diff options
Diffstat (limited to 'compiler')
| -rw-r--r-- | compiler/rustc_passes/src/reachable.rs | 72 |
1 files changed, 49 insertions, 23 deletions
diff --git a/compiler/rustc_passes/src/reachable.rs b/compiler/rustc_passes/src/reachable.rs index b7135de08ba..7db91146590 100644 --- a/compiler/rustc_passes/src/reachable.rs +++ b/compiler/rustc_passes/src/reachable.rs @@ -1,9 +1,14 @@ -// Finds items that are externally reachable, to determine which items -// need to have their metadata (and possibly their AST) serialized. -// All items that can be referred to through an exported name are -// reachable, and when a reachable thing is inline or generic, it -// makes all other generics or inline functions that it references -// reachable as well. +//! Finds local items that are externally reachable, which means that other crates need access to +//! their compiled machine code or their MIR. +//! +//! An item is "externally reachable" if it is relevant for other crates. This obviously includes +//! all public items. However, some of these items cannot be compiled to machine code (because they +//! are generic), and for some the machine code is not sufficient (because we want to cross-crate +//! inline them). These items "need cross-crate MIR". When a reachable function `f` needs +//! cross-crate MIR, then all the functions it calls also become reachable, as they will be +//! necessary to use the MIR of `f` from another crate. Furthermore, an item can become "externally +//! reachable" by having a `const`/`const fn` return a pointer to that item, so we also need to +//! recurse into reachable `const`/`const fn`. use hir::def_id::LocalDefIdSet; use rustc_data_structures::stack::ensure_sufficient_stack; @@ -21,7 +26,9 @@ use rustc_privacy::DefIdVisitor; use rustc_session::config::CrateType; use rustc_target::spec::abi::Abi; -fn item_might_be_inlined(tcx: TyCtxt<'_>, def_id: DefId) -> bool { +/// Determines whether this item is recursive for reachability. See `is_recursively_reachable_local` +/// below for details. +fn recursively_reachable(tcx: TyCtxt<'_>, def_id: DefId) -> bool { tcx.generics_of(def_id).requires_monomorphization(tcx) || tcx.cross_crate_inlinable(def_id) || tcx.is_const_fn(def_id) @@ -54,12 +61,20 @@ impl<'tcx> Visitor<'tcx> for ReachableContext<'tcx> { fn visit_expr(&mut self, expr: &'tcx hir::Expr<'tcx>) { let res = match expr.kind { hir::ExprKind::Path(ref qpath) => { + // This covers fn ptr casts but also "non-method" calls. Some(self.typeck_results().qpath_res(qpath, expr.hir_id)) } - hir::ExprKind::MethodCall(..) => self - .typeck_results() - .type_dependent_def(expr.hir_id) - .map(|(kind, def_id)| Res::Def(kind, def_id)), + hir::ExprKind::MethodCall(..) => { + // Method calls don't involve a full "path", so we need to determine the callee + // based on the receiver type. + // If this is a method call on a generic type, we might not be able to find the + // callee. That's why `reachable_set` also adds all potential callees for such + // calls, i.e. all trait impl items, to the reachable set. So here we only worry + // about the calls we can identify. + self.typeck_results() + .type_dependent_def(expr.hir_id) + .map(|(kind, def_id)| Res::Def(kind, def_id)) + } hir::ExprKind::Closure(&hir::Closure { def_id, .. }) => { self.reachable_symbols.insert(def_id); None @@ -96,16 +111,24 @@ impl<'tcx> ReachableContext<'tcx> { .expect("`ReachableContext::typeck_results` called outside of body") } - // Returns true if the given def ID represents a local item that is - // eligible for inlining and false otherwise. - fn def_id_represents_local_inlined_item(&self, def_id: DefId) -> bool { + /// Returns true if the given def ID represents a local item that is recursive for reachability, + /// i.e. whether everything mentioned in here also needs to be considered reachable. + /// + /// There are two reasons why an item may be recursively reachable: + /// - It needs cross-crate MIR (see the module-level doc comment above). + /// - It is a `const` or `const fn`. This is *not* because we need the MIR to interpret them + /// (MIR for const-eval and MIR for codegen is separate, and MIR for const-eval is always + /// encoded). Instead, it is because `const fn` can create `fn()` pointers to other items + /// which end up in the evaluated result of the constant and can then be called from other + /// crates. Those items must be considered reachable. + fn is_recursively_reachable_local(&self, def_id: DefId) -> bool { let Some(def_id) = def_id.as_local() else { return false; }; match self.tcx.hir_node_by_def_id(def_id) { Node::Item(item) => match item.kind { - hir::ItemKind::Fn(..) => item_might_be_inlined(self.tcx, def_id.into()), + hir::ItemKind::Fn(..) => recursively_reachable(self.tcx, def_id.into()), _ => false, }, Node::TraitItem(trait_method) => match trait_method.kind { @@ -117,7 +140,7 @@ impl<'tcx> ReachableContext<'tcx> { Node::ImplItem(impl_item) => match impl_item.kind { hir::ImplItemKind::Const(..) => true, hir::ImplItemKind::Fn(..) => { - item_might_be_inlined(self.tcx, impl_item.hir_id().owner.to_def_id()) + recursively_reachable(self.tcx, impl_item.hir_id().owner.to_def_id()) } hir::ImplItemKind::Type(_) => false, }, @@ -174,7 +197,7 @@ impl<'tcx> ReachableContext<'tcx> { Node::Item(item) => { match item.kind { hir::ItemKind::Fn(.., body) => { - if item_might_be_inlined(self.tcx, item.owner_id.into()) { + if recursively_reachable(self.tcx, item.owner_id.into()) { self.visit_nested_body(body); } } @@ -228,7 +251,7 @@ impl<'tcx> ReachableContext<'tcx> { self.visit_nested_body(body); } hir::ImplItemKind::Fn(_, body) => { - if item_might_be_inlined(self.tcx, impl_item.hir_id().owner.to_def_id()) { + if recursively_reachable(self.tcx, impl_item.hir_id().owner.to_def_id()) { self.visit_nested_body(body) } } @@ -316,7 +339,7 @@ impl<'tcx> ReachableContext<'tcx> { self.worklist.push(def_id); } _ => { - if self.def_id_represents_local_inlined_item(def_id.to_def_id()) { + if self.is_recursively_reachable_local(def_id.to_def_id()) { self.worklist.push(def_id); } else { self.reachable_symbols.insert(def_id); @@ -394,6 +417,7 @@ fn has_custom_linkage(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool { || codegen_attrs.flags.contains(CodegenFnAttrFlags::USED_LINKER) } +/// See module-level doc comment above. fn reachable_set(tcx: TyCtxt<'_>, (): ()) -> LocalDefIdSet { let effective_visibilities = &tcx.effective_visibilities(()); @@ -427,14 +451,16 @@ fn reachable_set(tcx: TyCtxt<'_>, (): ()) -> LocalDefIdSet { } } { - // Some methods from non-exported (completely private) trait impls still have to be - // reachable if they are called from inlinable code. Generally, it's not known until - // monomorphization if a specific trait impl item can be reachable or not. So, we - // conservatively mark all of them as reachable. + // As explained above, we have to mark all functions called from reachable + // `item_might_be_inlined` items as reachable. The issue is, when those functions are + // generic and call a trait method, we have no idea where that call goes! So, we + // conservatively mark all trait impl items as reachable. // FIXME: One possible strategy for pruning the reachable set is to avoid marking impl // items of non-exported traits (or maybe all local traits?) unless their respective // trait items are used from inlinable code through method call syntax or UFCS, or their // trait is a lang item. + // (But if you implement this, don't forget to take into account that vtables can also + // make trait methods reachable!) let crate_items = tcx.hir_crate_items(()); for id in crate_items.items() { |