Auto merge of #95899 - petrochenkov:modchild2, r=cjgillot

rustc_metadata: Do not encode unnecessary module children

This should remove the syntax context shift and the special case for `ExternCrate` in decoder in https://github.com/rust-lang/rust/pull/95880.

This PR also shifts some work from decoding to encoding, which is typically useful for performance (but probably not much in this case).
r? `@cjgillot`

4 files changed, 96 insertions, 71 deletions

diff --git a/compiler/rustc_data_structures/src/lib.rs b/compiler/rustc_data_structures/src/lib.rs
index 98d4870e645..30a3ddc6003 100644
--- a/compiler/rustc_data_structures/src/lib.rs
+++ b/compiler/rustc_data_structures/src/lib.rs
@@ -14,6 +14,8 @@
 #![feature(control_flow_enum)]
 #![feature(core_intrinsics)]
 #![feature(extend_one)]
+#![feature(generator_trait)]
+#![feature(generators)]
 #![feature(let_else)]
 #![feature(hash_raw_entry)]
 #![feature(maybe_uninit_uninit_array)]
@@ -112,6 +114,9 @@ pub mod unhash;
 pub use ena::undo_log;
 pub use ena::unify;
 
+use std::ops::{Generator, GeneratorState};
+use std::pin::Pin;
+
 pub struct OnDrop<F: Fn()>(pub F);
 
 impl<F: Fn()> OnDrop<F> {
@@ -130,6 +135,26 @@ impl<F: Fn()> Drop for OnDrop<F> {
     }
 }
 
+struct IterFromGenerator<G>(G);
+
+impl<G: Generator<Return = ()> + Unpin> Iterator for IterFromGenerator<G> {
+    type Item = G::Yield;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        match Pin::new(&mut self.0).resume(()) {
+            GeneratorState::Yielded(n) => Some(n),
+            GeneratorState::Complete(_) => None,
+        }
+    }
+}
+
+/// An adapter for turning a generator closure into an iterator, similar to `iter::from_fn`.
+pub fn iter_from_generator<G: Generator<Return = ()> + Unpin>(
+    generator: G,
+) -> impl Iterator<Item = G::Yield> {
+    IterFromGenerator(generator)
+}
+
 // See comments in src/librustc_middle/lib.rs
 #[doc(hidden)]
 pub fn __noop_fix_for_27438() {}
diff --git a/compiler/rustc_metadata/src/lib.rs b/compiler/rustc_metadata/src/lib.rs
index aebd293f6c2..dfed9dd15a7 100644
--- a/compiler/rustc_metadata/src/lib.rs
+++ b/compiler/rustc_metadata/src/lib.rs
@@ -1,6 +1,7 @@
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![feature(crate_visibility_modifier)]
 #![feature(drain_filter)]
+#![feature(generators)]
 #![feature(let_else)]
 #![feature(nll)]
 #![feature(once_cell)]
diff --git a/compiler/rustc_metadata/src/rmeta/decoder.rs b/compiler/rustc_metadata/src/rmeta/decoder.rs
index 8d0e8467404..611694a0d30 100644
--- a/compiler/rustc_metadata/src/rmeta/decoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/decoder.rs
@@ -1089,63 +1089,32 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
         // Iterate over all children.
         if let Some(children) = self.root.tables.children.get(self, id) {
             for child_index in children.decode((self, sess)) {
-                if let Some(ident) = self.opt_item_ident(child_index, sess) {
-                    let kind = self.def_kind(child_index);
-                    let def_id = self.local_def_id(child_index);
-                    let res = Res::Def(kind, def_id);
-                    let vis = self.get_visibility(child_index);
-                    let span = self.get_span(child_index, sess);
-                    let macro_rules = match kind {
-                        DefKind::Macro(..) => match self.kind(child_index) {
-                            EntryKind::MacroDef(_, macro_rules) => macro_rules,
-                            _ => unreachable!(),
-                        },
-                        _ => false,
-                    };
-
-                    callback(ModChild { ident, res, vis, span, macro_rules });
-
-                    // For non-re-export structs and variants add their constructors to children.
-                    // Re-export lists automatically contain constructors when necessary.
-                    match kind {
-                        DefKind::Struct => {
-                            if let Some((ctor_def_id, ctor_kind)) =
-                                self.get_ctor_def_id_and_kind(child_index)
-                            {
-                                let ctor_res =
-                                    Res::Def(DefKind::Ctor(CtorOf::Struct, ctor_kind), ctor_def_id);
-                                let vis = self.get_visibility(ctor_def_id.index);
-                                callback(ModChild {
-                                    ident,
-                                    res: ctor_res,
-                                    vis,
-                                    span,
-                                    macro_rules: false,
-                                });
-                            }
-                        }
-                        DefKind::Variant => {
-                            // Braced variants, unlike structs, generate unusable names in
-                            // value namespace, they are reserved for possible future use.
-                            // It's ok to use the variant's id as a ctor id since an
-                            // error will be reported on any use of such resolution anyway.
-                            let (ctor_def_id, ctor_kind) = self
-                                .get_ctor_def_id_and_kind(child_index)
-                                .unwrap_or((def_id, CtorKind::Fictive));
+                let ident = self.item_ident(child_index, sess);
+                let kind = self.def_kind(child_index);
+                let def_id = self.local_def_id(child_index);
+                let res = Res::Def(kind, def_id);
+                let vis = self.get_visibility(child_index);
+                let span = self.get_span(child_index, sess);
+                let macro_rules = match kind {
+                    DefKind::Macro(..) => match self.kind(child_index) {
+                        EntryKind::MacroDef(_, macro_rules) => macro_rules,
+                        _ => unreachable!(),
+                    },
+                    _ => false,
+                };
+
+                callback(ModChild { ident, res, vis, span, macro_rules });
+
+                // For non-re-export structs and variants add their constructors to children.
+                // Re-export lists automatically contain constructors when necessary.
+                match kind {
+                    DefKind::Struct => {
+                        if let Some((ctor_def_id, ctor_kind)) =
+                            self.get_ctor_def_id_and_kind(child_index)
+                        {
                             let ctor_res =
-                                Res::Def(DefKind::Ctor(CtorOf::Variant, ctor_kind), ctor_def_id);
-                            let mut vis = self.get_visibility(ctor_def_id.index);
-                            if ctor_def_id == def_id && vis.is_public() {
-                                // For non-exhaustive variants lower the constructor visibility to
-                                // within the crate. We only need this for fictive constructors,
-                                // for other constructors correct visibilities
-                                // were already encoded in metadata.
-                                let mut attrs = self.get_item_attrs(def_id.index, sess);
-                                if attrs.any(|item| item.has_name(sym::non_exhaustive)) {
-                                    let crate_def_id = self.local_def_id(CRATE_DEF_INDEX);
-                                    vis = ty::Visibility::Restricted(crate_def_id);
-                                }
-                            }
+                                Res::Def(DefKind::Ctor(CtorOf::Struct, ctor_kind), ctor_def_id);
+                            let vis = self.get_visibility(ctor_def_id.index);
                             callback(ModChild {
                                 ident,
                                 res: ctor_res,
@@ -1154,8 +1123,32 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
                                 macro_rules: false,
                             });
                         }
-                        _ => {}
                     }
+                    DefKind::Variant => {
+                        // Braced variants, unlike structs, generate unusable names in
+                        // value namespace, they are reserved for possible future use.
+                        // It's ok to use the variant's id as a ctor id since an
+                        // error will be reported on any use of such resolution anyway.
+                        let (ctor_def_id, ctor_kind) = self
+                            .get_ctor_def_id_and_kind(child_index)
+                            .unwrap_or((def_id, CtorKind::Fictive));
+                        let ctor_res =
+                            Res::Def(DefKind::Ctor(CtorOf::Variant, ctor_kind), ctor_def_id);
+                        let mut vis = self.get_visibility(ctor_def_id.index);
+                        if ctor_def_id == def_id && vis.is_public() {
+                            // For non-exhaustive variants lower the constructor visibility to
+                            // within the crate. We only need this for fictive constructors,
+                            // for other constructors correct visibilities
+                            // were already encoded in metadata.
+                            let mut attrs = self.get_item_attrs(def_id.index, sess);
+                            if attrs.any(|item| item.has_name(sym::non_exhaustive)) {
+                                let crate_def_id = self.local_def_id(CRATE_DEF_INDEX);
+                                vis = ty::Visibility::Restricted(crate_def_id);
+                            }
+                        }
+                        callback(ModChild { ident, res: ctor_res, vis, span, macro_rules: false });
+                    }
+                    _ => {}
                 }
             }
         }
diff --git a/compiler/rustc_metadata/src/rmeta/encoder.rs b/compiler/rustc_metadata/src/rmeta/encoder.rs
index e967750aebb..732e7ef8a23 100644
--- a/compiler/rustc_metadata/src/rmeta/encoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/encoder.rs
@@ -4,6 +4,7 @@ use crate::rmeta::*;
 
 use rustc_data_structures::fingerprint::Fingerprint;
 use rustc_data_structures::fx::{FxHashMap, FxIndexSet};
+use rustc_data_structures::iter_from_generator;
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_data_structures::sync::{join, par_iter, Lrc, ParallelIterator};
 use rustc_hir as hir;
@@ -1107,21 +1108,26 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
             // Encode this here because we don't do it in encode_def_ids.
             record!(self.tables.expn_that_defined[def_id] <- tcx.expn_that_defined(local_def_id));
         } else {
-            let direct_children = md.item_ids.iter().map(|item_id| item_id.def_id.local_def_index);
-            // Foreign items are planted into their parent modules from name resolution point of view.
-            let tcx = self.tcx;
-            let foreign_item_children = md
-                .item_ids
-                .iter()
-                .filter_map(|item_id| match tcx.hir().item(*item_id).kind {
-                    hir::ItemKind::ForeignMod { items, .. } => {
-                        Some(items.iter().map(|fi_ref| fi_ref.id.def_id.local_def_index))
+            record!(self.tables.children[def_id] <- iter_from_generator(|| {
+                for item_id in md.item_ids {
+                    match tcx.hir().item(*item_id).kind {
+                        // Foreign items are planted into their parent modules
+                        // from name resolution point of view.
+                        hir::ItemKind::ForeignMod { items, .. } => {
+                            for foreign_item in items {
+                                yield foreign_item.id.def_id.local_def_index;
+                            }
+                        }
+                        // Only encode named non-reexport children, reexports are encoded
+                        // separately and unnamed items are not used by name resolution.
+                        hir::ItemKind::ExternCrate(..) => continue,
+                        _ if tcx.def_key(item_id.def_id.to_def_id()).get_opt_name().is_some() => {
+                            yield item_id.def_id.local_def_index;
+                        }
+                        _ => continue,
                     }
-                    _ => None,
-                })
-                .flatten();
-
-            record!(self.tables.children[def_id] <- direct_children.chain(foreign_item_children));
+                }
+            }));
         }
     }