

  • We may skip the update (or backout the update) if major problems are encountered (typically, we’ve had to do so because of build problems, crash reporting bustage, or performance issues).

Developer builds

Local developer builds use whatever Rust toolchain is available on the system.

Someone building Firefox can maintain the latest stable Rust with the rustup or mach bootstrap tools, or try other variations.

Minimum Supported Rust Version

We will update the Minimum Supported Rust Version (MSRV) when required.

The MSRV will generally remain unchanged, until a newer version is required by some code.

When that happens, we’ll prefer to update the MSRV to the strict minimum required at that moment (e.g. if we require version 1.47.0, the currently used Rust version is 1.51.0, and a crate needs 1.50.0, we’ll prefer to update the MSRV to 1.50.0 rather than 1.51.0).

The MSRV won’t be updated to a version of Rust that hasn’t been used for Firefox Nightly for at least 14 days.

We expect ESR releases will keep their MSRV, so backporting security fixes may require Rust compatibility work.

Rationale

Historically, the Rust ecosystem quickly required new features provided by new Rust compilers, which made it necessary to update the minimum supported version quite often, and as such, a scheduled update was deemed a better trade-off.

Fast-forward several years, and new Rust compiler releases more rarely sport ground-breaking new features, which has reduced the necessity to update quite significantly.

On the flip side, in some instances, we have had to stick to specific versions of the Rust compiler for extended periods of time because of e.g. regressions, going against the schedule.

Schedule

Here are the Rust versions for each Firefox version.

  • The “Uses” column indicates the version of Rust used to build releases shipped to users.

  • The “MSRV” column indicates the minimum supported Rust version to build the sources.

Firefox Version Uses MSRV Rust "Uses" release date Nightly Soft Freeze Firefox release date
Firefox 56 Rust 1.19.0 1.17.0 2017 April 27 2017 September 26
Firefox 57 Rust 1.19.0 1.19.0 2017 July 20 2017 November 14
Firefox 58 Rust 1.21.0 1.21.0 2017 October 12 2018 January 16
Firefox 59 Rust 1.22.1 1.22.1 2017 November 23 2018 March 13
Firefox 60 Rust 1.24.0 1.24.0 2018 February 15 2018 May 9
Firefox 61 Rust 1.24.0 1.24.0 2018 February 15 2018 June 26
Firefox 62 Rust 1.24.0 1.24.0 2018 February 15 2018 September 5
Firefox 63 Rust 1.28.0 1.28.0 2018 August 2 2018 October 23
Firefox 64 Rust 1.29.2 1.29.0 2018 September 13 2018 October 15 2018 December 11
Firefox 65 Rust 1.30.0 1.30.0 2018 October 25 2018 December 3 2019 January 29
Firefox 66 Rust 1.31.0 1.31.0 2018 December 6 2019 January 21 2019 March 19
Firefox 67 Rust 1.32.0 1.32.0 2019 January 17 2019 March 11 2019 May 21
Firefox 68 Rust 1.34.0 1.34.0 2019 April 11 2019 May 13 2019 July 9
Firefox 69 Rust 1.35.0 1.35.0 2019 May 23 2019 July 1 2019 September 3
Firefox 70 Rust 1.37.0 1.36.0 2019 July 4 2019 August 26 2019 October 22
Firefox 71 Rust 1.37.0 1.37.0 2019 August 15 2019 October 14 2019 December 3
Firefox 72 Rust 1.38.0 1.37.0 2019 August 15 2019 November 25 2020 January 7
Firefox 73 Rust 1.39.0 1.39.0 2019 November 7 2020 January 1 2020 February 11
Firefox 74 Rust 1.39.0 1.39.0 2019 November 7 2020 February 6 2020 March 10
Firefox 75 Rust 1.41.0 1.41.0 2020 January 30 2020 March 5 2020 April 7
Firefox 76 Rust 1.41.0 1.41.0 2020 January 30 2020 April 2 2020 May 5
Firefox 77 Rust 1.41.1 1.41.0 2020 January 30 2020 April 30 2020 June 2
Firefox 78 Rust 1.43.0 1.41.0 2020 April 23 2020 May 28 2020 June 30
Firefox 79 Rust 1.43.0 1.43.0 2020 April 23 2020 June 26 2020 July 28
Firefox 80 Rust 1.43.0 1.43.0 2020 April 23 2020 July 23 2020 August 25
Firefox 81 Rust 1.43.0 1.43.0 2020 April 23 2020 August 20 2020 September 22
Firefox 82 Rust 1.43.0 1.43.0 2020 April 23 2020 September 17 2020 October 20
Firefox 83 Rust 1.43.0 1.43.0 2020 April 23 2020 October 15 2020 November 17
Firefox 84 Rust 1.47.0 1.43.0 2020 October 8 2020 November 12 2020 December 15
Firefox 85 Rust 1.48.0 1.47.0 2020 November 19 2020 December 10 2021 January 26
Firefox 86 Rust 1.49.0 1.47.0 2020 December 31 2021 January 21 2021 February 23
Firefox 87 Rust 1.50.0 1.47.0 2021 February 11 2021 February 18 2021 March 23
Firefox 88 Rust 1.50.0 1.47.0 2021 February 11 2021 March 18 2021 April 19
Firefox 89 Rust 1.51.0 1.47.0 2021 March 25 2021 April 15 2021 June 1
Firefox 90 Rust 1.52.0 1.47.0 2021 May 6 2021 May 27 2021 June 29
Firefox 91 Rust 1.53.0 1.51.0 2021 June 17 2021 July 8 2021 August 10
Firefox 92 Rust 1.54.0 1.51.0 2021 July 29 2021 August 5 2021 September 7
Firefox 93 Rust 1.54.0 1.51.0 2021 July 29 2021 September 2 2021 October 5
Firefox 94 Rust 1.55.0 1.53.0 2021 September 9 2021 September 30 2021 November 2
Firefox 95 Rust 1.56.0 1.53.0 2021 October 21 2021 October 28 2021 December 7
Firefox 96 Rust 1.57.0 1.53.0 2021 December 2 2021 December 2 2022 January 11
Firefox 97 Rust 1.57.0 1.57.0 2021 December 2 2022 January 6 2022 February 8
Firefox 98 Rust 1.58.0 1.57.0 2022 January 13 2022 February 2 2022 March 8
Firefox 99 Rust 1.59.0 1.57.0 2022 February 24 2022 March 3 2022 April 5
Firefox 100 Rust 1.59.0 1.57.0 2022 February 24 2022 March 31 2022 May 3
Firefox 101 Rust 1.60.0 1.59.0 2022 April 7 2022 April 28 2022 May 31
Firefox 102 Rust 1.60.0 1.59.0 2022 April 7 2022 May 26 2022 June 28
Firefox 103 Rust 1.61.0 1.59.0 2022 May 19 2022 June 23 2022 July 27
Firefox 104 Rust 1.62.0 1.59.0 2022 June 30 2022 July 21 2022 August 23
Firefox 105 Rust 1.63.0 1.61.0 2022 August 11 2022 August 18 2022 September 20
Firefox 106 Rust 1.63.0 1.61.0 2022 August 11 2022 September 15 2022 October 18
Firefox 107 Rust 1.64.0 1.61.0 2022 September 22 2022 October 13 2022 November 15
Estimated
Firefox 108 Rust 1.65.0 1.63.0 2022 November 3 2022 November 10 2022 December 13
Firefox 109 Rust 1.65.0 ? 2022 November 3 2022 December 8 2023 January 17
Firefox 110 Rust 1.66.0 ? 2022 December 15 2023 January 5 2023 February 14
Firefox 111 Rust 1.67.0 ? 2022 January 26 2023 February 9 2023 March 14
Firefox 112 Rust 1.67.0 ? 2022 January 26 2023 March 9 2023 April 11