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//! Finds local items that are "reachable", which means that other crates need access to their
//! compiled code or their *runtime* MIR. (Compile-time MIR is always encoded anyway, so we don't
//! worry about that here.)
//!
//! An item is "reachable" if codegen that happens in downstream crates can end up referencing this
//! item. This obviously includes all public items. However, some of these items cannot be codegen'd
//! (because they are generic), and for some the compiled 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 its MIR may be codegen'd in a downstream crate, and hence items it
//! mentions need to be considered reachable.
//!
//! Furthermore, if a `const`/`const fn` is reachable, then it can return pointers to other items,
//! making those reachable as well. For instance, consider a `const fn` returning a pointer to an
//! otherwise entirely private function: if a downstream crate calls that `const fn` to compute the
//! initial value of a `static`, then it needs to generate a direct reference to this function --
//! i.e., the function is directly reachable from that downstream crate! Hence we have to recurse
//! into `const` and `const fn`.
//!
//! Conversely, reachability *stops* when it hits a monomorphic non-`const` function that we do not
//! want to cross-crate inline. That function will just be codegen'd in this crate, which means the
//! monomorphization collector will consider it a root and then do another graph traversal to
//! codegen everything called by this function -- but that's a very different graph from what we are
//! considering here as at that point, everything is monomorphic.
use hir::def_id::LocalDefIdSet;
use rustc_data_structures::stack::ensure_sufficient_stack;
use rustc_hir as hir;
use rustc_hir::def::{DefKind, Res};
use rustc_hir::def_id::{DefId, LocalDefId};
use rustc_hir::intravisit::{self, Visitor};
use rustc_hir::Node;
use rustc_middle::bug;
use rustc_middle::middle::codegen_fn_attrs::{CodegenFnAttrFlags, CodegenFnAttrs};
use rustc_middle::middle::privacy::{self, Level};
use rustc_middle::mir::interpret::{ConstAllocation, ErrorHandled, GlobalAlloc};
use rustc_middle::query::Providers;
use rustc_middle::ty::{self, ExistentialTraitRef, TyCtxt};
use rustc_privacy::DefIdVisitor;
use rustc_session::config::CrateType;
use tracing::debug;
/// 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)
}
// Information needed while computing reachability.
struct ReachableContext<'tcx> {
// The type context.
tcx: TyCtxt<'tcx>,
maybe_typeck_results: Option<&'tcx ty::TypeckResults<'tcx>>,
// The set of items which must be exported in the linkage sense.
reachable_symbols: LocalDefIdSet,
// A worklist of item IDs. Each item ID in this worklist will be inlined
// and will be scanned for further references.
// FIXME(eddyb) benchmark if this would be faster as a `VecDeque`.
worklist: Vec<LocalDefId>,
// Whether any output of this compilation is a library
any_library: bool,
}
impl<'tcx> Visitor<'tcx> for ReachableContext<'tcx> {
fn visit_nested_body(&mut self, body: hir::BodyId) {
let old_maybe_typeck_results =
self.maybe_typeck_results.replace(self.tcx.typeck_body(body));
let body = self.tcx.hir().body(body);
self.visit_body(body);
self.maybe_typeck_results = old_maybe_typeck_results;
}
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(..) => {
// 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
}
_ => None,
};
if let Some(res) = res {
self.propagate_item(res);
}
intravisit::walk_expr(self, expr)
}
fn visit_inline_asm(&mut self, asm: &'tcx hir::InlineAsm<'tcx>, id: hir::HirId) {
for (op, _) in asm.operands {
if let hir::InlineAsmOperand::SymStatic { def_id, .. } = op {
if let Some(def_id) = def_id.as_local() {
self.reachable_symbols.insert(def_id);
}
}
}
intravisit::walk_inline_asm(self, asm, id);
}
}
impl<'tcx> ReachableContext<'tcx> {
/// Gets the type-checking results for the current body.
/// As this will ICE if called outside bodies, only call when working with
/// `Expr` or `Pat` nodes (they are guaranteed to be found only in bodies).
#[track_caller]
fn typeck_results(&self) -> &'tcx ty::TypeckResults<'tcx> {
self.maybe_typeck_results
.expect("`ReachableContext::typeck_results` called outside of body")
}
/// 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(..) => recursively_reachable(self.tcx, def_id.into()),
_ => false,
},
Node::TraitItem(trait_method) => match trait_method.kind {
hir::TraitItemKind::Const(_, ref default) => default.is_some(),
hir::TraitItemKind::Fn(_, hir::TraitFn::Provided(_)) => true,
hir::TraitItemKind::Fn(_, hir::TraitFn::Required(_))
| hir::TraitItemKind::Type(..) => false,
},
Node::ImplItem(impl_item) => match impl_item.kind {
hir::ImplItemKind::Const(..) => true,
hir::ImplItemKind::Fn(..) => {
recursively_reachable(self.tcx, impl_item.hir_id().owner.to_def_id())
}
hir::ImplItemKind::Type(_) => false,
},
Node::Expr(&hir::Expr { kind: hir::ExprKind::Closure(..), .. }) => true,
_ => false,
}
}
// Step 2: Mark all symbols that the symbols on the worklist touch.
fn propagate(&mut self) {
let mut scanned = LocalDefIdSet::default();
while let Some(search_item) = self.worklist.pop() {
if !scanned.insert(search_item) {
continue;
}
self.propagate_node(&self.tcx.hir_node_by_def_id(search_item), search_item);
}
}
fn propagate_node(&mut self, node: &Node<'tcx>, search_item: LocalDefId) {
if !self.any_library {
// If we are building an executable, only explicitly extern
// types need to be exported.
let codegen_attrs = if self.tcx.def_kind(search_item).has_codegen_attrs() {
self.tcx.codegen_fn_attrs(search_item)
} else {
CodegenFnAttrs::EMPTY
};
let is_extern = codegen_attrs.contains_extern_indicator();
let std_internal =
codegen_attrs.flags.contains(CodegenFnAttrFlags::RUSTC_STD_INTERNAL_SYMBOL);
if is_extern || std_internal {
self.reachable_symbols.insert(search_item);
}
} else {
// If we are building a library, then reachable symbols will
// continue to participate in linkage after this product is
// produced. In this case, we traverse the ast node, recursing on
// all reachable nodes from this one.
self.reachable_symbols.insert(search_item);
}
match *node {
Node::Item(item) => {
match item.kind {
hir::ItemKind::Fn(.., body) => {
if recursively_reachable(self.tcx, item.owner_id.into()) {
self.visit_nested_body(body);
}
}
hir::ItemKind::Const(_, _, init) => {
// Only things actually ending up in the final constant value are reachable
// for codegen. Everything else is only needed during const-eval, so even if
// const-eval happens in a downstream crate, all they need is
// `mir_for_ctfe`.
match self.tcx.const_eval_poly_to_alloc(item.owner_id.def_id.into()) {
Ok(alloc) => {
let alloc = self.tcx.global_alloc(alloc.alloc_id).unwrap_memory();
self.propagate_from_alloc(alloc);
}
// We can't figure out which value the constant will evaluate to. In
// lieu of that, we have to consider everything mentioned in the const
// initializer reachable, since it *may* end up in the final value.
Err(ErrorHandled::TooGeneric(_)) => self.visit_nested_body(init),
// If there was an error evaluating the const, nothing can be reachable
// via it, and anyway compilation will fail.
Err(ErrorHandled::Reported(..)) => {}
}
}
hir::ItemKind::Static(..) => {
if let Ok(alloc) = self.tcx.eval_static_initializer(item.owner_id.def_id) {
self.propagate_from_alloc(alloc);
}
}
// These are normal, nothing reachable about these
// inherently and their children are already in the
// worklist, as determined by the privacy pass
hir::ItemKind::ExternCrate(_)
| hir::ItemKind::Use(..)
| hir::ItemKind::OpaqueTy(..)
| hir::ItemKind::TyAlias(..)
| hir::ItemKind::Macro(..)
| hir::ItemKind::Mod(..)
| hir::ItemKind::ForeignMod { .. }
| hir::ItemKind::Impl { .. }
| hir::ItemKind::Trait(..)
| hir::ItemKind::TraitAlias(..)
| hir::ItemKind::Struct(..)
| hir::ItemKind::Enum(..)
| hir::ItemKind::Union(..)
| hir::ItemKind::GlobalAsm(..) => {}
}
}
Node::TraitItem(trait_method) => {
match trait_method.kind {
hir::TraitItemKind::Const(_, None)
| hir::TraitItemKind::Fn(_, hir::TraitFn::Required(_)) => {
// Keep going, nothing to get exported
}
hir::TraitItemKind::Const(_, Some(body_id))
| hir::TraitItemKind::Fn(_, hir::TraitFn::Provided(body_id)) => {
self.visit_nested_body(body_id);
}
hir::TraitItemKind::Type(..) => {}
}
}
Node::ImplItem(impl_item) => match impl_item.kind {
hir::ImplItemKind::Const(_, body) => {
self.visit_nested_body(body);
}
hir::ImplItemKind::Fn(_, body) => {
if recursively_reachable(self.tcx, impl_item.hir_id().owner.to_def_id()) {
self.visit_nested_body(body)
}
}
hir::ImplItemKind::Type(_) => {}
},
Node::Expr(&hir::Expr {
kind: hir::ExprKind::Closure(&hir::Closure { body, .. }),
..
}) => {
self.visit_nested_body(body);
}
// Nothing to recurse on for these
Node::ForeignItem(_)
| Node::Variant(_)
| Node::Ctor(..)
| Node::Field(_)
| Node::Ty(_)
| Node::Crate(_)
| Node::Synthetic => {}
_ => {
bug!(
"found unexpected node kind in worklist: {} ({:?})",
self.tcx.hir().node_to_string(self.tcx.local_def_id_to_hir_id(search_item)),
node,
);
}
}
}
/// Finds things to add to `reachable_symbols` within allocations.
/// In contrast to visit_nested_body this ignores things that were only needed to evaluate
/// the allocation.
fn propagate_from_alloc(&mut self, alloc: ConstAllocation<'tcx>) {
if !self.any_library {
return;
}
for (_, prov) in alloc.0.provenance().ptrs().iter() {
match self.tcx.global_alloc(prov.alloc_id()) {
GlobalAlloc::Static(def_id) => {
self.propagate_item(Res::Def(self.tcx.def_kind(def_id), def_id))
}
GlobalAlloc::Function { instance, .. } => {
// Manually visit to actually see the instance's `DefId`. Type visitors won't see it
self.propagate_item(Res::Def(
self.tcx.def_kind(instance.def_id()),
instance.def_id(),
));
self.visit(instance.args);
}
GlobalAlloc::VTable(ty, trait_ref) => {
self.visit(ty);
// Manually visit to actually see the trait's `DefId`. Type visitors won't see it
if let Some(trait_ref) = trait_ref {
let ExistentialTraitRef { def_id, args } = trait_ref.skip_binder();
self.visit_def_id(def_id, "", &"");
self.visit(args);
}
}
GlobalAlloc::Memory(alloc) => self.propagate_from_alloc(alloc),
}
}
}
fn propagate_item(&mut self, res: Res) {
let Res::Def(kind, def_id) = res else { return };
let Some(def_id) = def_id.as_local() else { return };
match kind {
DefKind::Static { nested: true, .. } => {
// This is the main purpose of this function: add the def_id we find
// to `reachable_symbols`.
if self.reachable_symbols.insert(def_id) {
if let Ok(alloc) = self.tcx.eval_static_initializer(def_id) {
// This cannot cause infinite recursion, because we abort by inserting into the
// work list once we hit a normal static. Nested statics, even if they somehow
// become recursive, are also not infinitely recursing, because of the
// `reachable_symbols` check above.
// We still need to protect against stack overflow due to deeply nested statics.
ensure_sufficient_stack(|| self.propagate_from_alloc(alloc));
}
}
}
// Reachable constants and reachable statics can have their contents inlined
// into other crates. Mark them as reachable and recurse into their body.
DefKind::Const | DefKind::AssocConst | DefKind::Static { .. } => {
self.worklist.push(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);
}
}
}
}
}
impl<'tcx> DefIdVisitor<'tcx> for ReachableContext<'tcx> {
type Result = ();
fn tcx(&self) -> TyCtxt<'tcx> {
self.tcx
}
fn visit_def_id(
&mut self,
def_id: DefId,
_kind: &str,
_descr: &dyn std::fmt::Display,
) -> Self::Result {
self.propagate_item(Res::Def(self.tcx.def_kind(def_id), def_id))
}
}
fn check_item<'tcx>(
tcx: TyCtxt<'tcx>,
id: hir::ItemId,
worklist: &mut Vec<LocalDefId>,
effective_visibilities: &privacy::EffectiveVisibilities,
) {
if has_custom_linkage(tcx, id.owner_id.def_id) {
worklist.push(id.owner_id.def_id);
}
if !matches!(tcx.def_kind(id.owner_id), DefKind::Impl { of_trait: true }) {
return;
}
// We need only trait impls here, not inherent impls, and only non-exported ones
if effective_visibilities.is_reachable(id.owner_id.def_id) {
return;
}
let items = tcx.associated_item_def_ids(id.owner_id);
worklist.extend(items.iter().map(|ii_ref| ii_ref.expect_local()));
let Some(trait_def_id) = tcx.trait_id_of_impl(id.owner_id.to_def_id()) else {
unreachable!();
};
if !trait_def_id.is_local() {
return;
}
worklist
.extend(tcx.provided_trait_methods(trait_def_id).map(|assoc| assoc.def_id.expect_local()));
}
fn has_custom_linkage(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
// Anything which has custom linkage gets thrown on the worklist no
// matter where it is in the crate, along with "special std symbols"
// which are currently akin to allocator symbols.
if !tcx.def_kind(def_id).has_codegen_attrs() {
return false;
}
let codegen_attrs = tcx.codegen_fn_attrs(def_id);
codegen_attrs.contains_extern_indicator()
|| codegen_attrs.flags.contains(CodegenFnAttrFlags::RUSTC_STD_INTERNAL_SYMBOL)
// FIXME(nbdd0121): `#[used]` are marked as reachable here so it's picked up by
// `linked_symbols` in cg_ssa. They won't be exported in binary or cdylib due to their
// `SymbolExportLevel::Rust` export level but may end up being exported in dylibs.
|| codegen_attrs.flags.contains(CodegenFnAttrFlags::USED)
|| 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(());
let any_library = tcx
.crate_types()
.iter()
.any(|ty| *ty == CrateType::Rlib || *ty == CrateType::Dylib || *ty == CrateType::ProcMacro);
let mut reachable_context = ReachableContext {
tcx,
maybe_typeck_results: None,
reachable_symbols: Default::default(),
worklist: Vec::new(),
any_library,
};
// Step 1: Seed the worklist with all nodes which were found to be public as
// a result of the privacy pass along with all local lang items and impl items.
// If other crates link to us, they're going to expect to be able to
// use the lang items, so we need to be sure to mark them as
// exported.
reachable_context.worklist = effective_visibilities
.iter()
.filter_map(|(&id, effective_vis)| {
effective_vis.is_public_at_level(Level::ReachableThroughImplTrait).then_some(id)
})
.collect::<Vec<_>>();
for (_, def_id) in tcx.lang_items().iter() {
if let Some(def_id) = def_id.as_local() {
reachable_context.worklist.push(def_id);
}
}
{
// 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.free_items() {
check_item(tcx, id, &mut reachable_context.worklist, effective_visibilities);
}
for id in crate_items.impl_items() {
if has_custom_linkage(tcx, id.owner_id.def_id) {
reachable_context.worklist.push(id.owner_id.def_id);
}
}
}
// Step 2: Mark all symbols that the symbols on the worklist touch.
reachable_context.propagate();
debug!("Inline reachability shows: {:?}", reachable_context.reachable_symbols);
// Return the set of reachable symbols.
reachable_context.reachable_symbols
}
pub(crate) fn provide(providers: &mut Providers) {
*providers = Providers { reachable_set, ..*providers };
}