src/librustc_expand/config.rs - third_party/rust - Git at Google

 //! Conditional compilation stripping.

 use rustc_ast::ast::{self, AttrItem, Attribute, MetaItem};
 use rustc_ast::attr::HasAttrs;
 use rustc_ast::mut_visit::*;
 use rustc_ast::ptr::P;
 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, ParseSess};
 use rustc_span::edition::{Edition, ALL_EDITIONS};
 use rustc_span::symbol::{sym, Symbol};
 use rustc_span::{Span, DUMMY_SP};

 use smallvec::SmallVec;

 /// A folder that strips out items that do not belong in the current configuration.
 pub struct StripUnconfigured<'a> {
     pub sess: &'a ParseSess,
     pub features: Option<&'a Features>,
 }

 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");
         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();

     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 !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();

             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 !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) {
                 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) = 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(
     mut krate: ast::Crate,
     sess: &ParseSess,
     edition: Edition,
     allow_features: &Option<Vec<String>>,
 ) -> (ast::Crate, Features) {
     let mut strip_unconfigured = StripUnconfigured { sess, features: None };

     let unconfigured_attrs = krate.attrs.clone();
     let diag = &sess.span_diagnostic;
     let err_count = diag.err_count();
     let features = match strip_unconfigured.configure(krate.attrs) {
         None => {
             // The entire crate is unconfigured.
             krate.attrs = Vec::new();
             krate.module.items = Vec::new();
             Features::default()
         }
         Some(attrs) => {
             krate.attrs = attrs;
             let features = get_features(diag, &krate.attrs, edition, allow_features);
             if err_count == diag.err_count() {
                 // Avoid reconfiguring malformed `cfg_attr`s.
                 strip_unconfigured.features = Some(&features);
                 strip_unconfigured.configure(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: HasAttrs>(&mut self, mut node: T) -> Option<T> {
         self.process_cfg_attrs(&mut node);
         self.in_cfg(node.attrs()).then_some(node)
     }

     /// 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.
     pub fn process_cfg_attrs<T: HasAttrs>(&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.
         attr::mark_used(&attr);

         if !attr::cfg_matches(&cfg_predicate, self.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 attr = attr::mk_attr_from_item(attr.style, item, span);
                 self.process_cfg_attr(attr)
             })
             .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, dspan, delim, msg);
                 match parse_in(self.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
             .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.
     pub fn in_cfg(&self, attrs: &[Attribute]) -> bool {
         attrs.iter().all(|attr| {
             if !is_cfg(attr) {
                 return true;
             }
             let meta_item = match validate_attr::parse_meta(self.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.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, self.features),
                     None => error(single.span(), "`cfg` predicate key cannot be a literal", ""),
                 },
             }
         })
     }

     /// Visit attributes on expression and statements (but not attributes on items in blocks).
     fn visit_expr_attrs(&mut self, attrs: &[Attribute]) {
         // flag the offending attributes
         for attr in attrs.iter() {
             self.maybe_emit_expr_attr_err(attr);
         }
     }

     /// If attributes are not allowed on expressions, emit an error for `attr`
     pub fn maybe_emit_expr_attr_err(&self, attr: &Attribute) {
         if !self.features.map(|features| features.stmt_expr_attributes).unwrap_or(true) {
             let mut err = feature_err(
                 self.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_foreign_mod(&mut self, foreign_mod: &mut ast::ForeignMod) {
         let ast::ForeignMod { abi: _, items } = foreign_mod;
         items.flat_map_in_place(|item| self.configure(item));
     }

     pub fn configure_generic_params(&mut self, params: &mut Vec<ast::GenericParam>) {
         params.flat_map_in_place(|param| self.configure(param));
     }

     fn configure_variant_data(&mut self, vdata: &mut ast::VariantData) {
         match vdata {
             ast::VariantData::Struct(fields, ..) | ast::VariantData::Tuple(fields, _) => {
                 fields.flat_map_in_place(|field| self.configure(field))
             }
             ast::VariantData::Unit(_) => {}
         }
     }

     pub fn configure_item_kind(&mut self, item: &mut ast::ItemKind) {
         match item {
             ast::ItemKind::Struct(def, _generics) | ast::ItemKind::Union(def, _generics) => {
                 self.configure_variant_data(def)
             }
             ast::ItemKind::Enum(ast::EnumDef { variants }, _generics) => {
                 variants.flat_map_in_place(|variant| self.configure(variant));
                 for variant in variants {
                     self.configure_variant_data(&mut variant.data);
                 }
             }
             _ => {}
         }
     }

     pub fn configure_expr_kind(&mut self, expr_kind: &mut ast::ExprKind) {
         match expr_kind {
             ast::ExprKind::Match(_m, arms) => {
                 arms.flat_map_in_place(|arm| self.configure(arm));
             }
             ast::ExprKind::Struct(_path, fields, _base) => {
                 fields.flat_map_in_place(|field| self.configure(field));
             }
             _ => {}
         }
     }

     pub fn configure_expr(&mut self, expr: &mut P<ast::Expr>) {
         self.visit_expr_attrs(expr.attrs());

         // 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(a)) {
             let msg = "removing an expression is not supported in this position";
             self.sess.span_diagnostic.span_err(attr.span, msg);
         }

         self.process_cfg_attrs(expr)
     }

     pub fn configure_pat(&mut self, pat: &mut P<ast::Pat>) {
         if let ast::PatKind::Struct(_path, fields, _etc) = &mut pat.kind {
             fields.flat_map_in_place(|field| self.configure(field));
         }
     }

     pub fn configure_fn_decl(&mut self, fn_decl: &mut ast::FnDecl) {
         fn_decl.inputs.flat_map_in_place(|arg| self.configure(arg));
     }
 }

 impl<'a> MutVisitor for StripUnconfigured<'a> {
     fn visit_foreign_mod(&mut self, foreign_mod: &mut ast::ForeignMod) {
         self.configure_foreign_mod(foreign_mod);
         noop_visit_foreign_mod(foreign_mod, self);
     }

     fn visit_item_kind(&mut self, item: &mut ast::ItemKind) {
         self.configure_item_kind(item);
         noop_visit_item_kind(item, self);
     }

     fn visit_expr(&mut self, expr: &mut P<ast::Expr>) {
         self.configure_expr(expr);
         self.configure_expr_kind(&mut expr.kind);
         noop_visit_expr(expr, self);
     }

     fn filter_map_expr(&mut self, expr: P<ast::Expr>) -> Option<P<ast::Expr>> {
         let mut expr = configure!(self, expr);
         self.configure_expr_kind(&mut expr.kind);
         noop_visit_expr(&mut expr, self);
         Some(expr)
     }

     fn flat_map_stmt(&mut self, stmt: ast::Stmt) -> SmallVec<[ast::Stmt; 1]> {
         noop_flat_map_stmt(configure!(self, stmt), self)
     }

     fn flat_map_item(&mut self, item: P<ast::Item>) -> SmallVec<[P<ast::Item>; 1]> {
         noop_flat_map_item(configure!(self, item), self)
     }

     fn flat_map_impl_item(&mut self, item: P<ast::AssocItem>) -> SmallVec<[P<ast::AssocItem>; 1]> {
         noop_flat_map_assoc_item(configure!(self, item), self)
     }

     fn flat_map_trait_item(&mut self, item: P<ast::AssocItem>) -> SmallVec<[P<ast::AssocItem>; 1]> {
         noop_flat_map_assoc_item(configure!(self, item), self)
     }

     fn visit_mac(&mut self, _mac: &mut ast::MacCall) {
         // Don't configure interpolated AST (cf. issue #34171).
         // Interpolated AST will get configured once the surrounding tokens are parsed.
     }

     fn visit_pat(&mut self, pat: &mut P<ast::Pat>) {
         self.configure_pat(pat);
         noop_visit_pat(pat, self)
     }

     fn visit_fn_decl(&mut self, mut fn_decl: &mut P<ast::FnDecl>) {
         self.configure_fn_decl(&mut fn_decl);
         noop_visit_fn_decl(fn_decl, self);
     }
 }

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

	use rustc_ast::ast::{self, AttrItem, Attribute, MetaItem};
	use rustc_ast::attr::HasAttrs;
	use rustc_ast::mut_visit::*;
	use rustc_ast::ptr::P;
	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, ParseSess};
	use rustc_span::edition::{Edition, ALL_EDITIONS};
	use rustc_span::symbol::{sym, Symbol};
	use rustc_span::{Span, DUMMY_SP};

	use smallvec::SmallVec;

	/// A folder that strips out items that do not belong in the current configuration.
	pub struct StripUnconfigured<'a> {
	pub sess: &'a ParseSess,
	pub features: Option<&'a Features>,
	}

	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");
	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();

	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 !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();

	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 !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) {
	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) = 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(
	mut krate: ast::Crate,
	sess: &ParseSess,
	edition: Edition,
	allow_features: &Option<Vec<String>>,
	) -> (ast::Crate, Features) {
	let mut strip_unconfigured = StripUnconfigured { sess, features: None };

	let unconfigured_attrs = krate.attrs.clone();
	let diag = &sess.span_diagnostic;
	let err_count = diag.err_count();
	let features = match strip_unconfigured.configure(krate.attrs) {
	None => {
	// The entire crate is unconfigured.
	krate.attrs = Vec::new();
	krate.module.items = Vec::new();
	Features::default()
	}
	Some(attrs) => {
	krate.attrs = attrs;
	let features = get_features(diag, &krate.attrs, edition, allow_features);
	if err_count == diag.err_count() {
	// Avoid reconfiguring malformed `cfg_attr`s.
	strip_unconfigured.features = Some(&features);
	strip_unconfigured.configure(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: HasAttrs>(&mut self, mut node: T) -> Option<T> {
	self.process_cfg_attrs(&mut node);
	self.in_cfg(node.attrs()).then_some(node)
	}

	/// 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.
	pub fn process_cfg_attrs<T: HasAttrs>(&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.
	attr::mark_used(&attr);

	if !attr::cfg_matches(&cfg_predicate, self.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 attr = attr::mk_attr_from_item(attr.style, item, span);
	self.process_cfg_attr(attr)
	})
	.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, dspan, delim, msg);
	match parse_in(self.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
	.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.
	pub fn in_cfg(&self, attrs: &[Attribute]) -> bool {
	attrs.iter().all(\|attr\| {
	if !is_cfg(attr) {
	return true;
	}
	let meta_item = match validate_attr::parse_meta(self.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.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, self.features),
	None => error(single.span(), "`cfg` predicate key cannot be a literal", ""),
	},
	}
	})
	}

	/// Visit attributes on expression and statements (but not attributes on items in blocks).
	fn visit_expr_attrs(&mut self, attrs: &[Attribute]) {
	// flag the offending attributes
	for attr in attrs.iter() {
	self.maybe_emit_expr_attr_err(attr);
	}
	}

	/// If attributes are not allowed on expressions, emit an error for `attr`
	pub fn maybe_emit_expr_attr_err(&self, attr: &Attribute) {
	if !self.features.map(\|features\| features.stmt_expr_attributes).unwrap_or(true) {
	let mut err = feature_err(
	self.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_foreign_mod(&mut self, foreign_mod: &mut ast::ForeignMod) {
	let ast::ForeignMod { abi: _, items } = foreign_mod;
	items.flat_map_in_place(\|item\| self.configure(item));
	}

	pub fn configure_generic_params(&mut self, params: &mut Vec<ast::GenericParam>) {
	params.flat_map_in_place(\|param\| self.configure(param));
	}

	fn configure_variant_data(&mut self, vdata: &mut ast::VariantData) {
	match vdata {
	ast::VariantData::Struct(fields, ..) \| ast::VariantData::Tuple(fields, _) => {
	fields.flat_map_in_place(\|field\| self.configure(field))
	}
	ast::VariantData::Unit(_) => {}
	}
	}

	pub fn configure_item_kind(&mut self, item: &mut ast::ItemKind) {
	match item {
	ast::ItemKind::Struct(def, _generics) \| ast::ItemKind::Union(def, _generics) => {
	self.configure_variant_data(def)
	}
	ast::ItemKind::Enum(ast::EnumDef { variants }, _generics) => {
	variants.flat_map_in_place(\|variant\| self.configure(variant));
	for variant in variants {
	self.configure_variant_data(&mut variant.data);
	}
	}
	_ => {}
	}
	}

	pub fn configure_expr_kind(&mut self, expr_kind: &mut ast::ExprKind) {
	match expr_kind {
	ast::ExprKind::Match(_m, arms) => {
	arms.flat_map_in_place(\|arm\| self.configure(arm));
	}
	ast::ExprKind::Struct(_path, fields, _base) => {
	fields.flat_map_in_place(\|field\| self.configure(field));
	}
	_ => {}
	}
	}

	pub fn configure_expr(&mut self, expr: &mut P<ast::Expr>) {
	self.visit_expr_attrs(expr.attrs());

	// 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(a)) {
	let msg = "removing an expression is not supported in this position";
	self.sess.span_diagnostic.span_err(attr.span, msg);
	}

	self.process_cfg_attrs(expr)
	}

	pub fn configure_pat(&mut self, pat: &mut P<ast::Pat>) {
	if let ast::PatKind::Struct(_path, fields, _etc) = &mut pat.kind {
	fields.flat_map_in_place(\|field\| self.configure(field));
	}
	}

	pub fn configure_fn_decl(&mut self, fn_decl: &mut ast::FnDecl) {
	fn_decl.inputs.flat_map_in_place(\|arg\| self.configure(arg));
	}
	}

	impl<'a> MutVisitor for StripUnconfigured<'a> {
	fn visit_foreign_mod(&mut self, foreign_mod: &mut ast::ForeignMod) {
	self.configure_foreign_mod(foreign_mod);
	noop_visit_foreign_mod(foreign_mod, self);
	}

	fn visit_item_kind(&mut self, item: &mut ast::ItemKind) {
	self.configure_item_kind(item);
	noop_visit_item_kind(item, self);
	}

	fn visit_expr(&mut self, expr: &mut P<ast::Expr>) {
	self.configure_expr(expr);
	self.configure_expr_kind(&mut expr.kind);
	noop_visit_expr(expr, self);
	}

	fn filter_map_expr(&mut self, expr: P<ast::Expr>) -> Option<P<ast::Expr>> {
	let mut expr = configure!(self, expr);
	self.configure_expr_kind(&mut expr.kind);
	noop_visit_expr(&mut expr, self);
	Some(expr)
	}

	fn flat_map_stmt(&mut self, stmt: ast::Stmt) -> SmallVec<[ast::Stmt; 1]> {
	noop_flat_map_stmt(configure!(self, stmt), self)
	}

	fn flat_map_item(&mut self, item: P<ast::Item>) -> SmallVec<[P<ast::Item>; 1]> {
	noop_flat_map_item(configure!(self, item), self)
	}

	fn flat_map_impl_item(&mut self, item: P<ast::AssocItem>) -> SmallVec<[P<ast::AssocItem>; 1]> {
	noop_flat_map_assoc_item(configure!(self, item), self)
	}

	fn flat_map_trait_item(&mut self, item: P<ast::AssocItem>) -> SmallVec<[P<ast::AssocItem>; 1]> {
	noop_flat_map_assoc_item(configure!(self, item), self)
	}

	fn visit_mac(&mut self, _mac: &mut ast::MacCall) {
	// Don't configure interpolated AST (cf. issue #34171).
	// Interpolated AST will get configured once the surrounding tokens are parsed.
	}

	fn visit_pat(&mut self, pat: &mut P<ast::Pat>) {
	self.configure_pat(pat);
	noop_visit_pat(pat, self)
	}

	fn visit_fn_decl(&mut self, mut fn_decl: &mut P<ast::FnDecl>) {
	self.configure_fn_decl(&mut fn_decl);
	noop_visit_fn_decl(fn_decl, self);
	}
	}

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