# 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
| Firefox 108 | Rust 1.65.0 | 1.63.0 | 2022 November 3 | 2022 November 10 | 2022 December 13
| Firefox 109 | Rust 1.65.0 | 1.63.0 | 2022 November 3 | 2022 December 8 | 2023 January 17
| **Estimated** |
| Firefox 110 | Rust 1.66.0 | 1.65.0 | 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
| Firefox 113 | Rust 1.68.0 | ? | 2022 March 9 | 2023 April 6 | 2023 May 9
| Firefox 114 | Rust 1.69.0 | ? | 2022 April 20 | 2023 May 4 | 2023 June 6
| Firefox 115 | Rust 1.69.0 | ? | 2022 April 20 | 2023 June 1 | 2023 July 4
| Firefox 116 | Rust 1.70.0 | ? | 2022 June 1 | 2023 June 29 | 2023 August 1