rust/src/libsyntax/feature_gate.rs

//! # Feature gating
//!
//! This module implements the gating necessary for preventing certain compiler
//! features from being used by default. This module will crawl a pre-expanded
//! AST to ensure that there are no features which are used that are not
//! enabled.
//!
//! 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 code for detection of feature
//! gate usage is added, *do not remove it again* even once the feature
//! becomes stable.

mod accepted;
use accepted::ACCEPTED_FEATURES;
mod removed;
use removed::{REMOVED_FEATURES, STABLE_REMOVED_FEATURES};
mod active;
use active::{ACTIVE_FEATURES};
pub use active::{Features, INCOMPLETE_FEATURES};
mod builtin_attrs;
pub use builtin_attrs::{
    AttributeGate, AttributeType, GatedCfg,
    BuiltinAttribute, BUILTIN_ATTRIBUTES, BUILTIN_ATTRIBUTE_MAP,
    deprecated_attributes, is_builtin_attr,  is_builtin_attr_name,
};

use crate::ast::{
    self, AssocTyConstraint, AssocTyConstraintKind, NodeId, GenericParam, GenericParamKind,
    PatKind, RangeEnd,
};
use crate::attr::{self, check_builtin_attribute};
use crate::source_map::Spanned;
use crate::edition::{ALL_EDITIONS, Edition};
use crate::visit::{self, FnKind, Visitor};
use crate::parse::{token, ParseSess};
use crate::parse::parser::Parser;
use crate::symbol::{Symbol, sym};
use crate::tokenstream::TokenTree;

use errors::{Applicability, DiagnosticBuilder, Handler};
use rustc_data_structures::fx::FxHashMap;
use rustc_target::spec::abi::Abi;
use syntax_pos::{Span, DUMMY_SP, MultiSpan};
use log::debug;

use std::env;

#[derive(Copy, Clone, Debug)]
pub enum Stability {
    Unstable,
    // First argument is tracking issue link; second argument is an optional
    // help message, which defaults to "remove this attribute"
    Deprecated(&'static str, Option<&'static str>),
}

struct Context<'a> {
    features: &'a Features,
    parse_sess: &'a ParseSess,
    plugin_attributes: &'a [(Symbol, AttributeType)],
}

macro_rules! gate_feature_fn {
    ($cx: expr, $has_feature: expr, $span: expr, $name: expr, $explain: expr, $level: expr) => {{
        let (cx, has_feature, span,
             name, explain, level) = ($cx, $has_feature, $span, $name, $explain, $level);
        let has_feature: bool = has_feature(&$cx.features);
        debug!("gate_feature(feature = {:?}, span = {:?}); has? {}", name, span, has_feature);
        if !has_feature && !span.allows_unstable($name) {
            leveled_feature_err(cx.parse_sess, name, span, GateIssue::Language, explain, level)
                .emit();
        }
    }}
}

macro_rules! gate_feature {
    ($cx: expr, $feature: ident, $span: expr, $explain: expr) => {
        gate_feature_fn!($cx, |x:&Features| x.$feature, $span,
                         sym::$feature, $explain, GateStrength::Hard)
    };
    ($cx: expr, $feature: ident, $span: expr, $explain: expr, $level: expr) => {
        gate_feature_fn!($cx, |x:&Features| x.$feature, $span,
                         sym::$feature, $explain, $level)
    };
}

impl<'a> Context<'a> {
    fn check_attribute(
        &self,
        attr: &ast::Attribute,
        attr_info: Option<&BuiltinAttribute>,
        is_macro: bool
    ) {
        debug!("check_attribute(attr = {:?})", attr);
        if let Some(&(name, ty, _template, ref gateage)) = attr_info {
            if let AttributeGate::Gated(_, name, desc, ref has_feature) = *gateage {
                if !attr.span.allows_unstable(name) {
                    gate_feature_fn!(
                        self, has_feature, attr.span, name, desc, GateStrength::Hard
                    );
                }
            } else if name == sym::doc {
                if let Some(content) = attr.meta_item_list() {
                    if content.iter().any(|c| c.check_name(sym::include)) {
                        gate_feature!(self, external_doc, attr.span,
                            "`#[doc(include = \"...\")]` is experimental"
                        );
                    }
                }
            }
            debug!("check_attribute: {:?} is builtin, {:?}, {:?}", attr.path, ty, gateage);
            return;
        } else {
            for segment in &attr.path.segments {
                if segment.ident.as_str().starts_with("rustc") {
                    let msg = "attributes starting with `rustc` are \
                               reserved for use by the `rustc` compiler";
                    gate_feature!(self, rustc_attrs, segment.ident.span, msg);
                }
            }
        }
        for &(n, ty) in self.plugin_attributes {
            if attr.path == n {
                // Plugins can't gate attributes, so we don't check for it
                // unlike the code above; we only use this loop to
                // short-circuit to avoid the checks below.
                debug!("check_attribute: {:?} is registered by a plugin, {:?}", attr.path, ty);
                return;
            }
        }
        if !is_macro && !attr::is_known(attr) {
            // Only run the custom attribute lint during regular feature gate
            // checking. Macro gating runs before the plugin attributes are
            // registered, so we skip this in that case.
            let msg = format!("the attribute `{}` is currently unknown to the compiler and \
                               may have meaning added to it in the future", attr.path);
            gate_feature!(self, custom_attribute, attr.span, &msg);
        }
    }
}

pub fn check_attribute(attr: &ast::Attribute, parse_sess: &ParseSess, features: &Features) {
    let cx = Context { features, parse_sess, plugin_attributes: &[] };
    cx.check_attribute(
        attr,
        attr.ident().and_then(|ident| BUILTIN_ATTRIBUTE_MAP.get(&ident.name).map(|a| *a)),
        true
    );
}

fn find_lang_feature_issue(feature: Symbol) -> Option<u32> {
    if let Some(info) = ACTIVE_FEATURES.iter().find(|t| t.0 == feature) {
        let issue = info.2;
        // FIXME (#28244): enforce that active features have issue numbers
        // assert!(issue.is_some())
        issue
    } else {
        // search in Accepted, Removed, or Stable Removed features
        let found = ACCEPTED_FEATURES.iter().chain(REMOVED_FEATURES).chain(STABLE_REMOVED_FEATURES)
            .find(|t| t.0 == feature);
        match found {
            Some(&(_, _, issue, _)) => issue,
            None => panic!("Feature `{}` is not declared anywhere", feature),
        }
    }
}

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

#[derive(Debug, Copy, Clone, PartialEq)]
pub enum GateStrength {
    /// A hard error. (Most feature gates should use this.)
    Hard,
    /// Only a warning. (Use this only as backwards-compatibility demands.)
    Soft,
}

pub fn emit_feature_err(
    sess: &ParseSess,
    feature: Symbol,
    span: Span,
    issue: GateIssue,
    explain: &str,
) {
    feature_err(sess, feature, span, issue, explain).emit();
}

pub fn feature_err<'a, S: Into<MultiSpan>>(
    sess: &'a ParseSess,
    feature: Symbol,
    span: S,
    issue: GateIssue,
    explain: &str,
) -> DiagnosticBuilder<'a> {
    leveled_feature_err(sess, feature, span, issue, explain, GateStrength::Hard)
}

fn leveled_feature_err<'a, S: Into<MultiSpan>>(
    sess: &'a ParseSess,
    feature: Symbol,
    span: S,
    issue: GateIssue,
    explain: &str,
    level: GateStrength,
) -> DiagnosticBuilder<'a> {
    let diag = &sess.span_diagnostic;

    let issue = match issue {
        GateIssue::Language => find_lang_feature_issue(feature),
        GateIssue::Library(lib) => lib,
    };

    let mut err = match level {
        GateStrength::Hard => {
            diag.struct_span_err_with_code(span, explain, stringify_error_code!(E0658))
        }
        GateStrength::Soft => diag.struct_span_warn(span, explain),
    };

    match issue {
        None | Some(0) => {}  // We still accept `0` as a stand-in for backwards compatibility
        Some(n) => {
            err.note(&format!(
                "for more information, see https://github.com/rust-lang/rust/issues/{}",
                n,
            ));
        }
    }

    // #23973: do not suggest `#![feature(...)]` if we are in beta/stable
    if sess.unstable_features.is_nightly_build() {
        err.help(&format!("add `#![feature({})]` to the crate attributes to enable", feature));
    }

    // If we're on stable and only emitting a "soft" warning, add a note to
    // clarify that the feature isn't "on" (rather than being on but
    // warning-worthy).
    if !sess.unstable_features.is_nightly_build() && level == GateStrength::Soft {
        err.help("a nightly build of the compiler is required to enable this feature");
    }

    err

}

const EXPLAIN_BOX_SYNTAX: &str =
    "box expression syntax is experimental; you can call `Box::new` instead";

pub const EXPLAIN_STMT_ATTR_SYNTAX: &str =
    "attributes on expressions are experimental";

pub const EXPLAIN_ALLOW_INTERNAL_UNSTABLE: &str =
    "allow_internal_unstable side-steps feature gating and stability checks";
pub const EXPLAIN_ALLOW_INTERNAL_UNSAFE: &str =
    "allow_internal_unsafe side-steps the unsafe_code lint";

pub const EXPLAIN_UNSIZED_TUPLE_COERCION: &str =
    "unsized tuple coercion is not stable enough for use and is subject to change";

struct PostExpansionVisitor<'a> {
    context: &'a Context<'a>,
    builtin_attributes: &'static FxHashMap<Symbol, &'static BuiltinAttribute>,
}

macro_rules! gate_feature_post {
    ($cx: expr, $feature: ident, $span: expr, $explain: expr) => {{
        let (cx, span) = ($cx, $span);
        if !span.allows_unstable(sym::$feature) {
            gate_feature!(cx.context, $feature, span, $explain)
        }
    }};
    ($cx: expr, $feature: ident, $span: expr, $explain: expr, $level: expr) => {{
        let (cx, span) = ($cx, $span);
        if !span.allows_unstable(sym::$feature) {
            gate_feature!(cx.context, $feature, span, $explain, $level)
        }
    }}
}

impl<'a> PostExpansionVisitor<'a> {
    fn check_abi(&self, abi: Abi, span: Span) {
        match abi {
            Abi::RustIntrinsic => {
                gate_feature_post!(&self, intrinsics, span,
                                   "intrinsics are subject to change");
            },
            Abi::PlatformIntrinsic => {
                gate_feature_post!(&self, platform_intrinsics, span,
                                   "platform intrinsics are experimental and possibly buggy");
            },
            Abi::Vectorcall => {
                gate_feature_post!(&self, abi_vectorcall, span,
                                   "vectorcall is experimental and subject to change");
            },
            Abi::Thiscall => {
                gate_feature_post!(&self, abi_thiscall, span,
                                   "thiscall is experimental and subject to change");
            },
            Abi::RustCall => {
                gate_feature_post!(&self, unboxed_closures, span,
                                   "rust-call ABI is subject to change");
            },
            Abi::PtxKernel => {
                gate_feature_post!(&self, abi_ptx, span,
                                   "PTX ABIs are experimental and subject to change");
            },
            Abi::Unadjusted => {
                gate_feature_post!(&self, abi_unadjusted, span,
                                   "unadjusted ABI is an implementation detail and perma-unstable");
            },
            Abi::Msp430Interrupt => {
                gate_feature_post!(&self, abi_msp430_interrupt, span,
                                   "msp430-interrupt ABI is experimental and subject to change");
            },
            Abi::X86Interrupt => {
                gate_feature_post!(&self, abi_x86_interrupt, span,
                                   "x86-interrupt ABI is experimental and subject to change");
            },
            Abi::AmdGpuKernel => {
                gate_feature_post!(&self, abi_amdgpu_kernel, span,
                                   "amdgpu-kernel ABI is experimental and subject to change");
            },
            // Stable
            Abi::Cdecl |
            Abi::Stdcall |
            Abi::Fastcall |
            Abi::Aapcs |
            Abi::Win64 |
            Abi::SysV64 |
            Abi::Rust |
            Abi::C |
            Abi::System => {}
        }
    }
}

impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
    fn visit_attribute(&mut self, attr: &ast::Attribute) {
        let attr_info = attr.ident().and_then(|ident| {
            self.builtin_attributes.get(&ident.name).map(|a| *a)
        });

        // Check for gated attributes.
        self.context.check_attribute(attr, attr_info, false);

        if attr.check_name(sym::doc) {
            if let Some(content) = attr.meta_item_list() {
                if content.len() == 1 && content[0].check_name(sym::cfg) {
                    gate_feature_post!(&self, doc_cfg, attr.span,
                        "`#[doc(cfg(...))]` is experimental"
                    );
                } else if content.iter().any(|c| c.check_name(sym::masked)) {
                    gate_feature_post!(&self, doc_masked, attr.span,
                        "`#[doc(masked)]` is experimental"
                    );
                } else if content.iter().any(|c| c.check_name(sym::spotlight)) {
                    gate_feature_post!(&self, doc_spotlight, attr.span,
                        "`#[doc(spotlight)]` is experimental"
                    );
                } else if content.iter().any(|c| c.check_name(sym::alias)) {
                    gate_feature_post!(&self, doc_alias, attr.span,
                        "`#[doc(alias = \"...\")]` is experimental"
                    );
                } else if content.iter().any(|c| c.check_name(sym::keyword)) {
                    gate_feature_post!(&self, doc_keyword, attr.span,
                        "`#[doc(keyword = \"...\")]` is experimental"
                    );
                }
            }
        }

        match attr_info {
            // `rustc_dummy` doesn't have any restrictions specific to built-in attributes.
            Some(&(name, _, template, _)) if name != sym::rustc_dummy =>
                check_builtin_attribute(self.context.parse_sess, attr, name, template),
            _ => if let Some(TokenTree::Token(token)) = attr.tokens.trees().next() {
                if token == token::Eq {
                    // All key-value attributes are restricted to meta-item syntax.
                    attr.parse_meta(self.context.parse_sess).map_err(|mut err| err.emit()).ok();
                }
            }
        }
    }

    fn visit_name(&mut self, sp: Span, name: ast::Name) {
        if !name.as_str().is_ascii() {
            gate_feature_post!(
                &self,
                non_ascii_idents,
                self.context.parse_sess.source_map().def_span(sp),
                "non-ascii idents are not fully supported"
            );
        }
    }

    fn visit_item(&mut self, i: &'a ast::Item) {
        match i.node {
            ast::ItemKind::ForeignMod(ref foreign_module) => {
                self.check_abi(foreign_module.abi, i.span);
            }

            ast::ItemKind::Fn(..) => {
                if attr::contains_name(&i.attrs[..], sym::plugin_registrar) {
                    gate_feature_post!(&self, plugin_registrar, i.span,
                                       "compiler plugins are experimental and possibly buggy");
                }
                if attr::contains_name(&i.attrs[..], sym::start) {
                    gate_feature_post!(&self, start, i.span,
                                      "a `#[start]` function is an experimental \
                                       feature whose signature may change \
                                       over time");
                }
                if attr::contains_name(&i.attrs[..], sym::main) {
                    gate_feature_post!(&self, main, i.span,
                                       "declaration of a non-standard `#[main]` \
                                        function may change over time, for now \
                                        a top-level `fn main()` is required");
                }
            }

            ast::ItemKind::Struct(..) => {
                for attr in attr::filter_by_name(&i.attrs[..], sym::repr) {
                    for item in attr.meta_item_list().unwrap_or_else(Vec::new) {
                        if item.check_name(sym::simd) {
                            gate_feature_post!(&self, repr_simd, attr.span,
                                               "SIMD types are experimental and possibly buggy");
                        }
                    }
                }
            }

            ast::ItemKind::Enum(ast::EnumDef{ref variants, ..}, ..) => {
                for variant in variants {
                    match (&variant.data, &variant.disr_expr) {
                        (ast::VariantData::Unit(..), _) => {},
                        (_, Some(disr_expr)) =>
                            gate_feature_post!(
                                &self,
                                arbitrary_enum_discriminant,
                                disr_expr.value.span,
                                "discriminants on non-unit variants are experimental"),
                        _ => {},
                    }
                }

                let has_feature = self.context.features.arbitrary_enum_discriminant;
                if !has_feature && !i.span.allows_unstable(sym::arbitrary_enum_discriminant) {
                    Parser::maybe_report_invalid_custom_discriminants(
                        self.context.parse_sess,
                        &variants,
                    );
                }
            }

            ast::ItemKind::Impl(_, polarity, defaultness, _, _, _, _) => {
                if polarity == ast::ImplPolarity::Negative {
                    gate_feature_post!(&self, optin_builtin_traits,
                                       i.span,
                                       "negative trait bounds are not yet fully implemented; \
                                        use marker types for now");
                }

                if let ast::Defaultness::Default = defaultness {
                    gate_feature_post!(&self, specialization,
                                       i.span,
                                       "specialization is unstable");
                }
            }

            ast::ItemKind::Trait(ast::IsAuto::Yes, ..) => {
                gate_feature_post!(&self, optin_builtin_traits,
                                   i.span,
                                   "auto traits are experimental and possibly buggy");
            }

            ast::ItemKind::TraitAlias(..) => {
                gate_feature_post!(
                    &self,
                    trait_alias,
                    i.span,
                    "trait aliases are experimental"
                );
            }

            ast::ItemKind::MacroDef(ast::MacroDef { legacy: false, .. }) => {
                let msg = "`macro` is experimental";
                gate_feature_post!(&self, decl_macro, i.span, msg);
            }

            ast::ItemKind::OpaqueTy(..) => {
                gate_feature_post!(
                    &self,
                    type_alias_impl_trait,
                    i.span,
                    "`impl Trait` in type aliases is unstable"
                );
            }

            _ => {}
        }

        visit::walk_item(self, i);
    }

    fn visit_foreign_item(&mut self, i: &'a ast::ForeignItem) {
        match i.node {
            ast::ForeignItemKind::Fn(..) |
            ast::ForeignItemKind::Static(..) => {
                let link_name = attr::first_attr_value_str_by_name(&i.attrs, sym::link_name);
                let links_to_llvm = match link_name {
                    Some(val) => val.as_str().starts_with("llvm."),
                    _ => false
                };
                if links_to_llvm {
                    gate_feature_post!(&self, link_llvm_intrinsics, i.span,
                                       "linking to LLVM intrinsics is experimental");
                }
            }
            ast::ForeignItemKind::Ty => {
                    gate_feature_post!(&self, extern_types, i.span,
                                       "extern types are experimental");
            }
            ast::ForeignItemKind::Macro(..) => {}
        }

        visit::walk_foreign_item(self, i)
    }

    fn visit_ty(&mut self, ty: &'a ast::Ty) {
        match ty.node {
            ast::TyKind::BareFn(ref bare_fn_ty) => {
                self.check_abi(bare_fn_ty.abi, ty.span);
            }
            ast::TyKind::Never => {
                gate_feature_post!(&self, never_type, ty.span,
                                   "The `!` type is experimental");
            }
            _ => {}
        }
        visit::walk_ty(self, ty)
    }

    fn visit_fn_ret_ty(&mut self, ret_ty: &'a ast::FunctionRetTy) {
        if let ast::FunctionRetTy::Ty(ref output_ty) = *ret_ty {
            if let ast::TyKind::Never = output_ty.node {
                // Do nothing.
            } else {
                self.visit_ty(output_ty)
            }
        }
    }

    fn visit_expr(&mut self, e: &'a ast::Expr) {
        match e.node {
            ast::ExprKind::Box(_) => {
                gate_feature_post!(&self, box_syntax, e.span, EXPLAIN_BOX_SYNTAX);
            }
            ast::ExprKind::Type(..) => {
                // To avoid noise about type ascription in common syntax errors, only emit if it
                // is the *only* error.
                if self.context.parse_sess.span_diagnostic.err_count() == 0 {
                    gate_feature_post!(&self, type_ascription, e.span,
                                       "type ascription is experimental");
                }
            }
            ast::ExprKind::TryBlock(_) => {
                gate_feature_post!(&self, try_blocks, e.span, "`try` expression is experimental");
            }
            ast::ExprKind::Block(_, opt_label) => {
                if let Some(label) = opt_label {
                    gate_feature_post!(&self, label_break_value, label.ident.span,
                                    "labels on blocks are unstable");
                }
            }
            _ => {}
        }
        visit::walk_expr(self, e)
    }

    fn visit_arm(&mut self, arm: &'a ast::Arm) {
        visit::walk_arm(self, arm)
    }

    fn visit_pat(&mut self, pattern: &'a ast::Pat) {
        match &pattern.node {
            PatKind::Slice(pats) => {
                for pat in &*pats {
                    let span = pat.span;
                    let inner_pat = match &pat.node {
                        PatKind::Ident(.., Some(pat)) => pat,
                        _ => pat,
                    };
                    if inner_pat.is_rest() {
                        gate_feature_post!(
                            &self,
                            slice_patterns,
                            span,
                            "subslice patterns are unstable"
                        );
                    }
                }
            }
            PatKind::Box(..) => {
                gate_feature_post!(&self, box_patterns,
                                  pattern.span,
                                  "box pattern syntax is experimental");
            }
            PatKind::Range(_, _, Spanned { node: RangeEnd::Excluded, .. }) => {
                gate_feature_post!(&self, exclusive_range_pattern, pattern.span,
                                   "exclusive range pattern syntax is experimental");
            }
            _ => {}
        }
        visit::walk_pat(self, pattern)
    }

    fn visit_fn(&mut self,
                fn_kind: FnKind<'a>,
                fn_decl: &'a ast::FnDecl,
                span: Span,
                _node_id: NodeId) {
        if let Some(header) = fn_kind.header() {
            // Stability of const fn methods are covered in
            // `visit_trait_item` and `visit_impl_item` below; this is
            // because default methods don't pass through this point.
            self.check_abi(header.abi, span);
        }

        if fn_decl.c_variadic {
            gate_feature_post!(&self, c_variadic, span, "C-variadic functions are unstable");
        }

        visit::walk_fn(self, fn_kind, fn_decl, span)
    }

    fn visit_generic_param(&mut self, param: &'a GenericParam) {
        match param.kind {
            GenericParamKind::Const { .. } =>
                gate_feature_post!(&self, const_generics, param.ident.span,
                    "const generics are unstable"),
            _ => {}
        }
        visit::walk_generic_param(self, param)
    }

    fn visit_assoc_ty_constraint(&mut self, constraint: &'a AssocTyConstraint) {
        match constraint.kind {
            AssocTyConstraintKind::Bound { .. } =>
                gate_feature_post!(&self, associated_type_bounds, constraint.span,
                    "associated type bounds are unstable"),
            _ => {}
        }
        visit::walk_assoc_ty_constraint(self, constraint)
    }

    fn visit_trait_item(&mut self, ti: &'a ast::TraitItem) {
        match ti.node {
            ast::TraitItemKind::Method(ref sig, ref block) => {
                if block.is_none() {
                    self.check_abi(sig.header.abi, ti.span);
                }
                if sig.decl.c_variadic {
                    gate_feature_post!(&self, c_variadic, ti.span,
                                       "C-variadic functions are unstable");
                }
                if sig.header.constness.node == ast::Constness::Const {
                    gate_feature_post!(&self, const_fn, ti.span, "const fn is unstable");
                }
            }
            ast::TraitItemKind::Type(_, ref default) => {
                // We use three if statements instead of something like match guards so that all
                // of these errors can be emitted if all cases apply.
                if default.is_some() {
                    gate_feature_post!(&self, associated_type_defaults, ti.span,
                                       "associated type defaults are unstable");
                }
                if !ti.generics.params.is_empty() {
                    gate_feature_post!(&self, generic_associated_types, ti.span,
                                       "generic associated types are unstable");
                }
                if !ti.generics.where_clause.predicates.is_empty() {
                    gate_feature_post!(&self, generic_associated_types, ti.span,
                                       "where clauses on associated types are unstable");
                }
            }
            _ => {}
        }
        visit::walk_trait_item(self, ti)
    }

    fn visit_impl_item(&mut self, ii: &'a ast::ImplItem) {
        if ii.defaultness == ast::Defaultness::Default {
            gate_feature_post!(&self, specialization,
                              ii.span,
                              "specialization is unstable");
        }

        match ii.node {
            ast::ImplItemKind::Method(..) => {}
            ast::ImplItemKind::OpaqueTy(..) => {
                gate_feature_post!(
                    &self,
                    type_alias_impl_trait,
                    ii.span,
                    "`impl Trait` in type aliases is unstable"
                );
            }
            ast::ImplItemKind::TyAlias(_) => {
                if !ii.generics.params.is_empty() {
                    gate_feature_post!(&self, generic_associated_types, ii.span,
                                       "generic associated types are unstable");
                }
                if !ii.generics.where_clause.predicates.is_empty() {
                    gate_feature_post!(&self, generic_associated_types, ii.span,
                                       "where clauses on associated types are unstable");
                }
            }
            _ => {}
        }
        visit::walk_impl_item(self, ii)
    }

    fn visit_vis(&mut self, vis: &'a ast::Visibility) {
        if let ast::VisibilityKind::Crate(ast::CrateSugar::JustCrate) = vis.node {
            gate_feature_post!(&self, crate_visibility_modifier, vis.span,
                               "`crate` visibility modifier is experimental");
        }
        visit::walk_vis(self, vis)
    }
}

pub fn get_features(span_handler: &Handler, krate_attrs: &[ast::Attribute],
                    crate_edition: Edition, allow_features: &Option<Vec<String>>) -> Features {
    fn feature_removed(span_handler: &Handler, span: Span, reason: Option<&str>) {
        let mut err = struct_span_err!(span_handler, span, E0557, "feature has been removed");
        if let Some(reason) = reason {
            err.span_note(span, reason);
        } else {
            err.span_label(span, "feature has been removed");
        }
        err.emit();
    }

    let mut features = Features::new();
    let mut edition_enabled_features = FxHashMap::default();

    for &edition in ALL_EDITIONS {
        if edition <= crate_edition {
            // The `crate_edition` implies its respective umbrella feature-gate
            // (i.e., `#![feature(rust_20XX_preview)]` isn't needed on edition 20XX).
            edition_enabled_features.insert(edition.feature_name(), edition);
        }
    }

    for &(name, .., f_edition, set) in ACTIVE_FEATURES {
        if let Some(f_edition) = f_edition {
            if f_edition <= crate_edition {
                set(&mut features, DUMMY_SP);
                edition_enabled_features.insert(name, crate_edition);
            }
        }
    }

    // Process the edition umbrella feature-gates first, to ensure
    // `edition_enabled_features` is completed before it's queried.
    for attr in krate_attrs {
        if !attr.check_name(sym::feature) {
            continue
        }

        let list = match attr.meta_item_list() {
            Some(list) => list,
            None => continue,
        };

        for mi in list {
            if !mi.is_word() {
                continue;
            }

            let name = mi.name_or_empty();

            if let Some(edition) = ALL_EDITIONS.iter().find(|e| name == e.feature_name()) {
                if *edition <= crate_edition {
                    continue;
                }

                for &(name, .., f_edition, set) in ACTIVE_FEATURES {
                    if let Some(f_edition) = f_edition {
                        if f_edition <= *edition {
                            // FIXME(Manishearth) there is currently no way to set
                            // lib features by edition
                            set(&mut features, DUMMY_SP);
                            edition_enabled_features.insert(name, *edition);
                        }
                    }
                }
            }
        }
    }

    for attr in krate_attrs {
        if !attr.check_name(sym::feature) {
            continue
        }

        let list = match attr.meta_item_list() {
            Some(list) => list,
            None => continue,
        };

        let bad_input = |span| {
            struct_span_err!(span_handler, span, E0556, "malformed `feature` attribute input")
        };

        for mi in list {
            let name = match mi.ident() {
                Some(ident) if mi.is_word() => ident.name,
                Some(ident) => {
                    bad_input(mi.span()).span_suggestion(
                        mi.span(),
                        "expected just one word",
                        format!("{}", ident.name),
                        Applicability::MaybeIncorrect,
                    ).emit();
                    continue
                }
                None => {
                    bad_input(mi.span()).span_label(mi.span(), "expected just one word").emit();
                    continue
                }
            };

            if let Some(edition) = edition_enabled_features.get(&name) {
                struct_span_warn!(
                    span_handler,
                    mi.span(),
                    E0705,
                    "the feature `{}` is included in the Rust {} edition",
                    name,
                    edition,
                ).emit();
                continue;
            }

            if ALL_EDITIONS.iter().any(|e| name == e.feature_name()) {
                // Handled in the separate loop above.
                continue;
            }

            let removed = REMOVED_FEATURES.iter().find(|f| name == f.0);
            let stable_removed = STABLE_REMOVED_FEATURES.iter().find(|f| name == f.0);
            if let Some((.., reason)) = removed.or(stable_removed) {
                feature_removed(span_handler, mi.span(), *reason);
                continue;
            }

            if let Some((_, since, ..)) = ACCEPTED_FEATURES.iter().find(|f| name == f.0) {
                let since = Some(Symbol::intern(since));
                features.declared_lang_features.push((name, mi.span(), since));
                continue;
            }

            if let Some(allowed) = allow_features.as_ref() {
                if allowed.iter().find(|f| *f == name.as_str()).is_none() {
                    span_err!(span_handler, mi.span(), E0725,
                              "the feature `{}` is not in the list of allowed features",
                              name);
                    continue;
                }
            }

            if let Some((.., set)) = ACTIVE_FEATURES.iter().find(|f| name == f.0) {
                set(&mut features, mi.span());
                features.declared_lang_features.push((name, mi.span(), None));
                continue;
            }

            features.declared_lib_features.push((name, mi.span()));
        }
    }

    features
}

pub fn check_crate(krate: &ast::Crate,
                   sess: &ParseSess,
                   features: &Features,
                   plugin_attributes: &[(Symbol, AttributeType)],
                   unstable: UnstableFeatures) {
    maybe_stage_features(&sess.span_diagnostic, krate, unstable);
    let ctx = Context {
        features,
        parse_sess: sess,
        plugin_attributes,
    };

    macro_rules! gate_all {
        ($gate:ident, $msg:literal) => { gate_all!($gate, $gate, $msg); };
        ($spans:ident, $gate:ident, $msg:literal) => {
            for span in &*sess.gated_spans.$spans.borrow() {
                gate_feature!(&ctx, $gate, *span, $msg);
            }
        }
    }

    gate_all!(param_attrs, "attributes on function parameters are unstable");
    gate_all!(let_chains, "`let` expressions in this position are experimental");
    gate_all!(async_closure, "async closures are unstable");
    gate_all!(yields, generators, "yield syntax is experimental");
    gate_all!(or_patterns, "or-patterns syntax is experimental");

    let visitor = &mut PostExpansionVisitor {
        context: &ctx,
        builtin_attributes: &*BUILTIN_ATTRIBUTE_MAP,
    };
    visit::walk_crate(visitor, krate);
}

#[derive(Clone, Copy, Hash)]
pub enum UnstableFeatures {
    /// Hard errors for unstable features are active, as on beta/stable channels.
    Disallow,
    /// Allow features to be activated, 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 {
    pub fn from_environment() -> UnstableFeatures {
        // Whether 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();
        // Whether we should enable unstable features for bootstrapping
        let bootstrap = env::var("RUSTC_BOOTSTRAP").is_ok();
        match (disable_unstable_features, bootstrap) {
            (_, true) => UnstableFeatures::Cheat,
            (true, _) => UnstableFeatures::Disallow,
            (false, _) => UnstableFeatures::Allow
        }
    }

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

fn maybe_stage_features(span_handler: &Handler, krate: &ast::Crate,
                        unstable: UnstableFeatures) {
    let allow_features = match unstable {
        UnstableFeatures::Allow => true,
        UnstableFeatures::Disallow => false,
        UnstableFeatures::Cheat => true
    };
    if !allow_features {
        for attr in &krate.attrs {
            if attr.check_name(sym::feature) {
                let release_channel = option_env!("CFG_RELEASE_CHANNEL").unwrap_or("(unknown)");
                span_err!(span_handler, attr.span, E0554,
                          "`#![feature]` may not be used on the {} release channel",
                          release_channel);
            }
        }
    }
}