compiler/rustc_feature/src/lib.rs

   1 //! # Feature gates
   2 //!
   3 //! This crate declares the set of past and present unstable features in the compiler.
   4 //! Feature gate checking itself is done in `rustc_ast_passes/src/feature_gate.rs`
   5 //! at the moment.
   6 //!
   7 //! Features are enabled in programs via the crate-level attributes of
   8 //! `#![feature(...)]` with a comma-separated list of features.
   9 //!
  10 //! For the purpose of future feature-tracking, once a feature gate is added,
  11 //! even if it is stabilized or removed, *do not remove it*. Instead, move the
  12 //! symbol to the `accepted` or `removed` modules respectively.
  13
  14 #![feature(once_cell)]
  15
  16 mod accepted;
  17 mod active;
  18 mod builtin_attrs;
  19 mod removed;
  20
  21 #[cfg(test)]
  22 mod tests;
  23
  24 use rustc_span::{edition::Edition, symbol::Symbol, Span};
  25 use std::fmt;
  26 use std::num::NonZeroU32;
  27
  28 #[derive(Clone, Copy)]
  29 pub enum State {
  30     Accepted,
  31     Active { set: fn(&mut Features, Span) },
  32     Removed { reason: Option<&'static str> },
  33     Stabilized { reason: Option<&'static str> },
  34 }
  35
  36 impl fmt::Debug for State {
  37     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
  38         match self {
  39             State::Accepted { .. } => write!(f, "accepted"),
  40             State::Active { .. } => write!(f, "active"),
  41             State::Removed { .. } => write!(f, "removed"),
  42             State::Stabilized { .. } => write!(f, "stabilized"),
  43         }
  44     }
  45 }
  46
  47 #[derive(Debug, Clone)]
  48 pub struct Feature {
  49     pub state: State,
  50     pub name: Symbol,
  51     pub since: &'static str,
  52     issue: Option<NonZeroU32>,
  53     pub edition: Option<Edition>,
  54     description: &'static str,
  55 }
  56
  57 #[derive(Copy, Clone, Debug)]
  58 pub enum Stability {
  59     Unstable,
  60     // First argument is tracking issue link; second argument is an optional
  61     // help message, which defaults to "remove this attribute".
  62     Deprecated(&'static str, Option<&'static str>),
  63 }
  64
  65 #[derive(Clone, Copy, Debug, Hash)]
  66 pub enum UnstableFeatures {
  67     /// Hard errors for unstable features are active, as on beta/stable channels.
  68     Disallow,
  69     /// Allow features to be activated, as on nightly.
  70     Allow,
  71     /// Errors are bypassed for bootstrapping. This is required any time
  72     /// during the build that feature-related lints are set to warn or above
  73     /// because the build turns on warnings-as-errors and uses lots of unstable
  74     /// features. As a result, this is always required for building Rust itself.
  75     Cheat,
  76 }
  77
  78 impl UnstableFeatures {
  79     /// This takes into account `RUSTC_BOOTSTRAP`.
  80     ///
  81     /// If `krate` is [`Some`], then setting `RUSTC_BOOTSTRAP=krate` will enable the nightly features.
  82     /// Otherwise, only `RUSTC_BOOTSTRAP=1` will work.
  83     pub fn from_environment(krate: Option<&str>) -> Self {
  84         // `true` if this is a feature-staged build, i.e., on the beta or stable channel.
  85         let disable_unstable_features = option_env!("CFG_DISABLE_UNSTABLE_FEATURES").is_some();
  86         // Returns whether `krate` should be counted as unstable
  87         let is_unstable_crate = |var: &str| {
  88             krate.map_or(false, |name| var.split(',').any(|new_krate| new_krate == name))
  89         };
  90         // `true` if we should enable unstable features for bootstrapping.
  91         let bootstrap = std::env::var("RUSTC_BOOTSTRAP")
  92             .map_or(false, |var| var == "1" || is_unstable_crate(&var));
  93         match (disable_unstable_features, bootstrap) {
  94             (_, true) => UnstableFeatures::Cheat,
  95             (true, _) => UnstableFeatures::Disallow,
  96             (false, _) => UnstableFeatures::Allow,
  97         }
  98     }
  99
 100     pub fn is_nightly_build(&self) -> bool {
 101         match *self {
 102             UnstableFeatures::Allow | UnstableFeatures::Cheat => true,
 103             UnstableFeatures::Disallow => false,
 104         }
 105     }
 106 }
 107
 108 fn find_lang_feature_issue(feature: Symbol) -> Option<NonZeroU32> {
 109     if let Some(info) = ACTIVE_FEATURES.iter().find(|t| t.name == feature) {
 110         // FIXME (#28244): enforce that active features have issue numbers
 111         // assert!(info.issue.is_some())
 112         info.issue
 113     } else {
 114         // search in Accepted, Removed, or Stable Removed features
 115         let found = ACCEPTED_FEATURES
 116             .iter()
 117             .chain(REMOVED_FEATURES)
 118             .chain(STABLE_REMOVED_FEATURES)
 119             .find(|t| t.name == feature);
 120         match found {
 121             Some(found) => found.issue,
 122             None => panic!("feature `{}` is not declared anywhere", feature),
 123         }
 124     }
 125 }
 126
 127 const fn to_nonzero(n: Option<u32>) -> Option<NonZeroU32> {
 128     // Can be replaced with `n.and_then(NonZeroU32::new)` if that is ever usable
 129     // in const context. Requires https://github.com/rust-lang/rfcs/pull/2632.
 130     match n {
 131         None => None,
 132         Some(n) => NonZeroU32::new(n),
 133     }
 134 }
 135
 136 pub enum GateIssue {
 137     Language,
 138     Library(Option<NonZeroU32>),
 139 }
 140
 141 pub fn find_feature_issue(feature: Symbol, issue: GateIssue) -> Option<NonZeroU32> {
 142     match issue {
 143         GateIssue::Language => find_lang_feature_issue(feature),
 144         GateIssue::Library(lib) => lib,
 145     }
 146 }
 147
 148 pub use accepted::ACCEPTED_FEATURES;
 149 pub use active::{Features, ACTIVE_FEATURES, INCOMPATIBLE_FEATURES};
 150 pub use builtin_attrs::{
 151     deprecated_attributes, find_gated_cfg, is_builtin_attr_name, AttributeGate, AttributeTemplate,
 152     AttributeType, BuiltinAttribute, GatedCfg, BUILTIN_ATTRIBUTES, BUILTIN_ATTRIBUTE_MAP,
 153 };
 154 pub use removed::{REMOVED_FEATURES, STABLE_REMOVED_FEATURES};