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 }
  55
  56 #[derive(Copy, Clone, Debug)]
  57 pub enum Stability {
  58     Unstable,
  59     // First argument is tracking issue link; second argument is an optional
  60     // help message, which defaults to "remove this attribute".
  61     Deprecated(&'static str, Option<&'static str>),
  62 }
  63
  64 #[derive(Clone, Copy, Debug, Hash)]
  65 pub enum UnstableFeatures {
  66     /// Hard errors for unstable features are active, as on beta/stable channels.
  67     Disallow,
  68     /// Allow features to be activated, as on nightly.
  69     Allow,
  70     /// Errors are bypassed for bootstrapping. This is required any time
  71     /// during the build that feature-related lints are set to warn or above
  72     /// because the build turns on warnings-as-errors and uses lots of unstable
  73     /// features. As a result, this is always required for building Rust itself.
  74     Cheat,
  75 }
  76
  77 impl UnstableFeatures {
  78     /// This takes into account `RUSTC_BOOTSTRAP`.
  79     ///
  80     /// If `krate` is [`Some`], then setting `RUSTC_BOOTSTRAP=krate` will enable the nightly features.
  81     /// Otherwise, only `RUSTC_BOOTSTRAP=1` will work.
  82     pub fn from_environment(krate: Option<&str>) -> Self {
  83         // `true` if this is a feature-staged build, i.e., on the beta or stable channel.
  84         let disable_unstable_features = option_env!("CFG_DISABLE_UNSTABLE_FEATURES").is_some();
  85         // Returns whether `krate` should be counted as unstable
  86         let is_unstable_crate = |var: &str| {
  87             krate.map_or(false, |name| var.split(',').any(|new_krate| new_krate == name))
  88         };
  89         // `true` if we should enable unstable features for bootstrapping.
  90         let bootstrap = std::env::var("RUSTC_BOOTSTRAP")
  91             .map_or(false, |var| var == "1" || is_unstable_crate(&var));
  92         match (disable_unstable_features, bootstrap) {
  93             (_, true) => UnstableFeatures::Cheat,
  94             (true, _) => UnstableFeatures::Disallow,
  95             (false, _) => UnstableFeatures::Allow,
  96         }
  97     }
  98
  99     pub fn is_nightly_build(&self) -> bool {
 100         match *self {
 101             UnstableFeatures::Allow | UnstableFeatures::Cheat => true,
 102             UnstableFeatures::Disallow => false,
 103         }
 104     }
 105 }
 106
 107 fn find_lang_feature_issue(feature: Symbol) -> Option<NonZeroU32> {
 108     if let Some(info) = ACTIVE_FEATURES.iter().find(|t| t.name == feature) {
 109         // FIXME (#28244): enforce that active features have issue numbers
 110         // assert!(info.issue.is_some())
 111         info.issue
 112     } else {
 113         // search in Accepted, Removed, or Stable Removed features
 114         let found = ACCEPTED_FEATURES
 115             .iter()
 116             .chain(REMOVED_FEATURES)
 117             .chain(STABLE_REMOVED_FEATURES)
 118             .find(|t| t.name == feature);
 119         match found {
 120             Some(found) => found.issue,
 121             None => panic!("feature `{}` is not declared anywhere", feature),
 122         }
 123     }
 124 }
 125
 126 const fn to_nonzero(n: Option<u32>) -> Option<NonZeroU32> {
 127     // Can be replaced with `n.and_then(NonZeroU32::new)` if that is ever usable
 128     // in const context. Requires https://github.com/rust-lang/rfcs/pull/2632.
 129     match n {
 130         None => None,
 131         Some(n) => NonZeroU32::new(n),
 132     }
 133 }
 134
 135 pub enum GateIssue {
 136     Language,
 137     Library(Option<NonZeroU32>),
 138 }
 139
 140 pub fn find_feature_issue(feature: Symbol, issue: GateIssue) -> Option<NonZeroU32> {
 141     match issue {
 142         GateIssue::Language => find_lang_feature_issue(feature),
 143         GateIssue::Library(lib) => lib,
 144     }
 145 }
 146
 147 pub use accepted::ACCEPTED_FEATURES;
 148 pub use active::{Features, ACTIVE_FEATURES, INCOMPATIBLE_FEATURES};
 149 pub use builtin_attrs::AttributeDuplicates;
 150 pub use builtin_attrs::{
 151     deprecated_attributes, find_gated_cfg, is_builtin_attr_name, is_builtin_only_local,
 152     AttributeGate, AttributeTemplate, AttributeType, BuiltinAttribute, GatedCfg,
 153     BUILTIN_ATTRIBUTES, BUILTIN_ATTRIBUTE_MAP,
 154 };
 155 pub use removed::{REMOVED_FEATURES, STABLE_REMOVED_FEATURES};