compiler/rustc_expand/src/config.rs - third_party/rust - Git at Google

 //! Conditional compilation stripping.

 use rustc_ast::ptr::P;
 use rustc_ast::token::{DelimToken, Token, TokenKind};
 use rustc_ast::tokenstream::{AttrAnnotatedTokenStream, AttrAnnotatedTokenTree};
 use rustc_ast::tokenstream::{DelimSpan, Spacing};
 use rustc_ast::tokenstream::{LazyTokenStream, TokenTree};
 use rustc_ast::{self as ast, AstLike, AttrItem, AttrStyle, Attribute, MetaItem};
 use rustc_attr as attr;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::map_in_place::MapInPlace;
 use rustc_errors::{error_code, struct_span_err, Applicability, Handler};
 use rustc_feature::{Feature, Features, State as FeatureState};
 use rustc_feature::{
     ACCEPTED_FEATURES, ACTIVE_FEATURES, REMOVED_FEATURES, STABLE_REMOVED_FEATURES,
 };
 use rustc_parse::{parse_in, validate_attr};
 use rustc_session::parse::feature_err;
 use rustc_session::Session;
 use rustc_span::edition::{Edition, ALL_EDITIONS};
 use rustc_span::symbol::{sym, Symbol};
 use rustc_span::{Span, DUMMY_SP};

 /// A folder that strips out items that do not belong in the current configuration.
 pub struct StripUnconfigured<'a> {
     pub sess: &'a Session,
     pub features: Option<&'a Features>,
     /// If `true`, perform cfg-stripping on attached tokens.
     /// This is only used for the input to derive macros,
     /// which needs eager expansion of `cfg` and `cfg_attr`
     pub config_tokens: bool,
 }

 fn get_features(
     sess: &Session,
     span_handler: &Handler,
     krate_attrs: &[ast::Attribute],
 ) -> 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");
         err.span_label(span, "feature has been removed");
         if let Some(reason) = reason {
             err.note(reason);
         }
         err.emit();
     }

     fn active_features_up_to(edition: Edition) -> impl Iterator<Item = &'static Feature> {
         ACTIVE_FEATURES.iter().filter(move |feature| {
             if let Some(feature_edition) = feature.edition {
                 feature_edition <= edition
             } else {
                 false
             }
         })
     }

     let mut features = Features::default();
     let mut edition_enabled_features = FxHashMap::default();
     let crate_edition = sess.edition();

     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 feature in active_features_up_to(crate_edition) {
         feature.set(&mut features, DUMMY_SP);
         edition_enabled_features.insert(feature.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 !sess.check_name(attr, 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();

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

                 for feature in active_features_up_to(edition) {
                     // FIXME(Manishearth) there is currently no way to set
                     // lib features by edition
                     feature.set(&mut features, DUMMY_SP);
                     edition_enabled_features.insert(feature.name, edition);
                 }
             }
         }
     }

     for attr in krate_attrs {
         if !sess.check_name(attr, 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) {
                 let msg =
                     &format!("the feature `{}` is included in the Rust {} edition", name, edition);
                 span_handler.struct_span_warn_with_code(mi.span(), msg, error_code!(E0705)).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.name);
             let stable_removed = STABLE_REMOVED_FEATURES.iter().find(|f| name == f.name);
             if let Some(Feature { state, .. }) = removed.or(stable_removed) {
                 if let FeatureState::Removed { reason } | FeatureState::Stabilized { reason } =
                     state
                 {
                     feature_removed(span_handler, mi.span(), *reason);
                     continue;
                 }
             }

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

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

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

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

     features
 }

 // `cfg_attr`-process the crate's attributes and compute the crate's features.
 pub fn features(sess: &Session, mut krate: ast::Crate) -> (ast::Crate, Features) {
     let mut strip_unconfigured = StripUnconfigured { sess, features: None, config_tokens: false };

     let unconfigured_attrs = krate.attrs.clone();
     let diag = &sess.parse_sess.span_diagnostic;
     let err_count = diag.err_count();
     let features = match strip_unconfigured.configure_krate_attrs(krate.attrs) {
         None => {
             // The entire crate is unconfigured.
             krate.attrs = Vec::new();
             krate.items = Vec::new();
             Features::default()
         }
         Some(attrs) => {
             krate.attrs = attrs;
             let features = get_features(sess, diag, &krate.attrs);
             if err_count == diag.err_count() {
                 // Avoid reconfiguring malformed `cfg_attr`s.
                 strip_unconfigured.features = Some(&features);
                 // Run configuration again, this time with features available
                 // so that we can perform feature-gating.
                 strip_unconfigured.configure_krate_attrs(unconfigured_attrs);
             }
             features
         }
     };
     (krate, features)
 }

 #[macro_export]
 macro_rules! configure {
     ($this:ident, $node:ident) => {
         match $this.configure($node) {
             Some(node) => node,
             None => return Default::default(),
         }
     };
 }

 const CFG_ATTR_GRAMMAR_HELP: &str = "#[cfg_attr(condition, attribute, other_attribute, ...)]";
 const CFG_ATTR_NOTE_REF: &str = "for more information, visit \
     <https://doc.rust-lang.org/reference/conditional-compilation.html\
     #the-cfg_attr-attribute>";

 impl<'a> StripUnconfigured<'a> {
     pub fn configure<T: AstLike>(&mut self, mut node: T) -> Option<T> {
         self.process_cfg_attrs(&mut node);
         if self.in_cfg(node.attrs()) {
             self.try_configure_tokens(&mut node);
             Some(node)
         } else {
             None
         }
     }

     fn try_configure_tokens<T: AstLike>(&mut self, node: &mut T) {
         if self.config_tokens {
             if let Some(Some(tokens)) = node.tokens_mut() {
                 let attr_annotated_tokens = tokens.create_token_stream();
                 *tokens = LazyTokenStream::new(self.configure_tokens(&attr_annotated_tokens));
             }
         }
     }

     fn configure_krate_attrs(
         &mut self,
         mut attrs: Vec<ast::Attribute>,
     ) -> Option<Vec<ast::Attribute>> {
         attrs.flat_map_in_place(|attr| self.process_cfg_attr(attr));
         if self.in_cfg(&attrs) { Some(attrs) } else { None }
     }

     /// Performs cfg-expansion on `stream`, producing a new `AttrAnnotatedTokenStream`.
     /// This is only used during the invocation of `derive` proc-macros,
     /// which require that we cfg-expand their entire input.
     /// Normal cfg-expansion operates on parsed AST nodes via the `configure` method
     fn configure_tokens(&mut self, stream: &AttrAnnotatedTokenStream) -> AttrAnnotatedTokenStream {
         fn can_skip(stream: &AttrAnnotatedTokenStream) -> bool {
             stream.0.iter().all(|(tree, _spacing)| match tree {
                 AttrAnnotatedTokenTree::Attributes(_) => false,
                 AttrAnnotatedTokenTree::Token(_) => true,
                 AttrAnnotatedTokenTree::Delimited(_, _, inner) => can_skip(inner),
             })
         }

         if can_skip(stream) {
             return stream.clone();
         }

         let trees: Vec<_> = stream
             .0
             .iter()
             .flat_map(|(tree, spacing)| match tree.clone() {
                 AttrAnnotatedTokenTree::Attributes(mut data) => {
                     let mut attrs: Vec<_> = std::mem::take(&mut data.attrs).into();
                     attrs.flat_map_in_place(|attr| self.process_cfg_attr(attr));
                     data.attrs = attrs.into();

                     if self.in_cfg(&data.attrs) {
                         data.tokens = LazyTokenStream::new(
                             self.configure_tokens(&data.tokens.create_token_stream()),
                         );
                         Some((AttrAnnotatedTokenTree::Attributes(data), *spacing)).into_iter()
                     } else {
                         None.into_iter()
                     }
                 }
                 AttrAnnotatedTokenTree::Delimited(sp, delim, mut inner) => {
                     inner = self.configure_tokens(&inner);
                     Some((AttrAnnotatedTokenTree::Delimited(sp, delim, inner), *spacing))
                         .into_iter()
                 }
                 AttrAnnotatedTokenTree::Token(token) => {
                     if let TokenKind::Interpolated(nt) = token.kind {
                         panic!(
                             "Nonterminal should have been flattened at {:?}: {:?}",
                             token.span, nt
                         );
                     } else {
                         Some((AttrAnnotatedTokenTree::Token(token), *spacing)).into_iter()
                     }
                 }
             })
             .collect();
         AttrAnnotatedTokenStream::new(trees)
     }

     /// Parse and expand all `cfg_attr` attributes into a list of attributes
     /// that are within each `cfg_attr` that has a true configuration predicate.
     ///
     /// Gives compiler warnings if any `cfg_attr` does not contain any
     /// attributes and is in the original source code. Gives compiler errors if
     /// the syntax of any `cfg_attr` is incorrect.
     fn process_cfg_attrs<T: AstLike>(&mut self, node: &mut T) {
         node.visit_attrs(|attrs| {
             attrs.flat_map_in_place(|attr| self.process_cfg_attr(attr));
         });
     }

     /// Parse and expand a single `cfg_attr` attribute into a list of attributes
     /// when the configuration predicate is true, or otherwise expand into an
     /// empty list of attributes.
     ///
     /// Gives a compiler warning when the `cfg_attr` contains no attributes and
     /// is in the original source file. Gives a compiler error if the syntax of
     /// the attribute is incorrect.
     fn process_cfg_attr(&mut self, attr: Attribute) -> Vec<Attribute> {
         if !attr.has_name(sym::cfg_attr) {
             return vec![attr];
         }

         let (cfg_predicate, expanded_attrs) = match self.parse_cfg_attr(&attr) {
             None => return vec![],
             Some(r) => r,
         };

         // Lint on zero attributes in source.
         if expanded_attrs.is_empty() {
             return vec![attr];
         }

         // At this point we know the attribute is considered used.
         self.sess.mark_attr_used(&attr);

         if !attr::cfg_matches(&cfg_predicate, &self.sess.parse_sess, self.features) {
             return vec![];
         }

         // We call `process_cfg_attr` recursively in case there's a
         // `cfg_attr` inside of another `cfg_attr`. E.g.
         //  `#[cfg_attr(false, cfg_attr(true, some_attr))]`.
         expanded_attrs
             .into_iter()
             .flat_map(|(item, span)| {
                 let orig_tokens = attr.tokens().to_tokenstream();

                 // We are taking an attribute of the form `#[cfg_attr(pred, attr)]`
                 // and producing an attribute of the form `#[attr]`. We
                 // have captured tokens for `attr` itself, but we need to
                 // synthesize tokens for the wrapper `#` and `[]`, which
                 // we do below.

                 // Use the `#` in `#[cfg_attr(pred, attr)]` as the `#` token
                 // for `attr` when we expand it to `#[attr]`
                 let mut orig_trees = orig_tokens.trees();
                 let pound_token = match orig_trees.next().unwrap() {
                     TokenTree::Token(token @ Token { kind: TokenKind::Pound, .. }) => token,
                     _ => panic!("Bad tokens for attribute {:?}", attr),
                 };
                 let pound_span = pound_token.span;

                 let mut trees = vec![(AttrAnnotatedTokenTree::Token(pound_token), Spacing::Alone)];
                 if attr.style == AttrStyle::Inner {
                     // For inner attributes, we do the same thing for the `!` in `#![some_attr]`
                     let bang_token = match orig_trees.next().unwrap() {
                         TokenTree::Token(token @ Token { kind: TokenKind::Not, .. }) => token,
                         _ => panic!("Bad tokens for attribute {:?}", attr),
                     };
                     trees.push((AttrAnnotatedTokenTree::Token(bang_token), Spacing::Alone));
                 }
                 // We don't really have a good span to use for the syntheized `[]`
                 // in `#[attr]`, so just use the span of the `#` token.
                 let bracket_group = AttrAnnotatedTokenTree::Delimited(
                     DelimSpan::from_single(pound_span),
                     DelimToken::Bracket,
                     item.tokens
                         .as_ref()
                         .unwrap_or_else(|| panic!("Missing tokens for {:?}", item))
                         .create_token_stream(),
                 );
                 trees.push((bracket_group, Spacing::Alone));
                 let tokens = Some(LazyTokenStream::new(AttrAnnotatedTokenStream::new(trees)));
                 self.process_cfg_attr(attr::mk_attr_from_item(item, tokens, attr.style, span))
             })
             .collect()
     }

     fn parse_cfg_attr(&self, attr: &Attribute) -> Option<(MetaItem, Vec<(AttrItem, Span)>)> {
         match attr.get_normal_item().args {
             ast::MacArgs::Delimited(dspan, delim, ref tts) if !tts.is_empty() => {
                 let msg = "wrong `cfg_attr` delimiters";
                 validate_attr::check_meta_bad_delim(&self.sess.parse_sess, dspan, delim, msg);
                 match parse_in(&self.sess.parse_sess, tts.clone(), "`cfg_attr` input", |p| {
                     p.parse_cfg_attr()
                 }) {
                     Ok(r) => return Some(r),
                     Err(mut e) => {
                         e.help(&format!("the valid syntax is `{}`", CFG_ATTR_GRAMMAR_HELP))
                             .note(CFG_ATTR_NOTE_REF)
                             .emit();
                     }
                 }
             }
             _ => self.error_malformed_cfg_attr_missing(attr.span),
         }
         None
     }

     fn error_malformed_cfg_attr_missing(&self, span: Span) {
         self.sess
             .parse_sess
             .span_diagnostic
             .struct_span_err(span, "malformed `cfg_attr` attribute input")
             .span_suggestion(
                 span,
                 "missing condition and attribute",
                 CFG_ATTR_GRAMMAR_HELP.to_string(),
                 Applicability::HasPlaceholders,
             )
             .note(CFG_ATTR_NOTE_REF)
             .emit();
     }

     /// Determines if a node with the given attributes should be included in this configuration.
     fn in_cfg(&self, attrs: &[Attribute]) -> bool {
         attrs.iter().all(|attr| {
             if !is_cfg(self.sess, attr) {
                 return true;
             }
             let meta_item = match validate_attr::parse_meta(&self.sess.parse_sess, attr) {
                 Ok(meta_item) => meta_item,
                 Err(mut err) => {
                     err.emit();
                     return true;
                 }
             };
             let error = |span, msg, suggestion: &str| {
                 let mut err = self.sess.parse_sess.span_diagnostic.struct_span_err(span, msg);
                 if !suggestion.is_empty() {
                     err.span_suggestion(
                         span,
                         "expected syntax is",
                         suggestion.into(),
                         Applicability::MaybeIncorrect,
                     );
                 }
                 err.emit();
                 true
             };
             let span = meta_item.span;
             match meta_item.meta_item_list() {
                 None => error(span, "`cfg` is not followed by parentheses", "cfg(/* predicate */)"),
                 Some([]) => error(span, "`cfg` predicate is not specified", ""),
                 Some([_, .., l]) => error(l.span(), "multiple `cfg` predicates are specified", ""),
                 Some([single]) => match single.meta_item() {
                     Some(meta_item) => {
                         attr::cfg_matches(meta_item, &self.sess.parse_sess, self.features)
                     }
                     None => error(single.span(), "`cfg` predicate key cannot be a literal", ""),
                 },
             }
         })
     }

     /// If attributes are not allowed on expressions, emit an error for `attr`
     crate fn maybe_emit_expr_attr_err(&self, attr: &Attribute) {
         if !self.features.map_or(true, |features| features.stmt_expr_attributes) {
             let mut err = feature_err(
                 &self.sess.parse_sess,
                 sym::stmt_expr_attributes,
                 attr.span,
                 "attributes on expressions are experimental",
             );

             if attr.is_doc_comment() {
                 err.help("`///` is for documentation comments. For a plain comment, use `//`.");
             }

             err.emit();
         }
     }

     pub fn configure_expr(&mut self, expr: &mut P<ast::Expr>) {
         for attr in expr.attrs.iter() {
             self.maybe_emit_expr_attr_err(attr);
         }

         // If an expr is valid to cfg away it will have been removed by the
         // outer stmt or expression folder before descending in here.
         // Anything else is always required, and thus has to error out
         // in case of a cfg attr.
         //
         // N.B., this is intentionally not part of the visit_expr() function
         //     in order for filter_map_expr() to be able to avoid this check
         if let Some(attr) = expr.attrs().iter().find(|a| is_cfg(self.sess, a)) {
             let msg = "removing an expression is not supported in this position";
             self.sess.parse_sess.span_diagnostic.span_err(attr.span, msg);
         }

         self.process_cfg_attrs(expr);
         self.try_configure_tokens(&mut *expr);
     }
 }

 fn is_cfg(sess: &Session, attr: &Attribute) -> bool {
     sess.check_name(attr, sym::cfg)
 }
	//! Conditional compilation stripping.

	use rustc_ast::ptr::P;
	use rustc_ast::token::{DelimToken, Token, TokenKind};
	use rustc_ast::tokenstream::{AttrAnnotatedTokenStream, AttrAnnotatedTokenTree};
	use rustc_ast::tokenstream::{DelimSpan, Spacing};
	use rustc_ast::tokenstream::{LazyTokenStream, TokenTree};
	use rustc_ast::{self as ast, AstLike, AttrItem, AttrStyle, Attribute, MetaItem};
	use rustc_attr as attr;
	use rustc_data_structures::fx::FxHashMap;
	use rustc_data_structures::map_in_place::MapInPlace;
	use rustc_errors::{error_code, struct_span_err, Applicability, Handler};
	use rustc_feature::{Feature, Features, State as FeatureState};
	use rustc_feature::{
	ACCEPTED_FEATURES, ACTIVE_FEATURES, REMOVED_FEATURES, STABLE_REMOVED_FEATURES,
	};
	use rustc_parse::{parse_in, validate_attr};
	use rustc_session::parse::feature_err;
	use rustc_session::Session;
	use rustc_span::edition::{Edition, ALL_EDITIONS};
	use rustc_span::symbol::{sym, Symbol};
	use rustc_span::{Span, DUMMY_SP};

	/// A folder that strips out items that do not belong in the current configuration.
	pub struct StripUnconfigured<'a> {
	pub sess: &'a Session,
	pub features: Option<&'a Features>,
	/// If `true`, perform cfg-stripping on attached tokens.
	/// This is only used for the input to derive macros,
	/// which needs eager expansion of `cfg` and `cfg_attr`
	pub config_tokens: bool,
	}

	fn get_features(
	sess: &Session,
	span_handler: &Handler,
	krate_attrs: &[ast::Attribute],
	) -> 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");
	err.span_label(span, "feature has been removed");
	if let Some(reason) = reason {
	err.note(reason);
	}
	err.emit();
	}

	fn active_features_up_to(edition: Edition) -> impl Iterator<Item = &'static Feature> {
	ACTIVE_FEATURES.iter().filter(move \|feature\| {
	if let Some(feature_edition) = feature.edition {
	feature_edition <= edition
	} else {
	false
	}
	})
	}

	let mut features = Features::default();
	let mut edition_enabled_features = FxHashMap::default();
	let crate_edition = sess.edition();

	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 feature in active_features_up_to(crate_edition) {
	feature.set(&mut features, DUMMY_SP);
	edition_enabled_features.insert(feature.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 !sess.check_name(attr, 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();

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

	for feature in active_features_up_to(edition) {
	// FIXME(Manishearth) there is currently no way to set
	// lib features by edition
	feature.set(&mut features, DUMMY_SP);
	edition_enabled_features.insert(feature.name, edition);
	}
	}
	}
	}

	for attr in krate_attrs {
	if !sess.check_name(attr, 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) {
	let msg =
	&format!("the feature `{}` is included in the Rust {} edition", name, edition);
	span_handler.struct_span_warn_with_code(mi.span(), msg, error_code!(E0705)).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.name);
	let stable_removed = STABLE_REMOVED_FEATURES.iter().find(\|f\| name == f.name);
	if let Some(Feature { state, .. }) = removed.or(stable_removed) {
	if let FeatureState::Removed { reason } \| FeatureState::Stabilized { reason } =
	state
	{
	feature_removed(span_handler, mi.span(), *reason);
	continue;
	}
	}

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

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

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

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

	features
	}

	// `cfg_attr`-process the crate's attributes and compute the crate's features.
	pub fn features(sess: &Session, mut krate: ast::Crate) -> (ast::Crate, Features) {
	let mut strip_unconfigured = StripUnconfigured { sess, features: None, config_tokens: false };

	let unconfigured_attrs = krate.attrs.clone();
	let diag = &sess.parse_sess.span_diagnostic;
	let err_count = diag.err_count();
	let features = match strip_unconfigured.configure_krate_attrs(krate.attrs) {
	None => {
	// The entire crate is unconfigured.
	krate.attrs = Vec::new();
	krate.items = Vec::new();
	Features::default()
	}
	Some(attrs) => {
	krate.attrs = attrs;
	let features = get_features(sess, diag, &krate.attrs);
	if err_count == diag.err_count() {
	// Avoid reconfiguring malformed `cfg_attr`s.
	strip_unconfigured.features = Some(&features);
	// Run configuration again, this time with features available
	// so that we can perform feature-gating.
	strip_unconfigured.configure_krate_attrs(unconfigured_attrs);
	}
	features
	}
	};
	(krate, features)
	}

	#[macro_export]
	macro_rules! configure {
	($this:ident, $node:ident) => {
	match $this.configure($node) {
	Some(node) => node,
	None => return Default::default(),
	}
	};
	}

	const CFG_ATTR_GRAMMAR_HELP: &str = "#[cfg_attr(condition, attribute, other_attribute, ...)]";
	const CFG_ATTR_NOTE_REF: &str = "for more information, visit \
	<https://doc.rust-lang.org/reference/conditional-compilation.html\
	#the-cfg_attr-attribute>";

	impl<'a> StripUnconfigured<'a> {
	pub fn configure<T: AstLike>(&mut self, mut node: T) -> Option<T> {
	self.process_cfg_attrs(&mut node);
	if self.in_cfg(node.attrs()) {
	self.try_configure_tokens(&mut node);
	Some(node)
	} else {
	None
	}
	}

	fn try_configure_tokens<T: AstLike>(&mut self, node: &mut T) {
	if self.config_tokens {
	if let Some(Some(tokens)) = node.tokens_mut() {
	let attr_annotated_tokens = tokens.create_token_stream();
	*tokens = LazyTokenStream::new(self.configure_tokens(&attr_annotated_tokens));
	}
	}
	}

	fn configure_krate_attrs(
	&mut self,
	mut attrs: Vec<ast::Attribute>,
	) -> Option<Vec<ast::Attribute>> {
	attrs.flat_map_in_place(\|attr\| self.process_cfg_attr(attr));
	if self.in_cfg(&attrs) { Some(attrs) } else { None }
	}

	/// Performs cfg-expansion on `stream`, producing a new `AttrAnnotatedTokenStream`.
	/// This is only used during the invocation of `derive` proc-macros,
	/// which require that we cfg-expand their entire input.
	/// Normal cfg-expansion operates on parsed AST nodes via the `configure` method
	fn configure_tokens(&mut self, stream: &AttrAnnotatedTokenStream) -> AttrAnnotatedTokenStream {
	fn can_skip(stream: &AttrAnnotatedTokenStream) -> bool {
	stream.0.iter().all(\|(tree, _spacing)\| match tree {
	AttrAnnotatedTokenTree::Attributes(_) => false,
	AttrAnnotatedTokenTree::Token(_) => true,
	AttrAnnotatedTokenTree::Delimited(_, _, inner) => can_skip(inner),
	})
	}

	if can_skip(stream) {
	return stream.clone();
	}

	let trees: Vec<_> = stream
	.0
	.iter()
	.flat_map(\|(tree, spacing)\| match tree.clone() {
	AttrAnnotatedTokenTree::Attributes(mut data) => {
	let mut attrs: Vec<_> = std::mem::take(&mut data.attrs).into();
	attrs.flat_map_in_place(\|attr\| self.process_cfg_attr(attr));
	data.attrs = attrs.into();

	if self.in_cfg(&data.attrs) {
	data.tokens = LazyTokenStream::new(
	self.configure_tokens(&data.tokens.create_token_stream()),
	);
	Some((AttrAnnotatedTokenTree::Attributes(data), *spacing)).into_iter()
	} else {
	None.into_iter()
	}
	}
	AttrAnnotatedTokenTree::Delimited(sp, delim, mut inner) => {
	inner = self.configure_tokens(&inner);
	Some((AttrAnnotatedTokenTree::Delimited(sp, delim, inner), *spacing))
	.into_iter()
	}
	AttrAnnotatedTokenTree::Token(token) => {
	if let TokenKind::Interpolated(nt) = token.kind {
	panic!(
	"Nonterminal should have been flattened at {:?}: {:?}",
	token.span, nt
	);
	} else {
	Some((AttrAnnotatedTokenTree::Token(token), *spacing)).into_iter()
	}
	}
	})
	.collect();
	AttrAnnotatedTokenStream::new(trees)
	}

	/// Parse and expand all `cfg_attr` attributes into a list of attributes
	/// that are within each `cfg_attr` that has a true configuration predicate.
	///
	/// Gives compiler warnings if any `cfg_attr` does not contain any
	/// attributes and is in the original source code. Gives compiler errors if
	/// the syntax of any `cfg_attr` is incorrect.
	fn process_cfg_attrs<T: AstLike>(&mut self, node: &mut T) {
	node.visit_attrs(\|attrs\| {
	attrs.flat_map_in_place(\|attr\| self.process_cfg_attr(attr));
	});
	}

	/// Parse and expand a single `cfg_attr` attribute into a list of attributes
	/// when the configuration predicate is true, or otherwise expand into an
	/// empty list of attributes.
	///
	/// Gives a compiler warning when the `cfg_attr` contains no attributes and
	/// is in the original source file. Gives a compiler error if the syntax of
	/// the attribute is incorrect.
	fn process_cfg_attr(&mut self, attr: Attribute) -> Vec<Attribute> {
	if !attr.has_name(sym::cfg_attr) {
	return vec![attr];
	}

	let (cfg_predicate, expanded_attrs) = match self.parse_cfg_attr(&attr) {
	None => return vec![],
	Some(r) => r,
	};

	// Lint on zero attributes in source.
	if expanded_attrs.is_empty() {
	return vec![attr];
	}

	// At this point we know the attribute is considered used.
	self.sess.mark_attr_used(&attr);

	if !attr::cfg_matches(&cfg_predicate, &self.sess.parse_sess, self.features) {
	return vec![];
	}

	// We call `process_cfg_attr` recursively in case there's a
	// `cfg_attr` inside of another `cfg_attr`. E.g.
	// `#[cfg_attr(false, cfg_attr(true, some_attr))]`.
	expanded_attrs
	.into_iter()
	.flat_map(\|(item, span)\| {
	let orig_tokens = attr.tokens().to_tokenstream();

	// We are taking an attribute of the form `#[cfg_attr(pred, attr)]`
	// and producing an attribute of the form `#[attr]`. We
	// have captured tokens for `attr` itself, but we need to
	// synthesize tokens for the wrapper `#` and `[]`, which
	// we do below.

	// Use the `#` in `#[cfg_attr(pred, attr)]` as the `#` token
	// for `attr` when we expand it to `#[attr]`
	let mut orig_trees = orig_tokens.trees();
	let pound_token = match orig_trees.next().unwrap() {
	TokenTree::Token(token @ Token { kind: TokenKind::Pound, .. }) => token,
	_ => panic!("Bad tokens for attribute {:?}", attr),
	};
	let pound_span = pound_token.span;

	let mut trees = vec![(AttrAnnotatedTokenTree::Token(pound_token), Spacing::Alone)];
	if attr.style == AttrStyle::Inner {
	// For inner attributes, we do the same thing for the `!` in `#![some_attr]`
	let bang_token = match orig_trees.next().unwrap() {
	TokenTree::Token(token @ Token { kind: TokenKind::Not, .. }) => token,
	_ => panic!("Bad tokens for attribute {:?}", attr),
	};
	trees.push((AttrAnnotatedTokenTree::Token(bang_token), Spacing::Alone));
	}
	// We don't really have a good span to use for the syntheized `[]`
	// in `#[attr]`, so just use the span of the `#` token.
	let bracket_group = AttrAnnotatedTokenTree::Delimited(
	DelimSpan::from_single(pound_span),
	DelimToken::Bracket,
	item.tokens
	.as_ref()
	.unwrap_or_else(\|\| panic!("Missing tokens for {:?}", item))
	.create_token_stream(),
	);
	trees.push((bracket_group, Spacing::Alone));
	let tokens = Some(LazyTokenStream::new(AttrAnnotatedTokenStream::new(trees)));
	self.process_cfg_attr(attr::mk_attr_from_item(item, tokens, attr.style, span))
	})
	.collect()
	}

	fn parse_cfg_attr(&self, attr: &Attribute) -> Option<(MetaItem, Vec<(AttrItem, Span)>)> {
	match attr.get_normal_item().args {
	ast::MacArgs::Delimited(dspan, delim, ref tts) if !tts.is_empty() => {
	let msg = "wrong `cfg_attr` delimiters";
	validate_attr::check_meta_bad_delim(&self.sess.parse_sess, dspan, delim, msg);
	match parse_in(&self.sess.parse_sess, tts.clone(), "`cfg_attr` input", \|p\| {
	p.parse_cfg_attr()
	}) {
	Ok(r) => return Some(r),
	Err(mut e) => {
	e.help(&format!("the valid syntax is `{}`", CFG_ATTR_GRAMMAR_HELP))
	.note(CFG_ATTR_NOTE_REF)
	.emit();
	}
	}
	}
	_ => self.error_malformed_cfg_attr_missing(attr.span),
	}
	None
	}

	fn error_malformed_cfg_attr_missing(&self, span: Span) {
	self.sess
	.parse_sess
	.span_diagnostic
	.struct_span_err(span, "malformed `cfg_attr` attribute input")
	.span_suggestion(
	span,
	"missing condition and attribute",
	CFG_ATTR_GRAMMAR_HELP.to_string(),
	Applicability::HasPlaceholders,
	)
	.note(CFG_ATTR_NOTE_REF)
	.emit();
	}

	/// Determines if a node with the given attributes should be included in this configuration.
	fn in_cfg(&self, attrs: &[Attribute]) -> bool {
	attrs.iter().all(\|attr\| {
	if !is_cfg(self.sess, attr) {
	return true;
	}
	let meta_item = match validate_attr::parse_meta(&self.sess.parse_sess, attr) {
	Ok(meta_item) => meta_item,
	Err(mut err) => {
	err.emit();
	return true;
	}
	};
	let error = \|span, msg, suggestion: &str\| {
	let mut err = self.sess.parse_sess.span_diagnostic.struct_span_err(span, msg);
	if !suggestion.is_empty() {
	err.span_suggestion(
	span,
	"expected syntax is",
	suggestion.into(),
	Applicability::MaybeIncorrect,
	);
	}
	err.emit();
	true
	};
	let span = meta_item.span;
	match meta_item.meta_item_list() {
	None => error(span, "`cfg` is not followed by parentheses", "cfg(/* predicate */)"),
	Some([]) => error(span, "`cfg` predicate is not specified", ""),
	Some([_, .., l]) => error(l.span(), "multiple `cfg` predicates are specified", ""),
	Some([single]) => match single.meta_item() {
	Some(meta_item) => {
	attr::cfg_matches(meta_item, &self.sess.parse_sess, self.features)
	}
	None => error(single.span(), "`cfg` predicate key cannot be a literal", ""),
	},
	}
	})
	}

	/// If attributes are not allowed on expressions, emit an error for `attr`
	crate fn maybe_emit_expr_attr_err(&self, attr: &Attribute) {
	if !self.features.map_or(true, \|features\| features.stmt_expr_attributes) {
	let mut err = feature_err(
	&self.sess.parse_sess,
	sym::stmt_expr_attributes,
	attr.span,
	"attributes on expressions are experimental",
	);

	if attr.is_doc_comment() {
	err.help("`///` is for documentation comments. For a plain comment, use `//`.");
	}

	err.emit();
	}
	}

	pub fn configure_expr(&mut self, expr: &mut P<ast::Expr>) {
	for attr in expr.attrs.iter() {
	self.maybe_emit_expr_attr_err(attr);
	}

	// If an expr is valid to cfg away it will have been removed by the
	// outer stmt or expression folder before descending in here.
	// Anything else is always required, and thus has to error out
	// in case of a cfg attr.
	//
	// N.B., this is intentionally not part of the visit_expr() function
	// in order for filter_map_expr() to be able to avoid this check
	if let Some(attr) = expr.attrs().iter().find(\|a\| is_cfg(self.sess, a)) {
	let msg = "removing an expression is not supported in this position";
	self.sess.parse_sess.span_diagnostic.span_err(attr.span, msg);
	}

	self.process_cfg_attrs(expr);
	self.try_configure_tokens(&mut *expr);
	}
	}

	fn is_cfg(sess: &Session, attr: &Attribute) -> bool {
	sess.check_name(attr, sym::cfg)
	}