Remember names of cfg-ed out items to mention them in diagnostics

`#[cfg]`s are frequently used to gate crate content behind cargo
features. This can lead to very confusing errors when features are
missing. For example, `serde` doesn't have the `derive` feature by
default. Therefore, `serde::Serialize` fails to resolve with a generic
error, even though the macro is present in the docs.

This commit adds a list of all stripped item names to metadata. This is
filled during macro expansion and then, through a fed query, persisted
in metadata. The downstream resolver can then access the metadata to
look at possible candidates for mentioning in the errors.

This slightly increases metadata (800k->809k for the feature-heavy
windows crate), but not enough to really matter.

Author: Noratrieb

compiler/rustc_ast/src/expand/mod.rs

@@ -1,3 +1,20 @@
 //! Definitions shared by macros / syntax extensions and e.g. `rustc_middle`.
 
+use rustc_span::{def_id::DefId, symbol::Ident};
+
+use crate::MetaItem;
+
 pub mod allocator;
+
+#[derive(Debug, Clone, Encodable, Decodable, HashStable_Generic)]
+pub struct StrippedCfgItem<ModId = DefId> {
+    pub parent_module: ModId,
+    pub name: Ident,
+    pub cfg: MetaItem,
+}
+
+impl<ModId> StrippedCfgItem<ModId> {
+    pub fn map_mod_id<New>(self, f: impl FnOnce(ModId) -> New) -> StrippedCfgItem<New> {
+        StrippedCfgItem { parent_module: f(self.parent_module), name: self.name, cfg: self.cfg }
+    }
+}

compiler/rustc_expand/src/base.rs

@@ -947,6 +947,8 @@ pub trait ResolverExpand {
     /// HIR proc macros items back to their harness items.
     fn declare_proc_macro(&mut self, id: NodeId);
 
+    fn append_stripped_cfg_item(&mut self, parent_node: NodeId, name: Ident, cfg: ast::MetaItem);
+
     /// Tools registered with `#![register_tool]` and used by tool attributes and lints.
     fn registered_tools(&self) -> &RegisteredTools;
 }
@@ -965,7 +967,7 @@ pub trait LintStoreExpand {
 
 type LintStoreExpandDyn<'a> = Option<&'a (dyn LintStoreExpand + 'a)>;
 
-#[derive(Clone, Default)]
+#[derive(Debug, Clone, Default)]
 pub struct ModuleData {
     /// Path to the module starting from the crate name, like `my_crate::foo::bar`.
     pub mod_path: Vec<Ident>,

compiler/rustc_expand/src/config.rs

@@ -416,20 +416,28 @@ impl<'a> StripUnconfigured<'a> {
 
     /// 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| !is_cfg(attr) || self.cfg_true(attr))
+        attrs.iter().all(|attr| !is_cfg(attr) || self.cfg_true(attr).0)
     }
 
-    pub(crate) fn cfg_true(&self, attr: &Attribute) -> bool {
+    pub(crate) fn cfg_true(&self, attr: &Attribute) -> (bool, Option<MetaItem>) {
         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;
+                return (true, None);
             }
         };
-        parse_cfg(&meta_item, &self.sess).map_or(true, |meta_item| {
-            attr::cfg_matches(&meta_item, &self.sess.parse_sess, self.lint_node_id, self.features)
-        })
+        (
+            parse_cfg(&meta_item, &self.sess).map_or(true, |meta_item| {
+                attr::cfg_matches(
+                    &meta_item,
+                    &self.sess.parse_sess,
+                    self.lint_node_id,
+                    self.features,
+                )
+            }),
+            Some(meta_item),
+        )
     }
 
     /// If attributes are not allowed on expressions, emit an error for `attr`

compiler/rustc_expand/src/expand.rs

@@ -1042,6 +1042,12 @@ trait InvocationCollectorNode: HasAttrs + HasNodeId + Sized {
     fn expand_cfg_false(&mut self, collector: &mut InvocationCollector<'_, '_>, span: Span) {
         collector.cx.emit_err(RemoveNodeNotSupported { span, descr: Self::descr() });
     }
+
+    /// All of the names (items) declared by this node.
+    /// This is an approximation and should only be used for diagnostics.
+    fn declared_names(&self) -> Vec<Ident> {
+        vec![]
+    }
 }
 
 impl InvocationCollectorNode for P<ast::Item> {
@@ -1148,6 +1154,27 @@ impl InvocationCollectorNode for P<ast::Item> {
         collector.cx.current_expansion.module = orig_module;
         res
     }
+    fn declared_names(&self) -> Vec<Ident> {
+        if let ItemKind::Use(ut) = &self.kind {
+            fn collect_use_tree_leaves(ut: &ast::UseTree, idents: &mut Vec<Ident>) {
+                match &ut.kind {
+                    ast::UseTreeKind::Glob => {}
+                    ast::UseTreeKind::Simple(_) => idents.push(ut.ident()),
+                    ast::UseTreeKind::Nested(nested) => {
+                        for (ut, _) in nested {
+                            collect_use_tree_leaves(&ut, idents);
+                        }
+                    }
+                }
+            }
+
+            let mut idents = Vec::new();
+            collect_use_tree_leaves(&ut, &mut idents);
+            return idents;
+        }
+
+        vec![self.ident]
+    }
 }
 
 struct TraitItemTag;
@@ -1685,16 +1712,17 @@ impl<'a, 'b> InvocationCollector<'a, 'b> {
         node: &mut impl HasAttrs,
         attr: ast::Attribute,
         pos: usize,
-    ) -> bool {
-        let res = self.cfg().cfg_true(&attr);
+    ) -> (bool, Option<ast::MetaItem>) {
+        let (res, meta_item) = self.cfg().cfg_true(&attr);
         if res {
             // FIXME: `cfg(TRUE)` attributes do not currently remove themselves during expansion,
             // and some tools like rustdoc and clippy rely on that. Find a way to remove them
             // while keeping the tools working.
             self.cx.expanded_inert_attrs.mark(&attr);
             node.visit_attrs(|attrs| attrs.insert(pos, attr));
         }
-        res
+
+        (res, meta_item)
     }
 
     fn expand_cfg_attr(&self, node: &mut impl HasAttrs, attr: &ast::Attribute, pos: usize) {
@@ -1715,9 +1743,20 @@ impl<'a, 'b> InvocationCollector<'a, 'b> {
             return match self.take_first_attr(&mut node) {
                 Some((attr, pos, derives)) => match attr.name_or_empty() {
                     sym::cfg => {
-                        if self.expand_cfg_true(&mut node, attr, pos) {
+                        let (res, meta_item) = self.expand_cfg_true(&mut node, attr, pos);
+                        if res {
                             continue;
                         }
+
+                        if let Some(meta_item) = meta_item {
+                            for name in node.declared_names() {
+                                self.cx.resolver.append_stripped_cfg_item(
+                                    self.cx.current_expansion.lint_node_id,
+                                    name,
+                                    meta_item.clone(),
+                                )
+                            }
+                        }
                         Default::default()
                     }
                     sym::cfg_attr => {
@@ -1761,7 +1800,7 @@ impl<'a, 'b> InvocationCollector<'a, 'b> {
                 Some((attr, pos, derives)) => match attr.name_or_empty() {
                     sym::cfg => {
                         let span = attr.span;
-                        if self.expand_cfg_true(node, attr, pos) {
+                        if self.expand_cfg_true(node, attr, pos).0 {
                             continue;
                         }
 

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -995,6 +995,15 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
         )
     }
 
+    fn get_stripped_cfg_items(self, cnum: CrateNum, tcx: TyCtxt<'tcx>) -> &'tcx [StrippedCfgItem] {
+        let item_names = self
+            .root
+            .stripped_cfg_items
+            .decode((self, tcx))
+            .map(|item| item.map_mod_id(|index| DefId { krate: cnum, index }));
+        tcx.arena.alloc_from_iter(item_names)
+    }
+
     /// Iterates over the diagnostic items in the given crate.
     fn get_diagnostic_items(self) -> DiagnosticItems {
         let mut id_to_name = FxHashMap::default();

compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs

@@ -345,6 +345,7 @@ provide! { tcx, def_id, other, cdata,
     stability_implications => {
         cdata.get_stability_implications(tcx).iter().copied().collect()
     }
+    stripped_cfg_items => { cdata.get_stripped_cfg_items(cdata.cnum, tcx) }
     is_intrinsic => { cdata.get_is_intrinsic(def_id.index) }
     defined_lang_items => { cdata.get_lang_items(tcx) }
     diagnostic_items => { cdata.get_diagnostic_items() }

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -3,6 +3,7 @@ use crate::rmeta::def_path_hash_map::DefPathHashMapRef;
 use crate::rmeta::table::TableBuilder;
 use crate::rmeta::*;
 
+use rustc_ast::expand::StrippedCfgItem;
 use rustc_ast::Attribute;
 use rustc_data_structures::fingerprint::Fingerprint;
 use rustc_data_structures::fx::{FxHashMap, FxIndexSet};
@@ -584,6 +585,8 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
             (self.encode_lang_items(), self.encode_lang_items_missing())
         });
 
+        let stripped_cfg_items = stat!("stripped-cfg-items", || self.encode_stripped_cfg_items());
+
         let diagnostic_items = stat!("diagnostic-items", || self.encode_diagnostic_items());
 
         let native_libraries = stat!("native-libs", || self.encode_native_libraries());
@@ -694,6 +697,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
                 lang_items,
                 diagnostic_items,
                 lang_items_missing,
+                stripped_cfg_items,
                 native_libraries,
                 foreign_modules,
                 source_map,
@@ -1940,6 +1944,15 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
         self.lazy_array(&tcx.lang_items().missing)
     }
 
+    fn encode_stripped_cfg_items(&mut self) -> LazyArray<StrippedCfgItem<DefIndex>> {
+        self.lazy_array(
+            self.tcx
+                .stripped_cfg_items(LOCAL_CRATE)
+                .into_iter()
+                .map(|item| item.clone().map_mod_id(|def_id| def_id.index)),
+        )
+    }
+
     fn encode_traits(&mut self) -> LazyArray<DefIndex> {
         empty_proc_macro!(self);
         self.lazy_array(self.tcx.traits(LOCAL_CRATE).iter().map(|def_id| def_id.index))

compiler/rustc_metadata/src/rmeta/mod.rs

@@ -6,6 +6,7 @@ use rustc_middle::middle::debugger_visualizer::DebuggerVisualizerFile;
 use table::TableBuilder;
 
 use rustc_ast as ast;
+use rustc_ast::expand::StrippedCfgItem;
 use rustc_attr as attr;
 use rustc_data_structures::svh::Svh;
 use rustc_hir as hir;
@@ -256,6 +257,7 @@ pub(crate) struct CrateRoot {
     stability_implications: LazyArray<(Symbol, Symbol)>,
     lang_items: LazyArray<(DefIndex, LangItem)>,
     lang_items_missing: LazyArray<LangItem>,
+    stripped_cfg_items: LazyArray<StrippedCfgItem<DefIndex>>,
     diagnostic_items: LazyArray<(Symbol, DefIndex)>,
     native_libraries: LazyArray<NativeLib>,
     foreign_modules: LazyArray<ForeignModule>,

compiler/rustc_middle/src/arena.rs

@@ -124,6 +124,7 @@ macro_rules! arena_types {
             [] predefined_opaques_in_body: rustc_middle::traits::solve::PredefinedOpaquesData<'tcx>,
             [decode] doc_link_resolutions: rustc_hir::def::DocLinkResMap,
             [] closure_kind_origin: (rustc_span::Span, rustc_middle::hir::place::Place<'tcx>),
+            [] stripped_cfg_items: rustc_ast::expand::StrippedCfgItem,
             [] mod_child: rustc_middle::metadata::ModChild,
         ]);
     )

compiler/rustc_middle/src/query/mod.rs

@@ -54,7 +54,7 @@ use crate::ty::{
 };
 use rustc_arena::TypedArena;
 use rustc_ast as ast;
-use rustc_ast::expand::allocator::AllocatorKind;
+use rustc_ast::expand::{allocator::AllocatorKind, StrippedCfgItem};
 use rustc_attr as attr;
 use rustc_data_structures::fingerprint::Fingerprint;
 use rustc_data_structures::fx::{FxHashMap, FxIndexMap, FxIndexSet};
@@ -2173,6 +2173,15 @@ rustc_queries! {
     query check_tys_might_be_eq(arg: Canonical<'tcx, (ty::ParamEnv<'tcx>, Ty<'tcx>, Ty<'tcx>)>) -> Result<(), NoSolution> {
         desc { "check whether two const param are definitely not equal to eachother"}
     }
+
+    /// Get all item paths that were stripped by a `#[cfg]` in a particular crate.
+    /// Should not be called for the local crate before the resolver outputs are created, as it
+    /// is only fed there.
+    query stripped_cfg_items(cnum: CrateNum) -> &'tcx [StrippedCfgItem] {
+        feedable
+        desc { "getting cfg-ed out item names" }
+        separate_provide_extern
+    }
 }
 
 rustc_query_append! { define_callbacks! }

compiler/rustc_middle/src/ty/parameterized.rs

@@ -73,6 +73,7 @@ trivially_parameterized_over_tcx! {
     ty::fast_reject::SimplifiedType,
     rustc_ast::Attribute,
     rustc_ast::DelimArgs,
+    rustc_ast::expand::StrippedCfgItem<rustc_hir::def_id::DefIndex>,
     rustc_attr::ConstStability,
     rustc_attr::DefaultBodyStability,
     rustc_attr::Deprecation,