src/doc/rustc-dev-guide/src/implementing_new_features.md

   1 # Implement New Feature
   2
   3 When you want to implement a new significant feature in the compiler,
   4 you need to go through this process to make sure everything goes
   5 smoothly.
   6
   7 ## The @rfcbot (p)FCP process
   8
   9 When the change is small and uncontroversial, then it can be done
  10 with just writing a PR and getting r+ from someone who knows that
  11 part of the code. However, if the change is potentially controversial,
  12 it would be a bad idea to push it without consensus from the rest
  13 of the team (both in the "distributed system" sense to make sure
  14 you don't break anything you don't know about, and in the social
  15 sense to avoid PR fights).
  16
  17 If such a change seems to be too small to require a full formal RFC
  18 process (e.g. a big refactoring of the code, or a
  19 "technically-breaking" change, or a "big bugfix" that basically
  20 amounts to a small feature) but is still too controversial or
  21 big to get by with a single r+, you can start a pFCP (or, if you
  22 don't have r+ rights, ask someone who has them to start one - and
  23 unless they have a concern themselves, they should). pFCP stands for
  24 "proposed final comment period".
  25
  26 Again, the pFCP process is only needed if you need consensus - if you
  27 don't think anyone would have a problem with your change, it's ok to
  28 get by with only an r+. For example, it is OK to add or modify
  29 unstable command-line flags or attributes without a pFCP for
  30 compiler development or standard library use, as long as you don't
  31 expect them to be in wide use in the nightly ecosystem.
  32
  33 You don't need to have the implementation fully ready for r+ to ask
  34 for a pFCP, but it is generally a good idea to have at least a proof
  35 of concept so that people can see what you are talking about.
  36
  37 When a pFCP is started, it requires all members of the team to sign off
  38 the FCP. After they all do so, there's a 10 day long "final comment
  39 period" where everybody can comment, and if no new concerns are raised,
  40 the PR/issue gets FCP approval.
  41
  42 ## The logistics of writing features
  43
  44 There are a few "logistic" hoops you might need to go through in
  45 order to implement a feature in a working way.
  46
  47 ### Warning Cycles
  48
  49 In some cases, a feature or bugfix might break some existing programs
  50 in some edge cases. In that case, you might want to do a crater run
  51 to assess the impact and possibly add a future-compatibility lint,
  52 similar to those used for
  53 [edition-gated lints](diagnostics.md#edition-gated-lints).
  54
  55 ### Stability
  56
  57 We [value the stability of Rust]. Code that works and runs on stable
  58 should (mostly) not break. Because of that, we don't want to release
  59 a feature to the world with only team consensus and code review -
  60 we want to gain real-world experience on using that feature on nightly,
  61 and we might want to change the feature based on that experience.
  62
  63 To allow for that, we must make sure users don't accidentally depend
  64 on that new feature - otherwise, especially if experimentation takes
  65 time or is delayed and the feature takes the trains to stable,
  66 it would end up de facto stable and we'll not be able to make changes
  67 in it without breaking people's code.
  68
  69 The way we do that is that we make sure all new features are feature
  70 gated - they can't be used without enabling a feature gate
  71 (`#[feature(foo)]`), which can't be done in a stable/beta compiler.
  72 See the [stability in code] section for the technical details.
  73
  74 Eventually, after we gain enough experience using the feature,
  75 make the necessary changes, and are satisfied, we expose it to
  76 the world using the stabilization process described [here].
  77 Until then, the feature is not set in stone: every part of the
  78 feature can be changed, or the feature might be completely
  79 rewritten or removed. Features are not supposed to gain tenure
  80 by being unstable and unchanged for a year.
  81
  82 <a name = "tracking-issue"></a>
  83 ###  Tracking Issues
  84
  85 To keep track of the status of an unstable feature, the
  86 experience we get while using it on nightly, and of the
  87 concerns that block its stabilization, every feature-gate
  88 needs a tracking issue.
  89
  90 General discussions about the feature should be done on
  91 the tracking issue.
  92
  93 For features that have an RFC, you should use the RFC's
  94 tracking issue for the feature.
  95
  96 For other features, you'll have to make a tracking issue
  97 for that feature. The issue title should be "Tracking issue
  98 for YOUR FEATURE".
  99
 100 For tracking issues for features (as opposed to future-compat
 101 warnings), I don't think the description has to contain
 102 anything specific. Generally we put the list of items required
 103 for stabilization in a checklist, e.g.,
 104
 105 ```txt
 106 **Steps:**
 107
 108 - [ ] Implement the RFC. (CC @rust-lang/compiler -- can anyone write
 109       up mentoring instructions?)
 110 - [ ] Adjust the documentation. ([See instructions on rustc-dev-guide.](https://rustc-dev-guide.rust-lang.org/stabilization_guide.html#documentation-prs))
 111 - [ ] Stabilize the feature. ([See instructions on rustc-dev-guide.](https://rustc-dev-guide.rust-lang.org/stabilization_guide.html#stabilization-pr))
 112 ```
 113
 114 <a name="stability-in-code"></a>
 115 ##  Stability in code
 116
 117 The below steps needs to be followed in order to implement
 118 a new unstable feature:
 119
 120 1. Open a [tracking issue] -
 121    if you have an RFC, you can use the tracking issue for the RFC.
 122
 123    The tracking issue should be labeled with at least `C-tracking-issue`.
 124    For a language feature, a label `F-feature_name` should be added as well.
 125
 126 2. Pick a name for the feature gate (for RFCs, use the name
 127    in the RFC).
 128
 129 3. Add a feature gate declaration to `rustc_feature/src/active.rs`
 130    in the active `declare_features` block. See [here][add-feature-gate] for
 131    detailed instructions.
 132
 133 4. Prevent usage of the new feature unless the feature gate is set.
 134    You can check it in most places in the compiler using the
 135    expression `tcx.features().$feature_name` (or
 136    `sess.features_untracked().$feature_name` if the
 137    tcx is unavailable)
 138
 139     If the feature gate is not set, you should either maintain
 140     the pre-feature behavior or raise an error, depending on
 141     what makes sense. Errors should generally use [`rustc_session::parse::feature_err`].
 142     For an example of adding an error, see [#81015].
 143
 144    For features introducing new syntax, pre-expansion gating should be used instead.
 145    To do so, extend the [`GatedSpans`] struct, add spans to it during parsing,
 146    and then finally feature-gate all the spans in
 147    [`rustc_ast_passes::feature_gate::check_crate`].
 148
 149 5. Add a test to ensure the feature cannot be used without
 150    a feature gate, by creating `feature-gate-$feature_name.rs`
 151    and `feature-gate-$feature_name.stderr` files under the
 152    directory where the other tests for your feature reside.
 153
 154 6. Add a section to the unstable book, in
 155    `src/doc/unstable-book/src/language-features/$feature_name.md`.
 156
 157 7. Write a lot of tests for the new feature.
 158    PRs without tests will not be accepted!
 159
 160 8. Get your PR reviewed and land it. You have now successfully
 161    implemented a feature in Rust!
 162
 163 [`GatedSpans`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_session/parse/struct.GatedSpans.html
 164 [#81015]: https://github.com/rust-lang/rust/pull/81015
 165 [`rustc_session::parse::feature_err`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_session/parse/fn.feature_err.html
 166 [`rustc_ast_passes::feature_gate::check_crate`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast_passes/feature_gate/fn.check_crate.html
 167 [value the stability of Rust]: https://github.com/rust-lang/rfcs/blob/master/text/1122-language-semver.md
 168 [stability in code]: #stability-in-code
 169 [here]: ./stabilization_guide.md
 170 [tracking issue]: #tracking-issue
 171 [add-feature-gate]: ./feature-gates.md#adding-a-feature-gate