Auto merge of #109500 - petrochenkov:modchainld, r=oli-obk

resolve: Preserve reexport chains in `ModChild`ren

This may be potentially useful for
- avoiding uses of `hir::ItemKind::Use` (which usually lead to correctness issues)
- preserving documentation comments on all reexports, including those from other crates
- preserving and checking stability/deprecation info on reexports
- all kinds of diagnostics

The second commit then migrates some hacky logic from rustdoc to `module_reexports` to make it simpler and more correct.
Ideally rustdoc should use `module_reexports` immediately at the top level, so `hir::ItemKind::Use`s are never used.
The second commit also fixes issues with https://github.com/rust-lang/rust/pull/109330 and therefore
Fixes https://github.com/rust-lang/rust/issues/109631
Fixes https://github.com/rust-lang/rust/issues/109614
Fixes https://github.com/rust-lang/rust/issues/109424

compiler/rustc_middle/src/arena.rs

@@ -119,6 +119,7 @@ macro_rules! arena_types {
             [] external_constraints: rustc_middle::traits::solve::ExternalConstraintsData<'tcx>,
             [decode] doc_link_resolutions: rustc_hir::def::DocLinkResMap,
             [] closure_kind_origin: (rustc_span::Span, rustc_middle::hir::place::Place<'tcx>),
+            [] mod_child: rustc_middle::metadata::ModChild,
         ]);
     )
 }

compiler/rustc_middle/src/metadata.rs

@@ -5,13 +5,34 @@ use rustc_macros::HashStable;
 use rustc_span::def_id::DefId;
 use rustc_span::symbol::Ident;
 use rustc_span::Span;
+use smallvec::SmallVec;
+
+/// A simplified version of `ImportKind` from resolve.
+/// `DefId`s here correspond to `use` and `extern crate` items themselves, not their targets.
+#[derive(Clone, Copy, Debug, TyEncodable, TyDecodable, HashStable)]
+pub enum Reexport {
+    Single(DefId),
+    Glob(DefId),
+    ExternCrate(DefId),
+    MacroUse,
+    MacroExport,
+}
+
+impl Reexport {
+    pub fn id(self) -> Option<DefId> {
+        match self {
+            Reexport::Single(id) | Reexport::Glob(id) | Reexport::ExternCrate(id) => Some(id),
+            Reexport::MacroUse | Reexport::MacroExport => None,
+        }
+    }
+}
 
 /// This structure is supposed to keep enough data to re-create `NameBinding`s for other crates
 /// during name resolution. Right now the bindings are not recreated entirely precisely so we may
 /// need to add more data in the future to correctly support macros 2.0, for example.
 /// Module child can be either a proper item or a reexport (including private imports).
 /// In case of reexport all the fields describe the reexport item itself, not what it refers to.
-#[derive(Copy, Clone, Debug, TyEncodable, TyDecodable, HashStable)]
+#[derive(Debug, TyEncodable, TyDecodable, HashStable)]
 pub struct ModChild {
     /// Name of the item.
     pub ident: Ident,
@@ -24,4 +45,7 @@ pub struct ModChild {
     pub span: Span,
     /// A proper `macro_rules` item (not a reexport).
     pub macro_rules: bool,
+    /// Reexport chain linking this module child to its original reexported item.
+    /// Empty if the module child is a proper item.
+    pub reexport_chain: SmallVec<[Reexport; 2]>,
 }

compiler/rustc_middle/src/query/erase.rs

@@ -235,7 +235,6 @@ trivial! {
     rustc_hir::OwnerId,
     rustc_hir::Upvar,
     rustc_index::bit_set::FiniteBitSet<u32>,
-    rustc_middle::metadata::ModChild,
     rustc_middle::middle::dependency_format::Linkage,
     rustc_middle::middle::exported_symbols::SymbolExportInfo,
     rustc_middle::middle::resolve_bound_vars::ObjectLifetimeDefault,

compiler/rustc_middle/src/query/mod.rs

@@ -1510,7 +1510,7 @@ rustc_queries! {
         desc { "getting traits in scope at a block" }
     }
 
-    query module_reexports(def_id: LocalDefId) -> Option<&'tcx [ModChild]> {
+    query module_reexports(def_id: LocalDefId) -> &'tcx [ModChild] {
         desc { |tcx| "looking up reexports of module `{}`", tcx.def_path_str(def_id.to_def_id()) }
     }
 

compiler/rustc_middle/src/ty/context.rs

@@ -2502,7 +2502,7 @@ pub struct DeducedParamAttrs {
 
 pub fn provide(providers: &mut ty::query::Providers) {
     providers.module_reexports =
-        |tcx, id| tcx.resolutions(()).reexport_map.get(&id).map(|v| &v[..]);
+        |tcx, id| tcx.resolutions(()).reexport_map.get(&id).map_or(&[], |v| &v[..]);
     providers.maybe_unused_trait_imports =
         |tcx, ()| &tcx.resolutions(()).maybe_unused_trait_imports;
     providers.names_imported_by_glob_use = |tcx, id| {