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(item.def_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(impl_item.def_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 codegen_attrs = self.tcx.codegen_fn_attrs(item.def_id);
 222                 let is_extern = codegen_attrs.contains_extern_indicator();
 223                 let std_internal =
 224                     codegen_attrs.flags.contains(CodegenFnAttrFlags::RUSTC_STD_INTERNAL_SYMBOL);
 225                 if reachable || is_extern || std_internal {
 226                     self.reachable_symbols.insert(search_item);
 227                 }
 228             }
 229         } else {
 230             // If we are building a library, then reachable symbols will
 231             // continue to participate in linkage after this product is
 232             // produced. In this case, we traverse the ast node, recursing on
 233             // all reachable nodes from this one.
 234             self.reachable_symbols.insert(search_item);
 235         }
 236
 237         match *node {
 238             Node::Item(item) => {
 239                 match item.kind {
 240                     hir::ItemKind::Fn(.., body) => {
 241                         if item_might_be_inlined(
 242                             self.tcx,
 243                             &item,
 244                             self.tcx.codegen_fn_attrs(item.def_id),
 245                         ) {
 246                             self.visit_nested_body(body);
 247                         }
 248                     }
 249
 250                     // Reachable constants will be inlined into other crates
 251                     // unconditionally, so we need to make sure that their
 252                     // contents are also reachable.
 253                     hir::ItemKind::Const(_, init) => {
 254                         self.visit_nested_body(init);
 255                     }
 256
 257                     // These are normal, nothing reachable about these
 258                     // inherently and their children are already in the
 259                     // worklist, as determined by the privacy pass
 260                     hir::ItemKind::ExternCrate(_)
 261                     | hir::ItemKind::Use(..)
 262                     | hir::ItemKind::OpaqueTy(..)
 263                     | hir::ItemKind::TyAlias(..)
 264                     | hir::ItemKind::Static(..)
 265                     | hir::ItemKind::Mod(..)
 266                     | hir::ItemKind::ForeignMod { .. }
 267                     | hir::ItemKind::Impl { .. }
 268                     | hir::ItemKind::Trait(..)
 269                     | hir::ItemKind::TraitAlias(..)
 270                     | hir::ItemKind::Struct(..)
 271                     | hir::ItemKind::Enum(..)
 272                     | hir::ItemKind::Union(..)
 273                     | hir::ItemKind::GlobalAsm(..) => {}
 274                 }
 275             }
 276             Node::TraitItem(trait_method) => {
 277                 match trait_method.kind {
 278                     hir::TraitItemKind::Const(_, None)
 279                     | hir::TraitItemKind::Fn(_, hir::TraitFn::Required(_)) => {
 280                         // Keep going, nothing to get exported
 281                     }
 282                     hir::TraitItemKind::Const(_, Some(body_id))
 283                     | hir::TraitItemKind::Fn(_, hir::TraitFn::Provided(body_id)) => {
 284                         self.visit_nested_body(body_id);
 285                     }
 286                     hir::TraitItemKind::Type(..) => {}
 287                 }
 288             }
 289             Node::ImplItem(impl_item) => match impl_item.kind {
 290                 hir::ImplItemKind::Const(_, body) => {
 291                     self.visit_nested_body(body);
 292                 }
 293                 hir::ImplItemKind::Fn(_, body) => {
 294                     let impl_def_id =
 295                         self.tcx.parent(search_item.to_def_id()).unwrap().expect_local();
 296                     if method_might_be_inlined(self.tcx, impl_item, impl_def_id) {
 297                         self.visit_nested_body(body)
 298                     }
 299                 }
 300                 hir::ImplItemKind::TyAlias(_) => {}
 301             },
 302             Node::Expr(&hir::Expr { kind: hir::ExprKind::Closure(.., body, _, _), .. }) => {
 303                 self.visit_nested_body(body);
 304             }
 305             // Nothing to recurse on for these
 306             Node::ForeignItem(_)
 307             | Node::Variant(_)
 308             | Node::Ctor(..)
 309             | Node::Field(_)
 310             | Node::Ty(_)
 311             | Node::Crate(_)
 312             | Node::MacroDef(_) => {}
 313             _ => {
 314                 bug!(
 315                     "found unexpected node kind in worklist: {} ({:?})",
 316                     self.tcx
 317                         .hir()
 318                         .node_to_string(self.tcx.hir().local_def_id_to_hir_id(search_item)),
 319                     node,
 320                 );
 321             }
 322         }
 323     }
 324 }
 325
 326 // Some methods from non-exported (completely private) trait impls still have to be
 327 // reachable if they are called from inlinable code. Generally, it's not known until
 328 // monomorphization if a specific trait impl item can be reachable or not. So, we
 329 // conservatively mark all of them as reachable.
 330 // FIXME: One possible strategy for pruning the reachable set is to avoid marking impl
 331 // items of non-exported traits (or maybe all local traits?) unless their respective
 332 // trait items are used from inlinable code through method call syntax or UFCS, or their
 333 // trait is a lang item.
 334 struct CollectPrivateImplItemsVisitor<'a, 'tcx> {
 335     tcx: TyCtxt<'tcx>,
 336     access_levels: &'a privacy::AccessLevels,
 337     worklist: &'a mut Vec<LocalDefId>,
 338 }
 339
 340 impl<'a, 'tcx> ItemLikeVisitor<'tcx> for CollectPrivateImplItemsVisitor<'a, 'tcx> {
 341     fn visit_item(&mut self, item: &hir::Item<'_>) {
 342         // Anything which has custom linkage gets thrown on the worklist no
 343         // matter where it is in the crate, along with "special std symbols"
 344         // which are currently akin to allocator symbols.
 345         let codegen_attrs = self.tcx.codegen_fn_attrs(item.def_id);
 346         if codegen_attrs.contains_extern_indicator()
 347             || codegen_attrs.flags.contains(CodegenFnAttrFlags::RUSTC_STD_INTERNAL_SYMBOL)
 348         {
 349             self.worklist.push(item.def_id);
 350         }
 351
 352         // We need only trait impls here, not inherent impls, and only non-exported ones
 353         if let hir::ItemKind::Impl(hir::Impl { of_trait: Some(ref trait_ref), ref items, .. }) =
 354             item.kind
 355         {
 356             if !self.access_levels.is_reachable(item.hir_id()) {
 357                 // FIXME(#53488) remove `let`
 358                 let tcx = self.tcx;
 359                 self.worklist.extend(items.iter().map(|ii_ref| ii_ref.id.def_id));
 360
 361                 let trait_def_id = match trait_ref.path.res {
 362                     Res::Def(DefKind::Trait, def_id) => def_id,
 363                     _ => unreachable!(),
 364                 };
 365
 366                 if !trait_def_id.is_local() {
 367                     return;
 368                 }
 369
 370                 self.worklist.extend(
 371                     tcx.provided_trait_methods(trait_def_id)
 372                         .map(|assoc| assoc.def_id.expect_local()),
 373                 );
 374             }
 375         }
 376     }
 377
 378     fn visit_trait_item(&mut self, _trait_item: &hir::TraitItem<'_>) {}
 379
 380     fn visit_impl_item(&mut self, _impl_item: &hir::ImplItem<'_>) {
 381         // processed in visit_item above
 382     }
 383
 384     fn visit_foreign_item(&mut self, _foreign_item: &hir::ForeignItem<'_>) {
 385         // We never export foreign functions as they have no body to export.
 386     }
 387 }
 388
 389 fn reachable_set<'tcx>(tcx: TyCtxt<'tcx>, crate_num: CrateNum) -> FxHashSet<LocalDefId> {
 390     debug_assert!(crate_num == LOCAL_CRATE);
 391
 392     let access_levels = &tcx.privacy_access_levels(LOCAL_CRATE);
 393
 394     let any_library =
 395         tcx.sess.crate_types().iter().any(|ty| {
 396             *ty == CrateType::Rlib || *ty == CrateType::Dylib || *ty == CrateType::ProcMacro
 397         });
 398     let mut reachable_context = ReachableContext {
 399         tcx,
 400         maybe_typeck_results: None,
 401         reachable_symbols: Default::default(),
 402         worklist: Vec::new(),
 403         any_library,
 404     };
 405
 406     // Step 1: Seed the worklist with all nodes which were found to be public as
 407     //         a result of the privacy pass along with all local lang items and impl items.
 408     //         If other crates link to us, they're going to expect to be able to
 409     //         use the lang items, so we need to be sure to mark them as
 410     //         exported.
 411     reachable_context
 412         .worklist
 413         .extend(access_levels.map.iter().map(|(id, _)| tcx.hir().local_def_id(*id)));
 414     for item in tcx.lang_items().items().iter() {
 415         if let Some(def_id) = *item {
 416             if let Some(def_id) = def_id.as_local() {
 417                 reachable_context.worklist.push(def_id);
 418             }
 419         }
 420     }
 421     {
 422         let mut collect_private_impl_items = CollectPrivateImplItemsVisitor {
 423             tcx,
 424             access_levels,
 425             worklist: &mut reachable_context.worklist,
 426         };
 427         tcx.hir().krate().visit_all_item_likes(&mut collect_private_impl_items);
 428     }
 429
 430     // Step 2: Mark all symbols that the symbols on the worklist touch.
 431     reachable_context.propagate();
 432
 433     debug!("Inline reachability shows: {:?}", reachable_context.reachable_symbols);
 434
 435     // Return the set of reachable symbols.
 436     reachable_context.reachable_symbols
 437 }
 438
 439 pub fn provide(providers: &mut Providers) {
 440     *providers = Providers { reachable_set, ..*providers };
 441 }