forked from mirrors/gecko-dev
73230b8c3e
Also update the table of Firefox requirements. It's worth noting that I actually upgraded to 1.57 earlier than I should have (too close to freeze), but what is done is done. Differential Revision: https://phabricator.services.mozilla.com/D134124
206 lines
11 KiB
Markdown
206 lines
11 KiB
Markdown
# Rust Update Policy
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We document here the decision making and planning around when we update the
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Rust toolchains used and required to build Firefox.
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This allows contributors to know when new features will be usable, and
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downstream packagers to know what toolchain will be required for each Firefox
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release. Both benefit from the predictability of a schedule.
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## Schedule
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Release Firefox requirements:
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| Firefox Version | Uses | Requires | Rust release date | Nightly Soft Freeze | Firefox release date |
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|-----------------|------|----------|-------------------|---------------------|----------------------|
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| Firefox 56 | Rust 1.19.0 | 1.17.0 | 2017 April 27 | | 2017 September 26
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| Firefox 57 | Rust 1.19.0 | 1.19.0 | 2017 July 20 | | 2017 November 14
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| Firefox 58 | Rust 1.21.0 | 1.21.0 | 2017 October 12 | | 2018 January 16
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| Firefox 59 | Rust 1.22.1 | 1.22.1 | 2017 November 23 | | 2018 March 13
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| Firefox 60 | Rust 1.24.0 | 1.24.0 | 2018 February 15 | | 2018 May 9
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| Firefox 61 | Rust 1.24.0 | 1.24.0 | 2018 February 15 | | 2018 June 26
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| Firefox 62 | Rust 1.24.0 | 1.24.0 | 2018 February 15 | | 2018 September 5
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| Firefox 63 | Rust 1.28.0 | 1.28.0 | 2018 August 2 | | 2018 October 23
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| Firefox 64 | Rust 1.29.2 | 1.29.0 | 2018 September 13 | 2018 October 15 | 2018 December 11
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| Firefox 65 | Rust 1.30.0 | 1.30.0 | 2018 October 25 | 2018 December 3 | 2019 January 29
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| Firefox 66 | Rust 1.31.0 | 1.31.0 | 2018 December 6 | 2019 January 21 | 2019 March 19
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| Firefox 67 | Rust 1.32.0 | 1.32.0 | 2019 January 17 | 2019 March 11 | 2019 May 21
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| Firefox 68 | Rust 1.34.0 | 1.34.0 | 2019 April 11 | 2019 May 13 | 2019 July 9
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| Firefox 69 | Rust 1.35.0 | 1.35.0 | 2019 May 23 | 2019 July 1 | 2019 September 3
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| Firefox 70 | Rust 1.37.0 | 1.36.0 | 2019 July 4 | 2019 August 26 | 2019 October 22
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| Firefox 71 | Rust 1.37.0 | 1.37.0 | 2019 August 15 | 2019 October 14 | 2019 December 3
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| Firefox 72 | Rust 1.38.0 | 1.37.0 | 2019 August 15 | 2019 November 25 | 2020 January 7
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| Firefox 73 | Rust 1.39.0 | 1.39.0 | 2019 November 7 | 2020 January 1 | 2020 February 11
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| Firefox 74 | Rust 1.39.0 | 1.39.0 | 2019 November 7 | 2020 February 6 | 2020 March 10
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| Firefox 75 | Rust 1.41.0 | 1.41.0 | 2020 January 30 | 2020 March 5 | 2020 April 7
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| Firefox 76 | Rust 1.41.0 | 1.41.0 | 2020 January 30 | 2020 April 2 | 2020 May 5
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| Firefox 77 | Rust 1.41.1 | 1.41.0 | 2020 January 30 | 2020 April 30 | 2020 June 2
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| Firefox 78 | Rust 1.43.0 | 1.41.0 | 2020 April 23 | 2020 May 28 | 2020 June 30
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| Firefox 79 | Rust 1.43.0 | 1.43.0 | 2020 April 23 | 2020 June 26 | 2020 July 28
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| Firefox 80 | Rust 1.43.0 | 1.43.0 | 2020 April 23 | 2020 July 23 | 2020 August 25
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| Firefox 81 | Rust 1.43.0 | 1.43.0 | 2020 April 23 | 2020 August 20 | 2020 September 22
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| Firefox 82 | Rust 1.43.0 | 1.43.0 | 2020 April 23 | 2020 September 17 | 2020 October 20
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| Firefox 83 | Rust 1.43.0 | 1.43.0 | 2020 April 23 | 2020 October 15 | 2020 November 17
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| Firefox 84 | Rust 1.47.0 | 1.43.0 | 2020 October 8 | 2020 November 12 | 2020 December 15
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| Firefox 85 | Rust 1.48.0 | 1.47.0 | 2020 November 19 | 2020 December 10 | 2021 January 26
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| Firefox 86 | Rust 1.49.0 | 1.47.0 | 2020 December 31 | 2021 January 21 | 2021 February 23
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| Firefox 87 | Rust 1.50.0 | 1.47.0 | 2021 February 11 | 2021 February 18 | 2021 March 23
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| Firefox 88 | Rust 1.50.0 | 1.47.0 | 2021 February 11 | 2021 March 18 | 2021 April 19
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| Firefox 89 | Rust 1.51.0 | 1.47.0 | 2021 March 25 | 2021 April 15 | 2021 June 1
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| Firefox 90 | Rust 1.52.0 | 1.47.0 | 2021 May 6 | 2021 May 27 | 2021 June 29
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| Firefox 91 | Rust 1.53.0 | 1.51.0 | 2021 June 17 | 2021 July 8 | 2021 August 10
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| Firefox 92 | Rust 1.54.0 | 1.51.0 | 2021 July 29 | 2021 August 5 | 2021 September 7
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| Firefox 93 | Rust 1.54.0 | 1.51.0 | 2021 July 29 | 2021 September 2 | 2021 October 5
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| Firefox 94 | Rust 1.55.0 | 1.53.0 | 2021 September 9 | 2021 September 30 | 2021 November 2
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| Firefox 95 | Rust 1.56.0 | 1.53.0 | 2021 October 21 | 2021 October 28 | 2021 December 7
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| Firefox 96 | Rust 1.57.0 | 1.53.0 | 2021 December 2 | 2021 December 2 | 2022 January 11
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| **Estimated** |
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| Firefox 97 | Rust 1.57.0 | 1.57.0 | 2021 December 2 | 2022 January 6 | 2022 February 8
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| Firefox 98 | Rust 1.58.0 | ? | 2021 January 13 | 2022 February 2 | 2022 March 8
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| Firefox 99 | Rust 1.59.0 | ? | 2021 February 24 | 2022 March 3 |
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New feature adoption schedule:
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| Mozilla-central can use | Starting on (Rust release + 2 weeks) |
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|-------------------------|--------------------------------------|
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| Rust 1.19.0 | 2017 August 3
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| Rust 1.20.0 | 2017 October 17 ([was](https://bugzilla.mozilla.org/show_bug.cgi?id=1396884) September 14)
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| Rust 1.21.0 | 2017 October 26
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| Rust 1.22.0 | 2017 December 7
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| Rust 1.23.0 | 2018 January 26 ([was](https://bugzilla.mozilla.org/show_bug.cgi?id=1418081#c8) January 18)
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| Rust 1.24.0 | 2018 March 1
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| Rust 1.25.0 | 2018 April 12
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| Rust 1.26.0 | 2018 May 24
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| Rust 1.27.0 | 2018 July 5
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| Rust 1.28.0 | 2018 August 16
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| Rust 1.29.0 | 2018 September 27
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| Rust 1.30.0 | 2018 November 8
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## Policy
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### Official builds
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_We ship official stable Firefox with stable Rust._
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Typically we build Firefox Nightly with the current stable Rust and let that
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configuration ride through beta to release. There have been frequent exceptions
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where we've adopted a beta Rust during the Nightly phase to address some
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platform support issue, and later updated when that Rust version became stable.
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For example, we needed to use beta Rust for our own builds when we first added
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Rust support to Firefox for Android.
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The policy provides good stability guarantees once Firefox is in release, while
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giving us the freedom to respond to the issues experience shows we'll need to
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address during development.
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### Developer builds
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_Local developer builds use whatever Rust toolchain is available on the
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system._
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Someone building Firefox can maintain the latest stable Rust with the `rustup`
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for `mach bootstrap` tools, or try other variations.
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### Required versions
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_We require a new stable Rust starting 2 weeks after it is released._
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This policy effectively removes the difference between the required Rust
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version and the default target for official builds, while still giving tier-3
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developers some time to process toolchain updates.
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Downstream packagers still have 2-3 months to package each stable Rust release
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before it’s required by a new Firefox release.
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The stable+two weeks requirement is a rough target. We expect to hit it within
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a few days for most Rust releases. But, for example, when the target date falls
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within a mozilla-central soft freeze (before branching for a beta release) we
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may update a week later.
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We expect esr releases will stay on the same minimum Rust version, so
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backporting security fixes may require Rust compatibility work too.
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### Rationale
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We need to be fast and efficient across the whole system which includes the
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Rust, Servo, and Firefox development teams. We realise that the decisions we
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make have an impact downstream, and we want to minimise the negative aspects of
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that impact. However, we cannot do so at the expense of our own productivity.
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There are many other stakeholders, of course, but our work in Gecko and Servo
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are major drivers for Rust language, compiler, and library development. That
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influence is more effective if we can iterate quickly. Servo itself often uses
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experimental language features to give the necessary early feedback on features
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as they evolve.
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Rust updates every six weeks, just like Firefox. This is more like web
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languages than native languages, but it's been great for building community. We
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(as Gecko developers) should embrace it.
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Many of us think of the toolchain and source code as being conceptually
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different. At this point in time, Rust is evolving much more quickly than
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Python or C++. For now it is easier to think of Rust as being part of Firefox,
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rather than thinking of it like a C++ toolchain with decades of history which
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we we have little influence on.
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Official build configurations are generally part of the Firefox code itself,
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subject to normal review, and don't affect anyone else. Therefore it's
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straightforward to manage the Rust toolchain we use directly like any other
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change.
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For contributors, we maintain a minimum supported Rust toolchain This helps
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those working on tier-3 platforms, where updating Rust can be more difficult
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than just running `rustup update`, by giving them time to adapt. However, it
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means other contributors must backport newer work to maintain compatibility. As
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more Rust code was merged into Firefox this became expensive.
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Historically we bumped this minimum every 3-4 Rust releases, which also helped
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contributors on slow network connections since they didn't have to download
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toolchains as often. Deciding to bump this way involved negotiating each
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change, which by late 2017 many contributors felt was a more significant
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burden. Delaying to give tier-3 platforms months instead of weeks to update
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their Rust packages is also not considered a good trade-off.
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My experience is that it takes about a week for version bumps to start pulling
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in dependent crates using new language features, so updating after a couple of
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weeks acts as only a moderate restraint on Servo developers working on Gecko
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modules. I think this is the correct trade-off between those two groups.
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### Alternate Proposals
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_We require nightly Rust_
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This would certainly speed up the feedback with language design. However, we
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would frequently encounter build breakage when unstable language features
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change. Even Servo pins to a specific nightly to avoid this problem. That’s
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appropriate for a smaller research project, but not something the size of
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Firefox.
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We should and do test building Firefox with nightly Rust to catch compiler
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issues, but that’s different from requiring everyone to use unstable Rust.
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_We require beta Rust_
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I don’t think anyone has suggested this, but I thought I’d include it for
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completeness. This shortens the feedback cycle for new features at the expense
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of more churn for contributors. It’s not a good trade-off because by the time a
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new feature is stabilized in beta Rust, it’s too late to change it much. This
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is better served by particular developers working with upstream directly. I
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believe beta is also something of an “excluded middle” in the Rust ecosystem,
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with most contributors working with either stable or unstable nightly Rust.
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_We require the previous stable Rust once a new stable Rust is released._
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That is, we bump the Firefox minimum to Rust 1.n.m when rust 1.(n+1).0 stable
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is released. This is roughly every 6 weeks, with six weeks to update before
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bumping into the new requirement. This policy might be simpler for contributors
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to remember. There's not much difference in the amount of time downstream
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packager receive for a particular Firefox release. This is a trade-off of more
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time for tier-3 developers against people working on Gecko/Servo components
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doing extra work to maintain compatibility. As I say above, I think this is the
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wrong choice.
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_We bump the minimum Rust version only when new features are compelling._
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This reduces churn for developers, but the need to negotiate each bump takes
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time better spent on development. See the motivation section above.
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