compiler/rustc_passes/src/reachable.rs

   1 // Finds items that are externally reachable, to determine which items
   2 // need to have their metadata (and possibly their AST) serialized.
   3 // All items that can be referred to through an exported name are
   4 // reachable, and when a reachable thing is inline or generic, it
   5 // makes all other generics or inline functions that it references
   6 // reachable as well.
   7
   8 use rustc_data_structures::fx::FxHashSet;
   9 use rustc_hir as hir;
  10 use rustc_hir::def::{DefKind, Res};
  11 use rustc_hir::def_id::LOCAL_CRATE;
  12 use rustc_hir::def_id::{CrateNum, DefId, LocalDefId};
  13 use rustc_hir::intravisit::{self, NestedVisitorMap, Visitor};
  14 use rustc_hir::itemlikevisit::ItemLikeVisitor;
  15 use rustc_hir::Node;
  16 use rustc_middle::middle::codegen_fn_attrs::{CodegenFnAttrFlags, CodegenFnAttrs};
  17 use rustc_middle::middle::privacy;
  18 use rustc_middle::ty::query::Providers;
  19 use rustc_middle::ty::{self, DefIdTree, TyCtxt};
  20 use rustc_session::config::CrateType;
  21 use rustc_target::spec::abi::Abi;
  22
  23 // Returns true if the given item must be inlined because it may be
  24 // monomorphized or it was marked with `#[inline]`. This will only return
  25 // true for functions.
  26 fn item_might_be_inlined(tcx: TyCtxt<'tcx>, item: &hir::Item<'_>, attrs: &CodegenFnAttrs) -> bool {
  27     if attrs.requests_inline() {
  28         return true;
  29     }
  30
  31     match item.kind {
  32         hir::ItemKind::Fn(ref sig, ..) if sig.header.is_const() => true,
  33         hir::ItemKind::Impl { .. } | hir::ItemKind::Fn(..) => {
  34             let generics = tcx.generics_of(tcx.hir().local_def_id(item.hir_id));
  35             generics.requires_monomorphization(tcx)
  36         }
  37         _ => false,
  38     }
  39 }
  40
  41 fn method_might_be_inlined(
  42     tcx: TyCtxt<'_>,
  43     impl_item: &hir::ImplItem<'_>,
  44     impl_src: LocalDefId,
  45 ) -> bool {
  46     let codegen_fn_attrs = tcx.codegen_fn_attrs(impl_item.hir_id.owner.to_def_id());
  47     let generics = tcx.generics_of(tcx.hir().local_def_id(impl_item.hir_id));
  48     if codegen_fn_attrs.requests_inline() || generics.requires_monomorphization(tcx) {
  49         return true;
  50     }
  51     if let hir::ImplItemKind::Fn(method_sig, _) = &impl_item.kind {
  52         if method_sig.header.is_const() {
  53             return true;
  54         }
  55     }
  56     match tcx.hir().find(tcx.hir().local_def_id_to_hir_id(impl_src)) {
  57         Some(Node::Item(item)) => item_might_be_inlined(tcx, &item, codegen_fn_attrs),
  58         Some(..) | None => span_bug!(impl_item.span, "impl did is not an item"),
  59     }
  60 }
  61
  62 // Information needed while computing reachability.
  63 struct ReachableContext<'tcx> {
  64     // The type context.
  65     tcx: TyCtxt<'tcx>,
  66     maybe_typeck_results: Option<&'tcx ty::TypeckResults<'tcx>>,
  67     // The set of items which must be exported in the linkage sense.
  68     reachable_symbols: FxHashSet<LocalDefId>,
  69     // A worklist of item IDs. Each item ID in this worklist will be inlined
  70     // and will be scanned for further references.
  71     // FIXME(eddyb) benchmark if this would be faster as a `VecDeque`.
  72     worklist: Vec<LocalDefId>,
  73     // Whether any output of this compilation is a library
  74     any_library: bool,
  75 }
  76
  77 impl<'tcx> Visitor<'tcx> for ReachableContext<'tcx> {
  78     type Map = intravisit::ErasedMap<'tcx>;
  79
  80     fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
  81         NestedVisitorMap::None
  82     }
  83
  84     fn visit_nested_body(&mut self, body: hir::BodyId) {
  85         let old_maybe_typeck_results =
  86             self.maybe_typeck_results.replace(self.tcx.typeck_body(body));
  87         let body = self.tcx.hir().body(body);
  88         self.visit_body(body);
  89         self.maybe_typeck_results = old_maybe_typeck_results;
  90     }
  91
  92     fn visit_expr(&mut self, expr: &'tcx hir::Expr<'tcx>) {
  93         let res = match expr.kind {
  94             hir::ExprKind::Path(ref qpath) => {
  95                 Some(self.typeck_results().qpath_res(qpath, expr.hir_id))
  96             }
  97             hir::ExprKind::MethodCall(..) => self
  98                 .typeck_results()
  99                 .type_dependent_def(expr.hir_id)
 100                 .map(|(kind, def_id)| Res::Def(kind, def_id)),
 101             _ => None,
 102         };
 103
 104         if let Some(res) = res {
 105             if let Some(def_id) = res.opt_def_id().and_then(|def_id| def_id.as_local()) {
 106                 if self.def_id_represents_local_inlined_item(def_id.to_def_id()) {
 107                     self.worklist.push(def_id);
 108                 } else {
 109                     match res {
 110                         // If this path leads to a constant, then we need to
 111                         // recurse into the constant to continue finding
 112                         // items that are reachable.
 113                         Res::Def(DefKind::Const | DefKind::AssocConst, _) => {
 114                             self.worklist.push(def_id);
 115                         }
 116
 117                         // If this wasn't a static, then the destination is
 118                         // surely reachable.
 119                         _ => {
 120                             self.reachable_symbols.insert(def_id);
 121                         }
 122                     }
 123                 }
 124             }
 125         }
 126
 127         intravisit::walk_expr(self, expr)
 128     }
 129 }
 130
 131 impl<'tcx> ReachableContext<'tcx> {
 132     /// Gets the type-checking results for the current body.
 133     /// As this will ICE if called outside bodies, only call when working with
 134     /// `Expr` or `Pat` nodes (they are guaranteed to be found only in bodies).
 135     #[track_caller]
 136     fn typeck_results(&self) -> &'tcx ty::TypeckResults<'tcx> {
 137         self.maybe_typeck_results
 138             .expect("`ReachableContext::typeck_results` called outside of body")
 139     }
 140
 141     // Returns true if the given def ID represents a local item that is
 142     // eligible for inlining and false otherwise.
 143     fn def_id_represents_local_inlined_item(&self, def_id: DefId) -> bool {
 144         let hir_id = match def_id.as_local() {
 145             Some(def_id) => self.tcx.hir().local_def_id_to_hir_id(def_id),
 146             None => {
 147                 return false;
 148             }
 149         };
 150
 151         match self.tcx.hir().find(hir_id) {
 152             Some(Node::Item(item)) => match item.kind {
 153                 hir::ItemKind::Fn(..) => {
 154                     item_might_be_inlined(self.tcx, &item, self.tcx.codegen_fn_attrs(def_id))
 155                 }
 156                 _ => false,
 157             },
 158             Some(Node::TraitItem(trait_method)) => match trait_method.kind {
 159                 hir::TraitItemKind::Const(_, ref default) => default.is_some(),
 160                 hir::TraitItemKind::Fn(_, hir::TraitFn::Provided(_)) => true,
 161                 hir::TraitItemKind::Fn(_, hir::TraitFn::Required(_))
 162                 | hir::TraitItemKind::Type(..) => false,
 163             },
 164             Some(Node::ImplItem(impl_item)) => {
 165                 match impl_item.kind {
 166                     hir::ImplItemKind::Const(..) => true,
 167                     hir::ImplItemKind::Fn(..) => {
 168                         let attrs = self.tcx.codegen_fn_attrs(def_id);
 169                         let generics = self.tcx.generics_of(def_id);
 170                         if generics.requires_monomorphization(self.tcx) || attrs.requests_inline() {
 171                             true
 172                         } else {
 173                             let impl_did = self.tcx.hir().get_parent_did(hir_id);
 174                             // Check the impl. If the generics on the self
 175                             // type of the impl require inlining, this method
 176                             // does too.
 177                             let impl_hir_id = self.tcx.hir().local_def_id_to_hir_id(impl_did);
 178                             match self.tcx.hir().expect_item(impl_hir_id).kind {
 179                                 hir::ItemKind::Impl { .. } => {
 180                                     let generics = self.tcx.generics_of(impl_did);
 181                                     generics.requires_monomorphization(self.tcx)
 182                                 }
 183                                 _ => false,
 184                             }
 185                         }
 186                     }
 187                     hir::ImplItemKind::TyAlias(_) => false,
 188                 }
 189             }
 190             Some(_) => false,
 191             None => false, // This will happen for default methods.
 192         }
 193     }
 194
 195     // Step 2: Mark all symbols that the symbols on the worklist touch.
 196     fn propagate(&mut self) {
 197         let mut scanned = FxHashSet::default();
 198         while let Some(search_item) = self.worklist.pop() {
 199             if !scanned.insert(search_item) {
 200                 continue;
 201             }
 202
 203             if let Some(ref item) =
 204                 self.tcx.hir().find(self.tcx.hir().local_def_id_to_hir_id(search_item))
 205             {
 206                 self.propagate_node(item, search_item);
 207             }
 208         }
 209     }
 210
 211     fn propagate_node(&mut self, node: &Node<'tcx>, search_item: LocalDefId) {
 212         if !self.any_library {
 213             // If we are building an executable, only explicitly extern
 214             // types need to be exported.
 215             if let Node::Item(item) = *node {
 216                 let reachable = if let hir::ItemKind::Fn(ref sig, ..) = item.kind {
 217                     sig.header.abi != Abi::Rust
 218                 } else {
 219                     false
 220                 };
 221                 let def_id = self.tcx.hir().local_def_id(item.hir_id);
 222                 let codegen_attrs = self.tcx.codegen_fn_attrs(def_id);
 223                 let is_extern = codegen_attrs.contains_extern_indicator();
 224                 let std_internal =
 225                     codegen_attrs.flags.contains(CodegenFnAttrFlags::RUSTC_STD_INTERNAL_SYMBOL);
 226                 if reachable || is_extern || std_internal {
 227                     self.reachable_symbols.insert(search_item);
 228                 }
 229             }
 230         } else {
 231             // If we are building a library, then reachable symbols will
 232             // continue to participate in linkage after this product is
 233             // produced. In this case, we traverse the ast node, recursing on
 234             // all reachable nodes from this one.
 235             self.reachable_symbols.insert(search_item);
 236         }
 237
 238         match *node {
 239             Node::Item(item) => {
 240                 match item.kind {
 241                     hir::ItemKind::Fn(.., body) => {
 242                         let def_id = self.tcx.hir().local_def_id(item.hir_id);
 243                         if item_might_be_inlined(self.tcx, &item, self.tcx.codegen_fn_attrs(def_id))
 244                         {
 245                             self.visit_nested_body(body);
 246                         }
 247                     }
 248
 249                     // Reachable constants will be inlined into other crates
 250                     // unconditionally, so we need to make sure that their
 251                     // contents are also reachable.
 252                     hir::ItemKind::Const(_, init) => {
 253                         self.visit_nested_body(init);
 254                     }
 255
 256                     // These are normal, nothing reachable about these
 257                     // inherently and their children are already in the
 258                     // worklist, as determined by the privacy pass
 259                     hir::ItemKind::ExternCrate(_)
 260                     | hir::ItemKind::Use(..)
 261                     | hir::ItemKind::OpaqueTy(..)
 262                     | hir::ItemKind::TyAlias(..)
 263                     | hir::ItemKind::Static(..)
 264                     | hir::ItemKind::Mod(..)
 265                     | hir::ItemKind::ForeignMod(..)
 266                     | hir::ItemKind::Impl { .. }
 267                     | hir::ItemKind::Trait(..)
 268                     | hir::ItemKind::TraitAlias(..)
 269                     | hir::ItemKind::Struct(..)
 270                     | hir::ItemKind::Enum(..)
 271                     | hir::ItemKind::Union(..)
 272                     | hir::ItemKind::GlobalAsm(..) => {}
 273                 }
 274             }
 275             Node::TraitItem(trait_method) => {
 276                 match trait_method.kind {
 277                     hir::TraitItemKind::Const(_, None)
 278                     | hir::TraitItemKind::Fn(_, hir::TraitFn::Required(_)) => {
 279                         // Keep going, nothing to get exported
 280                     }
 281                     hir::TraitItemKind::Const(_, Some(body_id))
 282                     | hir::TraitItemKind::Fn(_, hir::TraitFn::Provided(body_id)) => {
 283                         self.visit_nested_body(body_id);
 284                     }
 285                     hir::TraitItemKind::Type(..) => {}
 286                 }
 287             }
 288             Node::ImplItem(impl_item) => match impl_item.kind {
 289                 hir::ImplItemKind::Const(_, body) => {
 290                     self.visit_nested_body(body);
 291                 }
 292                 hir::ImplItemKind::Fn(_, body) => {
 293                     let impl_def_id =
 294                         self.tcx.parent(search_item.to_def_id()).unwrap().expect_local();
 295                     if method_might_be_inlined(self.tcx, impl_item, impl_def_id) {
 296                         self.visit_nested_body(body)
 297                     }
 298                 }
 299                 hir::ImplItemKind::TyAlias(_) => {}
 300             },
 301             Node::Expr(&hir::Expr { kind: hir::ExprKind::Closure(.., body, _, _), .. }) => {
 302                 self.visit_nested_body(body);
 303             }
 304             // Nothing to recurse on for these
 305             Node::ForeignItem(_)
 306             | Node::Variant(_)
 307             | Node::Ctor(..)
 308             | Node::Field(_)
 309             | Node::Ty(_)
 310             | Node::MacroDef(_) => {}
 311             _ => {
 312                 bug!(
 313                     "found unexpected node kind in worklist: {} ({:?})",
 314                     self.tcx
 315                         .hir()
 316                         .node_to_string(self.tcx.hir().local_def_id_to_hir_id(search_item)),
 317                     node,
 318                 );
 319             }
 320         }
 321     }
 322 }
 323
 324 // Some methods from non-exported (completely private) trait impls still have to be
 325 // reachable if they are called from inlinable code. Generally, it's not known until
 326 // monomorphization if a specific trait impl item can be reachable or not. So, we
 327 // conservatively mark all of them as reachable.
 328 // FIXME: One possible strategy for pruning the reachable set is to avoid marking impl
 329 // items of non-exported traits (or maybe all local traits?) unless their respective
 330 // trait items are used from inlinable code through method call syntax or UFCS, or their
 331 // trait is a lang item.
 332 struct CollectPrivateImplItemsVisitor<'a, 'tcx> {
 333     tcx: TyCtxt<'tcx>,
 334     access_levels: &'a privacy::AccessLevels,
 335     worklist: &'a mut Vec<LocalDefId>,
 336 }
 337
 338 impl<'a, 'tcx> ItemLikeVisitor<'tcx> for CollectPrivateImplItemsVisitor<'a, 'tcx> {
 339     fn visit_item(&mut self, item: &hir::Item<'_>) {
 340         // Anything which has custom linkage gets thrown on the worklist no
 341         // matter where it is in the crate, along with "special std symbols"
 342         // which are currently akin to allocator symbols.
 343         let def_id = self.tcx.hir().local_def_id(item.hir_id);
 344         let codegen_attrs = self.tcx.codegen_fn_attrs(def_id);
 345         if codegen_attrs.contains_extern_indicator()
 346             || codegen_attrs.flags.contains(CodegenFnAttrFlags::RUSTC_STD_INTERNAL_SYMBOL)
 347         {
 348             self.worklist.push(def_id);
 349         }
 350
 351         // We need only trait impls here, not inherent impls, and only non-exported ones
 352         if let hir::ItemKind::Impl { of_trait: Some(ref trait_ref), ref items, .. } = item.kind {
 353             if !self.access_levels.is_reachable(item.hir_id) {
 354                 // FIXME(#53488) remove `let`
 355                 let tcx = self.tcx;
 356                 self.worklist
 357                     .extend(items.iter().map(|ii_ref| tcx.hir().local_def_id(ii_ref.id.hir_id)));
 358
 359                 let trait_def_id = match trait_ref.path.res {
 360                     Res::Def(DefKind::Trait, def_id) => def_id,
 361                     _ => unreachable!(),
 362                 };
 363
 364                 if !trait_def_id.is_local() {
 365                     return;
 366                 }
 367
 368                 self.worklist.extend(
 369                     tcx.provided_trait_methods(trait_def_id)
 370                         .map(|assoc| assoc.def_id.expect_local()),
 371                 );
 372             }
 373         }
 374     }
 375
 376     fn visit_trait_item(&mut self, _trait_item: &hir::TraitItem<'_>) {}
 377
 378     fn visit_impl_item(&mut self, _impl_item: &hir::ImplItem<'_>) {
 379         // processed in visit_item above
 380     }
 381 }
 382
 383 fn reachable_set<'tcx>(tcx: TyCtxt<'tcx>, crate_num: CrateNum) -> FxHashSet<LocalDefId> {
 384     debug_assert!(crate_num == LOCAL_CRATE);
 385
 386     let access_levels = &tcx.privacy_access_levels(LOCAL_CRATE);
 387
 388     let any_library =
 389         tcx.sess.crate_types().iter().any(|ty| {
 390             *ty == CrateType::Rlib || *ty == CrateType::Dylib || *ty == CrateType::ProcMacro
 391         });
 392     let mut reachable_context = ReachableContext {
 393         tcx,
 394         maybe_typeck_results: None,
 395         reachable_symbols: Default::default(),
 396         worklist: Vec::new(),
 397         any_library,
 398     };
 399
 400     // Step 1: Seed the worklist with all nodes which were found to be public as
 401     //         a result of the privacy pass along with all local lang items and impl items.
 402     //         If other crates link to us, they're going to expect to be able to
 403     //         use the lang items, so we need to be sure to mark them as
 404     //         exported.
 405     reachable_context
 406         .worklist
 407         .extend(access_levels.map.iter().map(|(id, _)| tcx.hir().local_def_id(*id)));
 408     for item in tcx.lang_items().items().iter() {
 409         if let Some(def_id) = *item {
 410             if let Some(def_id) = def_id.as_local() {
 411                 reachable_context.worklist.push(def_id);
 412             }
 413         }
 414     }
 415     {
 416         let mut collect_private_impl_items = CollectPrivateImplItemsVisitor {
 417             tcx,
 418             access_levels,
 419             worklist: &mut reachable_context.worklist,
 420         };
 421         tcx.hir().krate().visit_all_item_likes(&mut collect_private_impl_items);
 422     }
 423
 424     // Step 2: Mark all symbols that the symbols on the worklist touch.
 425     reachable_context.propagate();
 426
 427     debug!("Inline reachability shows: {:?}", reachable_context.reachable_symbols);
 428
 429     // Return the set of reachable symbols.
 430     reachable_context.reachable_symbols
 431 }
 432
 433 pub fn provide(providers: &mut Providers) {
 434     *providers = Providers { reachable_set, ..*providers };
 435 }