frontend 0.4.1

//! # Feature gates
//!
//! This crate declares the set of past and present unstable features in the compiler.
//! Feature gate checking itself is done in `rustc_ast_passes/src/feature_gate.rs`
//! at the moment.
//!
//! Features are enabled in programs via the crate-level attributes of
//! `#![feature(...)]` with a comma-separated list of features.
//!
//! For the purpose of future feature-tracking, once a feature gate is added,
//! even if it is stabilized or removed, *do not remove it*. Instead, move the
//! symbol to the `accepted` or `removed` modules respectively.

// Real std use, and all of it is environment rather than computation:
// `env::var("RUSTC_BOOTSTRAP")` decides whether unstable features are permitted,
// `LazyLock` builds the builtin-attribute table, and `unstable.rs` stamps a file with
// `SystemTime`. The feature *decision* is pure; only its input is environmental, so this
// goes when the bootstrap flag is passed in rather than read.

// ---------------------------------------------------------------------------------------------
// STD IS BANNED IN THIS CRATE.
//
// `#![no_std]` above is the ban and the compiler is the enforcer: without `extern crate std;`
// there is no `std` in the extern prelude, so any `std::` path fails to resolve and the build
// stops. Do not add that line back to make an error go away - the error is the point. Whatever
// needed `std` either has a `core`/`alloc` equivalent, belongs in `ekostd`, or is a
// dependency that has to be replaced.
//
// The prelude is the part a grep cannot see: `Vec`, `String`, `Box`, `format!`, `vec!`,
// `thread_local!` and `println!` name no path. Under `#![no_std]` they resolve through `alloc`
// and `eko` instead, which is why those imports appear at the top of every file here.
// ---------------------------------------------------------------------------------------------
#[macro_use]
use alloc::string::String;
mod accepted;
mod builtin_attrs;
mod removed;
mod unstable;

#[cfg(test)]
mod tests;

use core::num::NonZero;

use crate::rustc_span::Symbol;

#[derive(Debug, Clone)]
pub struct Feature {
    pub name: Symbol,
    /// For unstable features: the version the feature was added in.
    /// For accepted features: the version the feature got stabilized in.
    /// For removed features we are inconsistent; sometimes this is the
    /// version it got added, sometimes the version it got removed.
    pub since: &'static str,
    issue: Option<NonZero<u32>>,
}

#[derive(Clone, Copy, Debug, Hash)]
pub enum UnstableFeatures {
    /// Disallow use of unstable features, as on beta/stable channels.
    Disallow,
    /// Allow use of unstable features, as on nightly.
    Allow,
    /// Errors are bypassed for bootstrapping. This is required any time
    /// during the build that feature-related lints are set to warn or above
    /// because the build turns on warnings-as-errors and uses lots of unstable
    /// features. As a result, this is always required for building Rust itself.
    Cheat,
}

impl UnstableFeatures {
    /// This takes into account `RUSTC_BOOTSTRAP`.
    ///
    /// If `krate` is [`Some`], then setting `RUSTC_BOOTSTRAP=krate` will enable the nightly
    /// features. Otherwise, only `RUSTC_BOOTSTRAP=1` will work.
    pub fn from_environment(krate: Option<&str>) -> Self {
        Self::from_environment_value(krate, eko::env::var("RUSTC_BOOTSTRAP"))
    }

    /// Avoid a racy `setenv` by allowing tests to inject `RUSTC_BOOTSTRAP`'s value with the
    /// `env_var_rustc_bootstrap` arg.
    ///
    /// `None` is both "unset" and "not UTF-8", which is the distinction this function collapsed
    /// on its own line anyway - it called `.ok()` on the `Result` and never looked at the error.
    fn from_environment_value(krate: Option<&str>, env_var_rustc_bootstrap: Option<String>) -> Self {
        // `true` if this is a feature-staged build, i.e., on the beta or stable channel.
        let disable_unstable_features =
            option_env!("CFG_DISABLE_UNSTABLE_FEATURES").is_some_and(|s| s != "0");
        // Returns whether `krate` should be counted as unstable
        let is_unstable_crate =
            |var: &str| krate.is_some_and(|name| var.split(',').any(|new_krate| new_krate == name));

        let bootstrap = env_var_rustc_bootstrap;
        if let Some(val) = bootstrap.as_deref() {
            match val {
                val if val == "1" || is_unstable_crate(val) => return UnstableFeatures::Cheat,
                // Hypnotize ourselves so that we think we are a stable compiler and thus don't
                // allow any unstable features.
                "-1" => return UnstableFeatures::Disallow,
                _ => {}
            }
        }

        if disable_unstable_features { UnstableFeatures::Disallow } else { UnstableFeatures::Allow }
    }

    pub fn is_nightly_build(&self) -> bool {
        match *self {
            UnstableFeatures::Allow | UnstableFeatures::Cheat => true,
            UnstableFeatures::Disallow => false,
        }
    }
}

fn find_lang_feature_issue(feature: Symbol) -> Option<NonZero<u32>> {
    // Search in all the feature lists.
    if let Some(f) = UNSTABLE_LANG_FEATURES.iter().find(|f| f.name == feature) {
        return f.issue;
    }
    if let Some(f) = ACCEPTED_LANG_FEATURES.iter().find(|f| f.name == feature) {
        return f.issue;
    }
    if let Some(f) = REMOVED_LANG_FEATURES.iter().find(|f| f.feature.name == feature) {
        return f.feature.issue;
    }
    panic!("feature `{feature}` is not declared anywhere");
}

const fn to_nonzero(n: Option<u32>) -> Option<NonZero<u32>> {
    // Can be replaced with `n.and_then(NonZero::new)` if that is ever usable
    // in const context. Requires https://github.com/rust-lang/rfcs/pull/2632.
    match n {
        None => None,
        Some(n) => NonZero::new(n),
    }
}

pub enum GateIssue {
    Language,
    Library(Option<NonZero<u32>>),
}

pub fn find_feature_issue(feature: Symbol, issue: GateIssue) -> Option<NonZero<u32>> {
    match issue {
        GateIssue::Language => find_lang_feature_issue(feature),
        GateIssue::Library(lib) => lib,
    }
}

pub use accepted::ACCEPTED_LANG_FEATURES;
pub use builtin_attrs::{
    AttributeStability, BUILTIN_ATTRIBUTE_MAP, BUILTIN_ATTRIBUTES, GatedCfg, find_gated_cfg,
    is_builtin_attr_name,
};
pub use removed::REMOVED_LANG_FEATURES;
pub use unstable::{
    DEPENDENT_FEATURES, EnabledLangFeature, EnabledLibFeature, Features, INCOMPATIBLE_FEATURES,
    TRACK_FEATURE, UNSTABLE_LANG_FEATURES,
};