rustc_monomorphize/
collector.rs

1//! Mono Item Collection
2//! ====================
3//!
4//! This module is responsible for discovering all items that will contribute
5//! to code generation of the crate. The important part here is that it not only
6//! needs to find syntax-level items (functions, structs, etc) but also all
7//! their monomorphized instantiations. Every non-generic, non-const function
8//! maps to one LLVM artifact. Every generic function can produce
9//! from zero to N artifacts, depending on the sets of type arguments it
10//! is instantiated with.
11//! This also applies to generic items from other crates: A generic definition
12//! in crate X might produce monomorphizations that are compiled into crate Y.
13//! We also have to collect these here.
14//!
15//! The following kinds of "mono items" are handled here:
16//!
17//! - Functions
18//! - Methods
19//! - Closures
20//! - Statics
21//! - Drop glue
22//!
23//! The following things also result in LLVM artifacts, but are not collected
24//! here, since we instantiate them locally on demand when needed in a given
25//! codegen unit:
26//!
27//! - Constants
28//! - VTables
29//! - Object Shims
30//!
31//! The main entry point is `collect_crate_mono_items`, at the bottom of this file.
32//!
33//! General Algorithm
34//! -----------------
35//! Let's define some terms first:
36//!
37//! - A "mono item" is something that results in a function or global in
38//!   the LLVM IR of a codegen unit. Mono items do not stand on their
39//!   own, they can use other mono items. For example, if function
40//!   `foo()` calls function `bar()` then the mono item for `foo()`
41//!   uses the mono item for function `bar()`. In general, the
42//!   definition for mono item A using a mono item B is that
43//!   the LLVM artifact produced for A uses the LLVM artifact produced
44//!   for B.
45//!
46//! - Mono items and the uses between them form a directed graph,
47//!   where the mono items are the nodes and uses form the edges.
48//!   Let's call this graph the "mono item graph".
49//!
50//! - The mono item graph for a program contains all mono items
51//!   that are needed in order to produce the complete LLVM IR of the program.
52//!
53//! The purpose of the algorithm implemented in this module is to build the
54//! mono item graph for the current crate. It runs in two phases:
55//!
56//! 1. Discover the roots of the graph by traversing the HIR of the crate.
57//! 2. Starting from the roots, find uses by inspecting the MIR
58//!    representation of the item corresponding to a given node, until no more
59//!    new nodes are found.
60//!
61//! ### Discovering roots
62//! The roots of the mono item graph correspond to the public non-generic
63//! syntactic items in the source code. We find them by walking the HIR of the
64//! crate, and whenever we hit upon a public function, method, or static item,
65//! we create a mono item consisting of the items DefId and, since we only
66//! consider non-generic items, an empty type-parameters set. (In eager
67//! collection mode, during incremental compilation, all non-generic functions
68//! are considered as roots, as well as when the `-Clink-dead-code` option is
69//! specified. Functions marked `#[no_mangle]` and functions called by inlinable
70//! functions also always act as roots.)
71//!
72//! ### Finding uses
73//! Given a mono item node, we can discover uses by inspecting its MIR. We walk
74//! the MIR to find other mono items used by each mono item. Since the mono
75//! item we are currently at is always monomorphic, we also know the concrete
76//! type arguments of its used mono items. The specific forms a use can take in
77//! MIR are quite diverse. Here is an overview:
78//!
79//! #### Calling Functions/Methods
80//! The most obvious way for one mono item to use another is a
81//! function or method call (represented by a CALL terminator in MIR). But
82//! calls are not the only thing that might introduce a use between two
83//! function mono items, and as we will see below, they are just a
84//! specialization of the form described next, and consequently will not get any
85//! special treatment in the algorithm.
86//!
87//! #### Taking a reference to a function or method
88//! A function does not need to actually be called in order to be used by
89//! another function. It suffices to just take a reference in order to introduce
90//! an edge. Consider the following example:
91//!
92//! ```
93//! # use core::fmt::Display;
94//! fn print_val<T: Display>(x: T) {
95//!     println!("{}", x);
96//! }
97//!
98//! fn call_fn(f: &dyn Fn(i32), x: i32) {
99//!     f(x);
100//! }
101//!
102//! fn main() {
103//!     let print_i32 = print_val::<i32>;
104//!     call_fn(&print_i32, 0);
105//! }
106//! ```
107//! The MIR of none of these functions will contain an explicit call to
108//! `print_val::<i32>`. Nonetheless, in order to mono this program, we need
109//! an instance of this function. Thus, whenever we encounter a function or
110//! method in operand position, we treat it as a use of the current
111//! mono item. Calls are just a special case of that.
112//!
113//! #### Drop glue
114//! Drop glue mono items are introduced by MIR drop-statements. The
115//! generated mono item will have additional drop-glue item uses if the
116//! type to be dropped contains nested values that also need to be dropped. It
117//! might also have a function item use for the explicit `Drop::drop`
118//! implementation of its type.
119//!
120//! #### Unsizing Casts
121//! A subtle way of introducing use edges is by casting to a trait object.
122//! Since the resulting wide-pointer contains a reference to a vtable, we need to
123//! instantiate all dyn-compatible methods of the trait, as we need to store
124//! pointers to these functions even if they never get called anywhere. This can
125//! be seen as a special case of taking a function reference.
126//!
127//!
128//! Interaction with Cross-Crate Inlining
129//! -------------------------------------
130//! The binary of a crate will not only contain machine code for the items
131//! defined in the source code of that crate. It will also contain monomorphic
132//! instantiations of any extern generic functions and of functions marked with
133//! `#[inline]`.
134//! The collection algorithm handles this more or less mono. If it is
135//! about to create a mono item for something with an external `DefId`,
136//! it will take a look if the MIR for that item is available, and if so just
137//! proceed normally. If the MIR is not available, it assumes that the item is
138//! just linked to and no node is created; which is exactly what we want, since
139//! no machine code should be generated in the current crate for such an item.
140//!
141//! Eager and Lazy Collection Strategy
142//! ----------------------------------
143//! Mono item collection can be performed with one of two strategies:
144//!
145//! - Lazy strategy means that items will only be instantiated when actually
146//!   used. The goal is to produce the least amount of machine code
147//!   possible.
148//!
149//! - Eager strategy is meant to be used in conjunction with incremental compilation
150//!   where a stable set of mono items is more important than a minimal
151//!   one. Thus, eager strategy will instantiate drop-glue for every drop-able type
152//!   in the crate, even if no drop call for that type exists (yet). It will
153//!   also instantiate default implementations of trait methods, something that
154//!   otherwise is only done on demand.
155//!
156//! Collection-time const evaluation and "mentioned" items
157//! ------------------------------------------------------
158//!
159//! One important role of collection is to evaluate all constants that are used by all the items
160//! which are being collected. Codegen can then rely on only encountering constants that evaluate
161//! successfully, and if a constant fails to evaluate, the collector has much better context to be
162//! able to show where this constant comes up.
163//!
164//! However, the exact set of "used" items (collected as described above), and therefore the exact
165//! set of used constants, can depend on optimizations. Optimizing away dead code may optimize away
166//! a function call that uses a failing constant, so an unoptimized build may fail where an
167//! optimized build succeeds. This is undesirable.
168//!
169//! To avoid this, the collector has the concept of "mentioned" items. Some time during the MIR
170//! pipeline, before any optimization-level-dependent optimizations, we compute a list of all items
171//! that syntactically appear in the code. These are considered "mentioned", and even if they are in
172//! dead code and get optimized away (which makes them no longer "used"), they are still
173//! "mentioned". For every used item, the collector ensures that all mentioned items, recursively,
174//! do not use a failing constant. This is reflected via the [`CollectionMode`], which determines
175//! whether we are visiting a used item or merely a mentioned item.
176//!
177//! The collector and "mentioned items" gathering (which lives in `rustc_mir_transform::mentioned_items`)
178//! need to stay in sync in the following sense:
179//!
180//! - For every item that the collector gather that could eventually lead to build failure (most
181//!   likely due to containing a constant that fails to evaluate), a corresponding mentioned item
182//!   must be added. This should use the exact same strategy as the ecollector to make sure they are
183//!   in sync. However, while the collector works on monomorphized types, mentioned items are
184//!   collected on generic MIR -- so any time the collector checks for a particular type (such as
185//!   `ty::FnDef`), we have to just onconditionally add this as a mentioned item.
186//! - In `visit_mentioned_item`, we then do with that mentioned item exactly what the collector
187//!   would have done during regular MIR visiting. Basically you can think of the collector having
188//!   two stages, a pre-monomorphization stage and a post-monomorphization stage (usually quite
189//!   literally separated by a call to `self.monomorphize`); the pre-monomorphizationn stage is
190//!   duplicated in mentioned items gathering and the post-monomorphization stage is duplicated in
191//!   `visit_mentioned_item`.
192//! - Finally, as a performance optimization, the collector should fill `used_mentioned_item` during
193//!   its MIR traversal with exactly what mentioned item gathering would have added in the same
194//!   situation. This detects mentioned items that have *not* been optimized away and hence don't
195//!   need a dedicated traversal.
196//!
197//! Open Issues
198//! -----------
199//! Some things are not yet fully implemented in the current version of this
200//! module.
201//!
202//! ### Const Fns
203//! Ideally, no mono item should be generated for const fns unless there
204//! is a call to them that cannot be evaluated at compile time. At the moment
205//! this is not implemented however: a mono item will be produced
206//! regardless of whether it is actually needed or not.
207
208use std::path::PathBuf;
209
210use rustc_attr_parsing::InlineAttr;
211use rustc_data_structures::fx::FxIndexMap;
212use rustc_data_structures::sync::{MTLock, par_for_each_in};
213use rustc_data_structures::unord::{UnordMap, UnordSet};
214use rustc_hir as hir;
215use rustc_hir::def::DefKind;
216use rustc_hir::def_id::{DefId, DefIdMap, LocalDefId};
217use rustc_hir::lang_items::LangItem;
218use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
219use rustc_middle::mir::interpret::{AllocId, ErrorHandled, GlobalAlloc, Scalar};
220use rustc_middle::mir::mono::{CollectionMode, InstantiationMode, MonoItem};
221use rustc_middle::mir::visit::Visitor as MirVisitor;
222use rustc_middle::mir::{self, Location, MentionedItem, traversal};
223use rustc_middle::query::TyCtxtAt;
224use rustc_middle::ty::adjustment::{CustomCoerceUnsized, PointerCoercion};
225use rustc_middle::ty::layout::ValidityRequirement;
226use rustc_middle::ty::print::{shrunk_instance_name, with_no_trimmed_paths};
227use rustc_middle::ty::{
228    self, GenericArgs, GenericParamDefKind, Instance, InstanceKind, Interner, Ty, TyCtxt,
229    TypeFoldable, TypeVisitableExt, VtblEntry,
230};
231use rustc_middle::util::Providers;
232use rustc_middle::{bug, span_bug};
233use rustc_session::Limit;
234use rustc_session::config::EntryFnType;
235use rustc_span::source_map::{Spanned, dummy_spanned, respan};
236use rustc_span::{DUMMY_SP, Span};
237use tracing::{debug, instrument, trace};
238
239use crate::errors::{self, EncounteredErrorWhileInstantiating, NoOptimizedMir, RecursionLimit};
240
241#[derive(PartialEq)]
242pub(crate) enum MonoItemCollectionStrategy {
243    Eager,
244    Lazy,
245}
246
247/// The state that is shared across the concurrent threads that are doing collection.
248struct SharedState<'tcx> {
249    /// Items that have been or are currently being recursively collected.
250    visited: MTLock<UnordSet<MonoItem<'tcx>>>,
251    /// Items that have been or are currently being recursively treated as "mentioned", i.e., their
252    /// consts are evaluated but nothing is added to the collection.
253    mentioned: MTLock<UnordSet<MonoItem<'tcx>>>,
254    /// Which items are being used where, for better errors.
255    usage_map: MTLock<UsageMap<'tcx>>,
256}
257
258pub(crate) struct UsageMap<'tcx> {
259    // Maps every mono item to the mono items used by it.
260    pub used_map: UnordMap<MonoItem<'tcx>, Vec<MonoItem<'tcx>>>,
261
262    // Maps every mono item to the mono items that use it.
263    user_map: UnordMap<MonoItem<'tcx>, Vec<MonoItem<'tcx>>>,
264}
265
266impl<'tcx> UsageMap<'tcx> {
267    fn new() -> UsageMap<'tcx> {
268        UsageMap { used_map: Default::default(), user_map: Default::default() }
269    }
270
271    fn record_used<'a>(&mut self, user_item: MonoItem<'tcx>, used_items: &'a MonoItems<'tcx>)
272    where
273        'tcx: 'a,
274    {
275        for used_item in used_items.items() {
276            self.user_map.entry(used_item).or_default().push(user_item);
277        }
278
279        assert!(self.used_map.insert(user_item, used_items.items().collect()).is_none());
280    }
281
282    pub(crate) fn get_user_items(&self, item: MonoItem<'tcx>) -> &[MonoItem<'tcx>] {
283        self.user_map.get(&item).map(|items| items.as_slice()).unwrap_or(&[])
284    }
285
286    /// Internally iterate over all inlined items used by `item`.
287    pub(crate) fn for_each_inlined_used_item<F>(
288        &self,
289        tcx: TyCtxt<'tcx>,
290        item: MonoItem<'tcx>,
291        mut f: F,
292    ) where
293        F: FnMut(MonoItem<'tcx>),
294    {
295        let used_items = self.used_map.get(&item).unwrap();
296        for used_item in used_items.iter() {
297            let is_inlined = used_item.instantiation_mode(tcx) == InstantiationMode::LocalCopy;
298            if is_inlined {
299                f(*used_item);
300            }
301        }
302    }
303}
304
305struct MonoItems<'tcx> {
306    // We want a set of MonoItem + Span where trying to re-insert a MonoItem with a different Span
307    // is ignored. Map does that, but it looks odd.
308    items: FxIndexMap<MonoItem<'tcx>, Span>,
309}
310
311impl<'tcx> MonoItems<'tcx> {
312    fn new() -> Self {
313        Self { items: FxIndexMap::default() }
314    }
315
316    fn is_empty(&self) -> bool {
317        self.items.is_empty()
318    }
319
320    fn push(&mut self, item: Spanned<MonoItem<'tcx>>) {
321        // Insert only if the entry does not exist. A normal insert would stomp the first span that
322        // got inserted.
323        self.items.entry(item.node).or_insert(item.span);
324    }
325
326    fn items(&self) -> impl Iterator<Item = MonoItem<'tcx>> {
327        self.items.keys().cloned()
328    }
329}
330
331impl<'tcx> IntoIterator for MonoItems<'tcx> {
332    type Item = Spanned<MonoItem<'tcx>>;
333    type IntoIter = impl Iterator<Item = Spanned<MonoItem<'tcx>>>;
334
335    fn into_iter(self) -> Self::IntoIter {
336        self.items.into_iter().map(|(item, span)| respan(span, item))
337    }
338}
339
340impl<'tcx> Extend<Spanned<MonoItem<'tcx>>> for MonoItems<'tcx> {
341    fn extend<I>(&mut self, iter: I)
342    where
343        I: IntoIterator<Item = Spanned<MonoItem<'tcx>>>,
344    {
345        for item in iter {
346            self.push(item)
347        }
348    }
349}
350
351/// Collect all monomorphized items reachable from `starting_point`, and emit a note diagnostic if a
352/// post-monomorphization error is encountered during a collection step.
353///
354/// `mode` determined whether we are scanning for [used items][CollectionMode::UsedItems]
355/// or [mentioned items][CollectionMode::MentionedItems].
356#[instrument(skip(tcx, state, recursion_depths, recursion_limit), level = "debug")]
357fn collect_items_rec<'tcx>(
358    tcx: TyCtxt<'tcx>,
359    starting_item: Spanned<MonoItem<'tcx>>,
360    state: &SharedState<'tcx>,
361    recursion_depths: &mut DefIdMap<usize>,
362    recursion_limit: Limit,
363    mode: CollectionMode,
364) {
365    if mode == CollectionMode::UsedItems {
366        if !state.visited.lock_mut().insert(starting_item.node) {
367            // We've been here already, no need to search again.
368            return;
369        }
370    } else {
371        if state.visited.lock().contains(&starting_item.node) {
372            // We've already done a *full* visit on this one, no need to do the "mention" visit.
373            return;
374        }
375        if !state.mentioned.lock_mut().insert(starting_item.node) {
376            // We've been here already, no need to search again.
377            return;
378        }
379        // There's some risk that we first do a 'mention' visit and then a full visit. But there's no
380        // harm in that, the mention visit will trigger all the queries and the results are cached.
381    }
382
383    let mut used_items = MonoItems::new();
384    let mut mentioned_items = MonoItems::new();
385    let recursion_depth_reset;
386
387    // Post-monomorphization errors MVP
388    //
389    // We can encounter errors while monomorphizing an item, but we don't have a good way of
390    // showing a complete stack of spans ultimately leading to collecting the erroneous one yet.
391    // (It's also currently unclear exactly which diagnostics and information would be interesting
392    // to report in such cases)
393    //
394    // This leads to suboptimal error reporting: a post-monomorphization error (PME) will be
395    // shown with just a spanned piece of code causing the error, without information on where
396    // it was called from. This is especially obscure if the erroneous mono item is in a
397    // dependency. See for example issue #85155, where, before minimization, a PME happened two
398    // crates downstream from libcore's stdarch, without a way to know which dependency was the
399    // cause.
400    //
401    // If such an error occurs in the current crate, its span will be enough to locate the
402    // source. If the cause is in another crate, the goal here is to quickly locate which mono
403    // item in the current crate is ultimately responsible for causing the error.
404    //
405    // To give at least _some_ context to the user: while collecting mono items, we check the
406    // error count. If it has changed, a PME occurred, and we trigger some diagnostics about the
407    // current step of mono items collection.
408    //
409    // FIXME: don't rely on global state, instead bubble up errors. Note: this is very hard to do.
410    let error_count = tcx.dcx().err_count();
411
412    // In `mentioned_items` we collect items that were mentioned in this MIR but possibly do not
413    // need to be monomorphized. This is done to ensure that optimizing away function calls does not
414    // hide const-eval errors that those calls would otherwise have triggered.
415    match starting_item.node {
416        MonoItem::Static(def_id) => {
417            recursion_depth_reset = None;
418
419            // Statics always get evaluated (which is possible because they can't be generic), so for
420            // `MentionedItems` collection there's nothing to do here.
421            if mode == CollectionMode::UsedItems {
422                let instance = Instance::mono(tcx, def_id);
423
424                // Sanity check whether this ended up being collected accidentally
425                debug_assert!(tcx.should_codegen_locally(instance));
426
427                let DefKind::Static { nested, .. } = tcx.def_kind(def_id) else { bug!() };
428                // Nested statics have no type.
429                if !nested {
430                    let ty = instance.ty(tcx, ty::TypingEnv::fully_monomorphized());
431                    visit_drop_use(tcx, ty, true, starting_item.span, &mut used_items);
432                }
433
434                if let Ok(alloc) = tcx.eval_static_initializer(def_id) {
435                    for &prov in alloc.inner().provenance().ptrs().values() {
436                        collect_alloc(tcx, prov.alloc_id(), &mut used_items);
437                    }
438                }
439
440                if tcx.needs_thread_local_shim(def_id) {
441                    used_items.push(respan(
442                        starting_item.span,
443                        MonoItem::Fn(Instance {
444                            def: InstanceKind::ThreadLocalShim(def_id),
445                            args: GenericArgs::empty(),
446                        }),
447                    ));
448                }
449            }
450
451            // mentioned_items stays empty since there's no codegen for statics. statics don't get
452            // optimized, and if they did then the const-eval interpreter would have to worry about
453            // mentioned_items.
454        }
455        MonoItem::Fn(instance) => {
456            // Sanity check whether this ended up being collected accidentally
457            debug_assert!(tcx.should_codegen_locally(instance));
458
459            // Keep track of the monomorphization recursion depth
460            recursion_depth_reset = Some(check_recursion_limit(
461                tcx,
462                instance,
463                starting_item.span,
464                recursion_depths,
465                recursion_limit,
466            ));
467
468            rustc_data_structures::stack::ensure_sufficient_stack(|| {
469                let (used, mentioned) = tcx.items_of_instance((instance, mode));
470                used_items.extend(used.into_iter().copied());
471                mentioned_items.extend(mentioned.into_iter().copied());
472            });
473        }
474        MonoItem::GlobalAsm(item_id) => {
475            assert!(
476                mode == CollectionMode::UsedItems,
477                "should never encounter global_asm when collecting mentioned items"
478            );
479            recursion_depth_reset = None;
480
481            let item = tcx.hir_item(item_id);
482            if let hir::ItemKind::GlobalAsm { asm, .. } = item.kind {
483                for (op, op_sp) in asm.operands {
484                    match *op {
485                        hir::InlineAsmOperand::Const { .. } => {
486                            // Only constants which resolve to a plain integer
487                            // are supported. Therefore the value should not
488                            // depend on any other items.
489                        }
490                        hir::InlineAsmOperand::SymFn { expr } => {
491                            let fn_ty = tcx.typeck(item_id.owner_id).expr_ty(expr);
492                            visit_fn_use(tcx, fn_ty, false, *op_sp, &mut used_items);
493                        }
494                        hir::InlineAsmOperand::SymStatic { path: _, def_id } => {
495                            let instance = Instance::mono(tcx, def_id);
496                            if tcx.should_codegen_locally(instance) {
497                                trace!("collecting static {:?}", def_id);
498                                used_items.push(dummy_spanned(MonoItem::Static(def_id)));
499                            }
500                        }
501                        hir::InlineAsmOperand::In { .. }
502                        | hir::InlineAsmOperand::Out { .. }
503                        | hir::InlineAsmOperand::InOut { .. }
504                        | hir::InlineAsmOperand::SplitInOut { .. }
505                        | hir::InlineAsmOperand::Label { .. } => {
506                            span_bug!(*op_sp, "invalid operand type for global_asm!")
507                        }
508                    }
509                }
510            } else {
511                span_bug!(item.span, "Mismatch between hir::Item type and MonoItem type")
512            }
513
514            // mention_items stays empty as nothing gets optimized here.
515        }
516    };
517
518    // Check for PMEs and emit a diagnostic if one happened. To try to show relevant edges of the
519    // mono item graph.
520    if tcx.dcx().err_count() > error_count
521        && starting_item.node.is_generic_fn()
522        && starting_item.node.is_user_defined()
523    {
524        let formatted_item = with_no_trimmed_paths!(starting_item.node.to_string());
525        tcx.dcx().emit_note(EncounteredErrorWhileInstantiating {
526            span: starting_item.span,
527            formatted_item,
528        });
529    }
530    // Only updating `usage_map` for used items as otherwise we may be inserting the same item
531    // multiple times (if it is first 'mentioned' and then later actuall used), and the usage map
532    // logic does not like that.
533    // This is part of the output of collection and hence only relevant for "used" items.
534    // ("Mentioned" items are only considered internally during collection.)
535    if mode == CollectionMode::UsedItems {
536        state.usage_map.lock_mut().record_used(starting_item.node, &used_items);
537    }
538
539    if mode == CollectionMode::MentionedItems {
540        assert!(used_items.is_empty(), "'mentioned' collection should never encounter used items");
541    } else {
542        for used_item in used_items {
543            collect_items_rec(
544                tcx,
545                used_item,
546                state,
547                recursion_depths,
548                recursion_limit,
549                CollectionMode::UsedItems,
550            );
551        }
552    }
553
554    // Walk over mentioned items *after* used items, so that if an item is both mentioned and used then
555    // the loop above has fully collected it, so this loop will skip it.
556    for mentioned_item in mentioned_items {
557        collect_items_rec(
558            tcx,
559            mentioned_item,
560            state,
561            recursion_depths,
562            recursion_limit,
563            CollectionMode::MentionedItems,
564        );
565    }
566
567    if let Some((def_id, depth)) = recursion_depth_reset {
568        recursion_depths.insert(def_id, depth);
569    }
570}
571
572fn check_recursion_limit<'tcx>(
573    tcx: TyCtxt<'tcx>,
574    instance: Instance<'tcx>,
575    span: Span,
576    recursion_depths: &mut DefIdMap<usize>,
577    recursion_limit: Limit,
578) -> (DefId, usize) {
579    let def_id = instance.def_id();
580    let recursion_depth = recursion_depths.get(&def_id).cloned().unwrap_or(0);
581    debug!(" => recursion depth={}", recursion_depth);
582
583    let adjusted_recursion_depth = if tcx.is_lang_item(def_id, LangItem::DropInPlace) {
584        // HACK: drop_in_place creates tight monomorphization loops. Give
585        // it more margin.
586        recursion_depth / 4
587    } else {
588        recursion_depth
589    };
590
591    // Code that needs to instantiate the same function recursively
592    // more than the recursion limit is assumed to be causing an
593    // infinite expansion.
594    if !recursion_limit.value_within_limit(adjusted_recursion_depth) {
595        let def_span = tcx.def_span(def_id);
596        let def_path_str = tcx.def_path_str(def_id);
597        let (shrunk, written_to_path) = shrunk_instance_name(tcx, instance);
598        let mut path = PathBuf::new();
599        let was_written = if let Some(written_to_path) = written_to_path {
600            path = written_to_path;
601            true
602        } else {
603            false
604        };
605        tcx.dcx().emit_fatal(RecursionLimit {
606            span,
607            shrunk,
608            def_span,
609            def_path_str,
610            was_written,
611            path,
612        });
613    }
614
615    recursion_depths.insert(def_id, recursion_depth + 1);
616
617    (def_id, recursion_depth)
618}
619
620struct MirUsedCollector<'a, 'tcx> {
621    tcx: TyCtxt<'tcx>,
622    body: &'a mir::Body<'tcx>,
623    used_items: &'a mut MonoItems<'tcx>,
624    /// See the comment in `collect_items_of_instance` for the purpose of this set.
625    /// Note that this contains *not-monomorphized* items!
626    used_mentioned_items: &'a mut UnordSet<MentionedItem<'tcx>>,
627    instance: Instance<'tcx>,
628}
629
630impl<'a, 'tcx> MirUsedCollector<'a, 'tcx> {
631    fn monomorphize<T>(&self, value: T) -> T
632    where
633        T: TypeFoldable<TyCtxt<'tcx>>,
634    {
635        trace!("monomorphize: self.instance={:?}", self.instance);
636        self.instance.instantiate_mir_and_normalize_erasing_regions(
637            self.tcx,
638            ty::TypingEnv::fully_monomorphized(),
639            ty::EarlyBinder::bind(value),
640        )
641    }
642
643    /// Evaluates a *not yet monomorphized* constant.
644    fn eval_constant(
645        &mut self,
646        constant: &mir::ConstOperand<'tcx>,
647    ) -> Option<mir::ConstValue<'tcx>> {
648        let const_ = self.monomorphize(constant.const_);
649        // Evaluate the constant. This makes const eval failure a collection-time error (rather than
650        // a codegen-time error). rustc stops after collection if there was an error, so this
651        // ensures codegen never has to worry about failing consts.
652        // (codegen relies on this and ICEs will happen if this is violated.)
653        match const_.eval(self.tcx, ty::TypingEnv::fully_monomorphized(), constant.span) {
654            Ok(v) => Some(v),
655            Err(ErrorHandled::TooGeneric(..)) => span_bug!(
656                constant.span,
657                "collection encountered polymorphic constant: {:?}",
658                const_
659            ),
660            Err(err @ ErrorHandled::Reported(..)) => {
661                err.emit_note(self.tcx);
662                return None;
663            }
664        }
665    }
666}
667
668impl<'a, 'tcx> MirVisitor<'tcx> for MirUsedCollector<'a, 'tcx> {
669    fn visit_rvalue(&mut self, rvalue: &mir::Rvalue<'tcx>, location: Location) {
670        debug!("visiting rvalue {:?}", *rvalue);
671
672        let span = self.body.source_info(location).span;
673
674        match *rvalue {
675            // When doing an cast from a regular pointer to a wide pointer, we
676            // have to instantiate all methods of the trait being cast to, so we
677            // can build the appropriate vtable.
678            mir::Rvalue::Cast(
679                mir::CastKind::PointerCoercion(PointerCoercion::Unsize, _)
680                | mir::CastKind::PointerCoercion(PointerCoercion::DynStar, _),
681                ref operand,
682                target_ty,
683            ) => {
684                let source_ty = operand.ty(self.body, self.tcx);
685                // *Before* monomorphizing, record that we already handled this mention.
686                self.used_mentioned_items
687                    .insert(MentionedItem::UnsizeCast { source_ty, target_ty });
688                let target_ty = self.monomorphize(target_ty);
689                let source_ty = self.monomorphize(source_ty);
690                let (source_ty, target_ty) =
691                    find_vtable_types_for_unsizing(self.tcx.at(span), source_ty, target_ty);
692                // This could also be a different Unsize instruction, like
693                // from a fixed sized array to a slice. But we are only
694                // interested in things that produce a vtable.
695                if (target_ty.is_trait() && !source_ty.is_trait())
696                    || (target_ty.is_dyn_star() && !source_ty.is_dyn_star())
697                {
698                    create_mono_items_for_vtable_methods(
699                        self.tcx,
700                        target_ty,
701                        source_ty,
702                        span,
703                        self.used_items,
704                    );
705                }
706            }
707            mir::Rvalue::Cast(
708                mir::CastKind::PointerCoercion(PointerCoercion::ReifyFnPointer, _),
709                ref operand,
710                _,
711            ) => {
712                let fn_ty = operand.ty(self.body, self.tcx);
713                // *Before* monomorphizing, record that we already handled this mention.
714                self.used_mentioned_items.insert(MentionedItem::Fn(fn_ty));
715                let fn_ty = self.monomorphize(fn_ty);
716                visit_fn_use(self.tcx, fn_ty, false, span, self.used_items);
717            }
718            mir::Rvalue::Cast(
719                mir::CastKind::PointerCoercion(PointerCoercion::ClosureFnPointer(_), _),
720                ref operand,
721                _,
722            ) => {
723                let source_ty = operand.ty(self.body, self.tcx);
724                // *Before* monomorphizing, record that we already handled this mention.
725                self.used_mentioned_items.insert(MentionedItem::Closure(source_ty));
726                let source_ty = self.monomorphize(source_ty);
727                if let ty::Closure(def_id, args) = *source_ty.kind() {
728                    let instance =
729                        Instance::resolve_closure(self.tcx, def_id, args, ty::ClosureKind::FnOnce);
730                    if self.tcx.should_codegen_locally(instance) {
731                        self.used_items.push(create_fn_mono_item(self.tcx, instance, span));
732                    }
733                } else {
734                    bug!()
735                }
736            }
737            mir::Rvalue::ThreadLocalRef(def_id) => {
738                assert!(self.tcx.is_thread_local_static(def_id));
739                let instance = Instance::mono(self.tcx, def_id);
740                if self.tcx.should_codegen_locally(instance) {
741                    trace!("collecting thread-local static {:?}", def_id);
742                    self.used_items.push(respan(span, MonoItem::Static(def_id)));
743                }
744            }
745            _ => { /* not interesting */ }
746        }
747
748        self.super_rvalue(rvalue, location);
749    }
750
751    /// This does not walk the MIR of the constant as that is not needed for codegen, all we need is
752    /// to ensure that the constant evaluates successfully and walk the result.
753    #[instrument(skip(self), level = "debug")]
754    fn visit_const_operand(&mut self, constant: &mir::ConstOperand<'tcx>, _location: Location) {
755        // No `super_constant` as we don't care about `visit_ty`/`visit_ty_const`.
756        let Some(val) = self.eval_constant(constant) else { return };
757        collect_const_value(self.tcx, val, self.used_items);
758    }
759
760    fn visit_terminator(&mut self, terminator: &mir::Terminator<'tcx>, location: Location) {
761        debug!("visiting terminator {:?} @ {:?}", terminator, location);
762        let source = self.body.source_info(location).span;
763
764        let tcx = self.tcx;
765        let push_mono_lang_item = |this: &mut Self, lang_item: LangItem| {
766            let instance = Instance::mono(tcx, tcx.require_lang_item(lang_item, Some(source)));
767            if tcx.should_codegen_locally(instance) {
768                this.used_items.push(create_fn_mono_item(tcx, instance, source));
769            }
770        };
771
772        match terminator.kind {
773            mir::TerminatorKind::Call { ref func, .. }
774            | mir::TerminatorKind::TailCall { ref func, .. } => {
775                let callee_ty = func.ty(self.body, tcx);
776                // *Before* monomorphizing, record that we already handled this mention.
777                self.used_mentioned_items.insert(MentionedItem::Fn(callee_ty));
778                let callee_ty = self.monomorphize(callee_ty);
779                visit_fn_use(self.tcx, callee_ty, true, source, &mut self.used_items)
780            }
781            mir::TerminatorKind::Drop { ref place, .. } => {
782                let ty = place.ty(self.body, self.tcx).ty;
783                // *Before* monomorphizing, record that we already handled this mention.
784                self.used_mentioned_items.insert(MentionedItem::Drop(ty));
785                let ty = self.monomorphize(ty);
786                visit_drop_use(self.tcx, ty, true, source, self.used_items);
787            }
788            mir::TerminatorKind::InlineAsm { ref operands, .. } => {
789                for op in operands {
790                    match *op {
791                        mir::InlineAsmOperand::SymFn { ref value } => {
792                            let fn_ty = value.const_.ty();
793                            // *Before* monomorphizing, record that we already handled this mention.
794                            self.used_mentioned_items.insert(MentionedItem::Fn(fn_ty));
795                            let fn_ty = self.monomorphize(fn_ty);
796                            visit_fn_use(self.tcx, fn_ty, false, source, self.used_items);
797                        }
798                        mir::InlineAsmOperand::SymStatic { def_id } => {
799                            let instance = Instance::mono(self.tcx, def_id);
800                            if self.tcx.should_codegen_locally(instance) {
801                                trace!("collecting asm sym static {:?}", def_id);
802                                self.used_items.push(respan(source, MonoItem::Static(def_id)));
803                            }
804                        }
805                        _ => {}
806                    }
807                }
808            }
809            mir::TerminatorKind::Assert { ref msg, .. } => match &**msg {
810                mir::AssertKind::BoundsCheck { .. } => {
811                    push_mono_lang_item(self, LangItem::PanicBoundsCheck);
812                }
813                mir::AssertKind::MisalignedPointerDereference { .. } => {
814                    push_mono_lang_item(self, LangItem::PanicMisalignedPointerDereference);
815                }
816                mir::AssertKind::NullPointerDereference => {
817                    push_mono_lang_item(self, LangItem::PanicNullPointerDereference);
818                }
819                _ => {
820                    push_mono_lang_item(self, msg.panic_function());
821                }
822            },
823            mir::TerminatorKind::UnwindTerminate(reason) => {
824                push_mono_lang_item(self, reason.lang_item());
825            }
826            mir::TerminatorKind::Goto { .. }
827            | mir::TerminatorKind::SwitchInt { .. }
828            | mir::TerminatorKind::UnwindResume
829            | mir::TerminatorKind::Return
830            | mir::TerminatorKind::Unreachable => {}
831            mir::TerminatorKind::CoroutineDrop
832            | mir::TerminatorKind::Yield { .. }
833            | mir::TerminatorKind::FalseEdge { .. }
834            | mir::TerminatorKind::FalseUnwind { .. } => bug!(),
835        }
836
837        if let Some(mir::UnwindAction::Terminate(reason)) = terminator.unwind() {
838            push_mono_lang_item(self, reason.lang_item());
839        }
840
841        self.super_terminator(terminator, location);
842    }
843}
844
845fn visit_drop_use<'tcx>(
846    tcx: TyCtxt<'tcx>,
847    ty: Ty<'tcx>,
848    is_direct_call: bool,
849    source: Span,
850    output: &mut MonoItems<'tcx>,
851) {
852    let instance = Instance::resolve_drop_in_place(tcx, ty);
853    visit_instance_use(tcx, instance, is_direct_call, source, output);
854}
855
856/// For every call of this function in the visitor, make sure there is a matching call in the
857/// `mentioned_items` pass!
858fn visit_fn_use<'tcx>(
859    tcx: TyCtxt<'tcx>,
860    ty: Ty<'tcx>,
861    is_direct_call: bool,
862    source: Span,
863    output: &mut MonoItems<'tcx>,
864) {
865    if let ty::FnDef(def_id, args) = *ty.kind() {
866        let instance = if is_direct_call {
867            ty::Instance::expect_resolve(
868                tcx,
869                ty::TypingEnv::fully_monomorphized(),
870                def_id,
871                args,
872                source,
873            )
874        } else {
875            match ty::Instance::resolve_for_fn_ptr(
876                tcx,
877                ty::TypingEnv::fully_monomorphized(),
878                def_id,
879                args,
880            ) {
881                Some(instance) => instance,
882                _ => bug!("failed to resolve instance for {ty}"),
883            }
884        };
885        visit_instance_use(tcx, instance, is_direct_call, source, output);
886    }
887}
888
889fn visit_instance_use<'tcx>(
890    tcx: TyCtxt<'tcx>,
891    instance: ty::Instance<'tcx>,
892    is_direct_call: bool,
893    source: Span,
894    output: &mut MonoItems<'tcx>,
895) {
896    debug!("visit_item_use({:?}, is_direct_call={:?})", instance, is_direct_call);
897    if !tcx.should_codegen_locally(instance) {
898        return;
899    }
900    if let Some(intrinsic) = tcx.intrinsic(instance.def_id()) {
901        if let Some(_requirement) = ValidityRequirement::from_intrinsic(intrinsic.name) {
902            // The intrinsics assert_inhabited, assert_zero_valid, and assert_mem_uninitialized_valid will
903            // be lowered in codegen to nothing or a call to panic_nounwind. So if we encounter any
904            // of those intrinsics, we need to include a mono item for panic_nounwind, else we may try to
905            // codegen a call to that function without generating code for the function itself.
906            let def_id = tcx.require_lang_item(LangItem::PanicNounwind, None);
907            let panic_instance = Instance::mono(tcx, def_id);
908            if tcx.should_codegen_locally(panic_instance) {
909                output.push(create_fn_mono_item(tcx, panic_instance, source));
910            }
911        } else if !intrinsic.must_be_overridden {
912            // Codegen the fallback body of intrinsics with fallback bodies.
913            // We explicitly skip this otherwise to ensure we get a linker error
914            // if anyone tries to call this intrinsic and the codegen backend did not
915            // override the implementation.
916            let instance = ty::Instance::new(instance.def_id(), instance.args);
917            if tcx.should_codegen_locally(instance) {
918                output.push(create_fn_mono_item(tcx, instance, source));
919            }
920        }
921    }
922
923    match instance.def {
924        ty::InstanceKind::Virtual(..) | ty::InstanceKind::Intrinsic(_) => {
925            if !is_direct_call {
926                bug!("{:?} being reified", instance);
927            }
928        }
929        ty::InstanceKind::ThreadLocalShim(..) => {
930            bug!("{:?} being reified", instance);
931        }
932        ty::InstanceKind::DropGlue(_, None) | ty::InstanceKind::AsyncDropGlueCtorShim(_, None) => {
933            // Don't need to emit noop drop glue if we are calling directly.
934            if !is_direct_call {
935                output.push(create_fn_mono_item(tcx, instance, source));
936            }
937        }
938        ty::InstanceKind::DropGlue(_, Some(_))
939        | ty::InstanceKind::AsyncDropGlueCtorShim(_, Some(_))
940        | ty::InstanceKind::VTableShim(..)
941        | ty::InstanceKind::ReifyShim(..)
942        | ty::InstanceKind::ClosureOnceShim { .. }
943        | ty::InstanceKind::ConstructCoroutineInClosureShim { .. }
944        | ty::InstanceKind::Item(..)
945        | ty::InstanceKind::FnPtrShim(..)
946        | ty::InstanceKind::CloneShim(..)
947        | ty::InstanceKind::FnPtrAddrShim(..) => {
948            output.push(create_fn_mono_item(tcx, instance, source));
949        }
950    }
951}
952
953/// Returns `true` if we should codegen an instance in the local crate, or returns `false` if we
954/// can just link to the upstream crate and therefore don't need a mono item.
955fn should_codegen_locally<'tcx>(tcx: TyCtxt<'tcx>, instance: Instance<'tcx>) -> bool {
956    let Some(def_id) = instance.def.def_id_if_not_guaranteed_local_codegen() else {
957        return true;
958    };
959
960    if tcx.is_foreign_item(def_id) {
961        // Foreign items are always linked against, there's no way of instantiating them.
962        return false;
963    }
964
965    if tcx.def_kind(def_id).has_codegen_attrs()
966        && matches!(tcx.codegen_fn_attrs(def_id).inline, InlineAttr::Force { .. })
967    {
968        // `#[rustc_force_inline]` items should never be codegened. This should be caught by
969        // the MIR validator.
970        tcx.delay_bug("attempt to codegen `#[rustc_force_inline]` item");
971    }
972
973    if def_id.is_local() {
974        // Local items cannot be referred to locally without monomorphizing them locally.
975        return true;
976    }
977
978    if tcx.is_reachable_non_generic(def_id) || instance.upstream_monomorphization(tcx).is_some() {
979        // We can link to the item in question, no instance needed in this crate.
980        return false;
981    }
982
983    if let DefKind::Static { .. } = tcx.def_kind(def_id) {
984        // We cannot monomorphize statics from upstream crates.
985        return false;
986    }
987
988    if !tcx.is_mir_available(def_id) {
989        tcx.dcx().emit_fatal(NoOptimizedMir {
990            span: tcx.def_span(def_id),
991            crate_name: tcx.crate_name(def_id.krate),
992        });
993    }
994
995    true
996}
997
998/// For a given pair of source and target type that occur in an unsizing coercion,
999/// this function finds the pair of types that determines the vtable linking
1000/// them.
1001///
1002/// For example, the source type might be `&SomeStruct` and the target type
1003/// might be `&dyn SomeTrait` in a cast like:
1004///
1005/// ```rust,ignore (not real code)
1006/// let src: &SomeStruct = ...;
1007/// let target = src as &dyn SomeTrait;
1008/// ```
1009///
1010/// Then the output of this function would be (SomeStruct, SomeTrait) since for
1011/// constructing the `target` wide-pointer we need the vtable for that pair.
1012///
1013/// Things can get more complicated though because there's also the case where
1014/// the unsized type occurs as a field:
1015///
1016/// ```rust
1017/// struct ComplexStruct<T: ?Sized> {
1018///    a: u32,
1019///    b: f64,
1020///    c: T
1021/// }
1022/// ```
1023///
1024/// In this case, if `T` is sized, `&ComplexStruct<T>` is a thin pointer. If `T`
1025/// is unsized, `&SomeStruct` is a wide pointer, and the vtable it points to is
1026/// for the pair of `T` (which is a trait) and the concrete type that `T` was
1027/// originally coerced from:
1028///
1029/// ```rust,ignore (not real code)
1030/// let src: &ComplexStruct<SomeStruct> = ...;
1031/// let target = src as &ComplexStruct<dyn SomeTrait>;
1032/// ```
1033///
1034/// Again, we want this `find_vtable_types_for_unsizing()` to provide the pair
1035/// `(SomeStruct, SomeTrait)`.
1036///
1037/// Finally, there is also the case of custom unsizing coercions, e.g., for
1038/// smart pointers such as `Rc` and `Arc`.
1039fn find_vtable_types_for_unsizing<'tcx>(
1040    tcx: TyCtxtAt<'tcx>,
1041    source_ty: Ty<'tcx>,
1042    target_ty: Ty<'tcx>,
1043) -> (Ty<'tcx>, Ty<'tcx>) {
1044    let ptr_vtable = |inner_source: Ty<'tcx>, inner_target: Ty<'tcx>| {
1045        let typing_env = ty::TypingEnv::fully_monomorphized();
1046        if tcx.type_has_metadata(inner_source, typing_env) {
1047            (inner_source, inner_target)
1048        } else {
1049            tcx.struct_lockstep_tails_for_codegen(inner_source, inner_target, typing_env)
1050        }
1051    };
1052
1053    match (source_ty.kind(), target_ty.kind()) {
1054        (&ty::Ref(_, a, _), &ty::Ref(_, b, _) | &ty::RawPtr(b, _))
1055        | (&ty::RawPtr(a, _), &ty::RawPtr(b, _)) => ptr_vtable(a, b),
1056        (_, _)
1057            if let Some(source_boxed) = source_ty.boxed_ty()
1058                && let Some(target_boxed) = target_ty.boxed_ty() =>
1059        {
1060            ptr_vtable(source_boxed, target_boxed)
1061        }
1062
1063        // T as dyn* Trait
1064        (_, &ty::Dynamic(_, _, ty::DynStar)) => ptr_vtable(source_ty, target_ty),
1065
1066        (&ty::Adt(source_adt_def, source_args), &ty::Adt(target_adt_def, target_args)) => {
1067            assert_eq!(source_adt_def, target_adt_def);
1068
1069            let CustomCoerceUnsized::Struct(coerce_index) =
1070                match crate::custom_coerce_unsize_info(tcx, source_ty, target_ty) {
1071                    Ok(ccu) => ccu,
1072                    Err(e) => {
1073                        let e = Ty::new_error(tcx.tcx, e);
1074                        return (e, e);
1075                    }
1076                };
1077
1078            let source_fields = &source_adt_def.non_enum_variant().fields;
1079            let target_fields = &target_adt_def.non_enum_variant().fields;
1080
1081            assert!(
1082                coerce_index.index() < source_fields.len()
1083                    && source_fields.len() == target_fields.len()
1084            );
1085
1086            find_vtable_types_for_unsizing(
1087                tcx,
1088                source_fields[coerce_index].ty(*tcx, source_args),
1089                target_fields[coerce_index].ty(*tcx, target_args),
1090            )
1091        }
1092        _ => bug!(
1093            "find_vtable_types_for_unsizing: invalid coercion {:?} -> {:?}",
1094            source_ty,
1095            target_ty
1096        ),
1097    }
1098}
1099
1100#[instrument(skip(tcx), level = "debug", ret)]
1101fn create_fn_mono_item<'tcx>(
1102    tcx: TyCtxt<'tcx>,
1103    instance: Instance<'tcx>,
1104    source: Span,
1105) -> Spanned<MonoItem<'tcx>> {
1106    let def_id = instance.def_id();
1107    if tcx.sess.opts.unstable_opts.profile_closures
1108        && def_id.is_local()
1109        && tcx.is_closure_like(def_id)
1110    {
1111        crate::util::dump_closure_profile(tcx, instance);
1112    }
1113
1114    respan(source, MonoItem::Fn(instance))
1115}
1116
1117/// Creates a `MonoItem` for each method that is referenced by the vtable for
1118/// the given trait/impl pair.
1119fn create_mono_items_for_vtable_methods<'tcx>(
1120    tcx: TyCtxt<'tcx>,
1121    trait_ty: Ty<'tcx>,
1122    impl_ty: Ty<'tcx>,
1123    source: Span,
1124    output: &mut MonoItems<'tcx>,
1125) {
1126    assert!(!trait_ty.has_escaping_bound_vars() && !impl_ty.has_escaping_bound_vars());
1127
1128    let ty::Dynamic(trait_ty, ..) = trait_ty.kind() else {
1129        bug!("create_mono_items_for_vtable_methods: {trait_ty:?} not a trait type");
1130    };
1131    if let Some(principal) = trait_ty.principal() {
1132        let trait_ref =
1133            tcx.instantiate_bound_regions_with_erased(principal.with_self_ty(tcx, impl_ty));
1134        assert!(!trait_ref.has_escaping_bound_vars());
1135
1136        // Walk all methods of the trait, including those of its supertraits
1137        let entries = tcx.vtable_entries(trait_ref);
1138        debug!(?entries);
1139        let methods = entries
1140            .iter()
1141            .filter_map(|entry| match entry {
1142                VtblEntry::MetadataDropInPlace
1143                | VtblEntry::MetadataSize
1144                | VtblEntry::MetadataAlign
1145                | VtblEntry::Vacant => None,
1146                VtblEntry::TraitVPtr(_) => {
1147                    // all super trait items already covered, so skip them.
1148                    None
1149                }
1150                VtblEntry::Method(instance) => {
1151                    Some(*instance).filter(|instance| tcx.should_codegen_locally(*instance))
1152                }
1153            })
1154            .map(|item| create_fn_mono_item(tcx, item, source));
1155        output.extend(methods);
1156    }
1157
1158    // Also add the destructor.
1159    visit_drop_use(tcx, impl_ty, false, source, output);
1160}
1161
1162/// Scans the CTFE alloc in order to find function pointers and statics that must be monomorphized.
1163fn collect_alloc<'tcx>(tcx: TyCtxt<'tcx>, alloc_id: AllocId, output: &mut MonoItems<'tcx>) {
1164    match tcx.global_alloc(alloc_id) {
1165        GlobalAlloc::Static(def_id) => {
1166            assert!(!tcx.is_thread_local_static(def_id));
1167            let instance = Instance::mono(tcx, def_id);
1168            if tcx.should_codegen_locally(instance) {
1169                trace!("collecting static {:?}", def_id);
1170                output.push(dummy_spanned(MonoItem::Static(def_id)));
1171            }
1172        }
1173        GlobalAlloc::Memory(alloc) => {
1174            trace!("collecting {:?} with {:#?}", alloc_id, alloc);
1175            let ptrs = alloc.inner().provenance().ptrs();
1176            // avoid `ensure_sufficient_stack` in the common case of "no pointers"
1177            if !ptrs.is_empty() {
1178                rustc_data_structures::stack::ensure_sufficient_stack(move || {
1179                    for &prov in ptrs.values() {
1180                        collect_alloc(tcx, prov.alloc_id(), output);
1181                    }
1182                });
1183            }
1184        }
1185        GlobalAlloc::Function { instance, .. } => {
1186            if tcx.should_codegen_locally(instance) {
1187                trace!("collecting {:?} with {:#?}", alloc_id, instance);
1188                output.push(create_fn_mono_item(tcx, instance, DUMMY_SP));
1189            }
1190        }
1191        GlobalAlloc::VTable(ty, dyn_ty) => {
1192            let alloc_id = tcx.vtable_allocation((
1193                ty,
1194                dyn_ty
1195                    .principal()
1196                    .map(|principal| tcx.instantiate_bound_regions_with_erased(principal)),
1197            ));
1198            collect_alloc(tcx, alloc_id, output)
1199        }
1200    }
1201}
1202
1203/// Scans the MIR in order to find function calls, closures, and drop-glue.
1204///
1205/// Anything that's found is added to `output`. Furthermore the "mentioned items" of the MIR are returned.
1206#[instrument(skip(tcx), level = "debug")]
1207fn collect_items_of_instance<'tcx>(
1208    tcx: TyCtxt<'tcx>,
1209    instance: Instance<'tcx>,
1210    mode: CollectionMode,
1211) -> (MonoItems<'tcx>, MonoItems<'tcx>) {
1212    // This item is getting monomorphized, do mono-time checks.
1213    tcx.ensure_ok().check_mono_item(instance);
1214
1215    let body = tcx.instance_mir(instance.def);
1216    // Naively, in "used" collection mode, all functions get added to *both* `used_items` and
1217    // `mentioned_items`. Mentioned items processing will then notice that they have already been
1218    // visited, but at that point each mentioned item has been monomorphized, added to the
1219    // `mentioned_items` worklist, and checked in the global set of visited items. To remove that
1220    // overhead, we have a special optimization that avoids adding items to `mentioned_items` when
1221    // they are already added in `used_items`. We could just scan `used_items`, but that's a linear
1222    // scan and not very efficient. Furthermore we can only do that *after* monomorphizing the
1223    // mentioned item. So instead we collect all pre-monomorphized `MentionedItem` that were already
1224    // added to `used_items` in a hash set, which can efficiently query in the
1225    // `body.mentioned_items` loop below without even having to monomorphize the item.
1226    let mut used_items = MonoItems::new();
1227    let mut mentioned_items = MonoItems::new();
1228    let mut used_mentioned_items = Default::default();
1229    let mut collector = MirUsedCollector {
1230        tcx,
1231        body,
1232        used_items: &mut used_items,
1233        used_mentioned_items: &mut used_mentioned_items,
1234        instance,
1235    };
1236
1237    if mode == CollectionMode::UsedItems {
1238        for (bb, data) in traversal::mono_reachable(body, tcx, instance) {
1239            collector.visit_basic_block_data(bb, data)
1240        }
1241    }
1242
1243    // Always visit all `required_consts`, so that we evaluate them and abort compilation if any of
1244    // them errors.
1245    for const_op in body.required_consts() {
1246        if let Some(val) = collector.eval_constant(const_op) {
1247            collect_const_value(tcx, val, &mut mentioned_items);
1248        }
1249    }
1250
1251    // Always gather mentioned items. We try to avoid processing items that we have already added to
1252    // `used_items` above.
1253    for item in body.mentioned_items() {
1254        if !collector.used_mentioned_items.contains(&item.node) {
1255            let item_mono = collector.monomorphize(item.node);
1256            visit_mentioned_item(tcx, &item_mono, item.span, &mut mentioned_items);
1257        }
1258    }
1259
1260    (used_items, mentioned_items)
1261}
1262
1263fn items_of_instance<'tcx>(
1264    tcx: TyCtxt<'tcx>,
1265    (instance, mode): (Instance<'tcx>, CollectionMode),
1266) -> (&'tcx [Spanned<MonoItem<'tcx>>], &'tcx [Spanned<MonoItem<'tcx>>]) {
1267    let (used_items, mentioned_items) = collect_items_of_instance(tcx, instance, mode);
1268
1269    let used_items = tcx.arena.alloc_from_iter(used_items);
1270    let mentioned_items = tcx.arena.alloc_from_iter(mentioned_items);
1271
1272    (used_items, mentioned_items)
1273}
1274
1275/// `item` must be already monomorphized.
1276#[instrument(skip(tcx, span, output), level = "debug")]
1277fn visit_mentioned_item<'tcx>(
1278    tcx: TyCtxt<'tcx>,
1279    item: &MentionedItem<'tcx>,
1280    span: Span,
1281    output: &mut MonoItems<'tcx>,
1282) {
1283    match *item {
1284        MentionedItem::Fn(ty) => {
1285            if let ty::FnDef(def_id, args) = *ty.kind() {
1286                let instance = Instance::expect_resolve(
1287                    tcx,
1288                    ty::TypingEnv::fully_monomorphized(),
1289                    def_id,
1290                    args,
1291                    span,
1292                );
1293                // `visit_instance_use` was written for "used" item collection but works just as well
1294                // for "mentioned" item collection.
1295                // We can set `is_direct_call`; that just means we'll skip a bunch of shims that anyway
1296                // can't have their own failing constants.
1297                visit_instance_use(tcx, instance, /*is_direct_call*/ true, span, output);
1298            }
1299        }
1300        MentionedItem::Drop(ty) => {
1301            visit_drop_use(tcx, ty, /*is_direct_call*/ true, span, output);
1302        }
1303        MentionedItem::UnsizeCast { source_ty, target_ty } => {
1304            let (source_ty, target_ty) =
1305                find_vtable_types_for_unsizing(tcx.at(span), source_ty, target_ty);
1306            // This could also be a different Unsize instruction, like
1307            // from a fixed sized array to a slice. But we are only
1308            // interested in things that produce a vtable.
1309            if (target_ty.is_trait() && !source_ty.is_trait())
1310                || (target_ty.is_dyn_star() && !source_ty.is_dyn_star())
1311            {
1312                create_mono_items_for_vtable_methods(tcx, target_ty, source_ty, span, output);
1313            }
1314        }
1315        MentionedItem::Closure(source_ty) => {
1316            if let ty::Closure(def_id, args) = *source_ty.kind() {
1317                let instance =
1318                    Instance::resolve_closure(tcx, def_id, args, ty::ClosureKind::FnOnce);
1319                if tcx.should_codegen_locally(instance) {
1320                    output.push(create_fn_mono_item(tcx, instance, span));
1321                }
1322            } else {
1323                bug!()
1324            }
1325        }
1326    }
1327}
1328
1329#[instrument(skip(tcx, output), level = "debug")]
1330fn collect_const_value<'tcx>(
1331    tcx: TyCtxt<'tcx>,
1332    value: mir::ConstValue<'tcx>,
1333    output: &mut MonoItems<'tcx>,
1334) {
1335    match value {
1336        mir::ConstValue::Scalar(Scalar::Ptr(ptr, _size)) => {
1337            collect_alloc(tcx, ptr.provenance.alloc_id(), output)
1338        }
1339        mir::ConstValue::Indirect { alloc_id, .. } => collect_alloc(tcx, alloc_id, output),
1340        mir::ConstValue::Slice { data, meta: _ } => {
1341            for &prov in data.inner().provenance().ptrs().values() {
1342                collect_alloc(tcx, prov.alloc_id(), output);
1343            }
1344        }
1345        _ => {}
1346    }
1347}
1348
1349//=-----------------------------------------------------------------------------
1350// Root Collection
1351//=-----------------------------------------------------------------------------
1352
1353// Find all non-generic items by walking the HIR. These items serve as roots to
1354// start monomorphizing from.
1355#[instrument(skip(tcx, mode), level = "debug")]
1356fn collect_roots(tcx: TyCtxt<'_>, mode: MonoItemCollectionStrategy) -> Vec<MonoItem<'_>> {
1357    debug!("collecting roots");
1358    let mut roots = MonoItems::new();
1359
1360    {
1361        let entry_fn = tcx.entry_fn(());
1362
1363        debug!("collect_roots: entry_fn = {:?}", entry_fn);
1364
1365        let mut collector = RootCollector { tcx, strategy: mode, entry_fn, output: &mut roots };
1366
1367        let crate_items = tcx.hir_crate_items(());
1368
1369        for id in crate_items.free_items() {
1370            collector.process_item(id);
1371        }
1372
1373        for id in crate_items.impl_items() {
1374            collector.process_impl_item(id);
1375        }
1376
1377        for id in crate_items.nested_bodies() {
1378            collector.process_nested_body(id);
1379        }
1380
1381        collector.push_extra_entry_roots();
1382    }
1383
1384    // We can only codegen items that are instantiable - items all of
1385    // whose predicates hold. Luckily, items that aren't instantiable
1386    // can't actually be used, so we can just skip codegenning them.
1387    roots
1388        .into_iter()
1389        .filter_map(|Spanned { node: mono_item, .. }| {
1390            mono_item.is_instantiable(tcx).then_some(mono_item)
1391        })
1392        .collect()
1393}
1394
1395struct RootCollector<'a, 'tcx> {
1396    tcx: TyCtxt<'tcx>,
1397    strategy: MonoItemCollectionStrategy,
1398    output: &'a mut MonoItems<'tcx>,
1399    entry_fn: Option<(DefId, EntryFnType)>,
1400}
1401
1402impl<'v> RootCollector<'_, 'v> {
1403    fn process_item(&mut self, id: hir::ItemId) {
1404        match self.tcx.def_kind(id.owner_id) {
1405            DefKind::Enum | DefKind::Struct | DefKind::Union => {
1406                if self.strategy == MonoItemCollectionStrategy::Eager
1407                    && !self.tcx.generics_of(id.owner_id).requires_monomorphization(self.tcx)
1408                {
1409                    debug!("RootCollector: ADT drop-glue for `{id:?}`",);
1410                    let id_args =
1411                        ty::GenericArgs::for_item(self.tcx, id.owner_id.to_def_id(), |param, _| {
1412                            match param.kind {
1413                                GenericParamDefKind::Lifetime => {
1414                                    self.tcx.lifetimes.re_erased.into()
1415                                }
1416                                GenericParamDefKind::Type { .. }
1417                                | GenericParamDefKind::Const { .. } => {
1418                                    unreachable!(
1419                                        "`own_requires_monomorphization` check means that \
1420                                we should have no type/const params"
1421                                    )
1422                                }
1423                            }
1424                        });
1425
1426                    // This type is impossible to instantiate, so we should not try to
1427                    // generate a `drop_in_place` instance for it.
1428                    if self.tcx.instantiate_and_check_impossible_predicates((
1429                        id.owner_id.to_def_id(),
1430                        id_args,
1431                    )) {
1432                        return;
1433                    }
1434
1435                    let ty =
1436                        self.tcx.type_of(id.owner_id.to_def_id()).instantiate(self.tcx, id_args);
1437                    assert!(!ty.has_non_region_param());
1438                    visit_drop_use(self.tcx, ty, true, DUMMY_SP, self.output);
1439                }
1440            }
1441            DefKind::GlobalAsm => {
1442                debug!(
1443                    "RootCollector: ItemKind::GlobalAsm({})",
1444                    self.tcx.def_path_str(id.owner_id)
1445                );
1446                self.output.push(dummy_spanned(MonoItem::GlobalAsm(id)));
1447            }
1448            DefKind::Static { .. } => {
1449                let def_id = id.owner_id.to_def_id();
1450                debug!("RootCollector: ItemKind::Static({})", self.tcx.def_path_str(def_id));
1451                self.output.push(dummy_spanned(MonoItem::Static(def_id)));
1452            }
1453            DefKind::Const => {
1454                // Const items only generate mono items if they are actually used somewhere.
1455                // Just declaring them is insufficient.
1456
1457                // But even just declaring them must collect the items they refer to
1458                // unless their generics require monomorphization.
1459                if !self.tcx.generics_of(id.owner_id).requires_monomorphization(self.tcx)
1460                    && let Ok(val) = self.tcx.const_eval_poly(id.owner_id.to_def_id())
1461                {
1462                    collect_const_value(self.tcx, val, self.output);
1463                }
1464            }
1465            DefKind::Impl { .. } => {
1466                if self.strategy == MonoItemCollectionStrategy::Eager {
1467                    create_mono_items_for_default_impls(self.tcx, id, self.output);
1468                }
1469            }
1470            DefKind::Fn => {
1471                self.push_if_root(id.owner_id.def_id);
1472            }
1473            _ => {}
1474        }
1475    }
1476
1477    fn process_impl_item(&mut self, id: hir::ImplItemId) {
1478        if matches!(self.tcx.def_kind(id.owner_id), DefKind::AssocFn) {
1479            self.push_if_root(id.owner_id.def_id);
1480        }
1481    }
1482
1483    fn process_nested_body(&mut self, def_id: LocalDefId) {
1484        match self.tcx.def_kind(def_id) {
1485            DefKind::Closure => {
1486                if self.strategy == MonoItemCollectionStrategy::Eager
1487                    && !self
1488                        .tcx
1489                        .generics_of(self.tcx.typeck_root_def_id(def_id.to_def_id()))
1490                        .requires_monomorphization(self.tcx)
1491                {
1492                    let instance = match *self.tcx.type_of(def_id).instantiate_identity().kind() {
1493                        ty::Closure(def_id, args)
1494                        | ty::Coroutine(def_id, args)
1495                        | ty::CoroutineClosure(def_id, args) => {
1496                            Instance::new(def_id, self.tcx.erase_regions(args))
1497                        }
1498                        _ => unreachable!(),
1499                    };
1500                    let Ok(instance) = self.tcx.try_normalize_erasing_regions(
1501                        ty::TypingEnv::fully_monomorphized(),
1502                        instance,
1503                    ) else {
1504                        // Don't ICE on an impossible-to-normalize closure.
1505                        return;
1506                    };
1507                    let mono_item = create_fn_mono_item(self.tcx, instance, DUMMY_SP);
1508                    if mono_item.node.is_instantiable(self.tcx) {
1509                        self.output.push(mono_item);
1510                    }
1511                }
1512            }
1513            _ => {}
1514        }
1515    }
1516
1517    fn is_root(&self, def_id: LocalDefId) -> bool {
1518        !self.tcx.generics_of(def_id).requires_monomorphization(self.tcx)
1519            && match self.strategy {
1520                MonoItemCollectionStrategy::Eager => {
1521                    !matches!(self.tcx.codegen_fn_attrs(def_id).inline, InlineAttr::Force { .. })
1522                }
1523                MonoItemCollectionStrategy::Lazy => {
1524                    self.entry_fn.and_then(|(id, _)| id.as_local()) == Some(def_id)
1525                        || self.tcx.is_reachable_non_generic(def_id)
1526                        || self
1527                            .tcx
1528                            .codegen_fn_attrs(def_id)
1529                            .flags
1530                            .contains(CodegenFnAttrFlags::RUSTC_STD_INTERNAL_SYMBOL)
1531                }
1532            }
1533    }
1534
1535    /// If `def_id` represents a root, pushes it onto the list of
1536    /// outputs. (Note that all roots must be monomorphic.)
1537    #[instrument(skip(self), level = "debug")]
1538    fn push_if_root(&mut self, def_id: LocalDefId) {
1539        if self.is_root(def_id) {
1540            debug!("found root");
1541
1542            let instance = Instance::mono(self.tcx, def_id.to_def_id());
1543            self.output.push(create_fn_mono_item(self.tcx, instance, DUMMY_SP));
1544        }
1545    }
1546
1547    /// As a special case, when/if we encounter the
1548    /// `main()` function, we also have to generate a
1549    /// monomorphized copy of the start lang item based on
1550    /// the return type of `main`. This is not needed when
1551    /// the user writes their own `start` manually.
1552    fn push_extra_entry_roots(&mut self) {
1553        let Some((main_def_id, EntryFnType::Main { .. })) = self.entry_fn else {
1554            return;
1555        };
1556
1557        let Some(start_def_id) = self.tcx.lang_items().start_fn() else {
1558            self.tcx.dcx().emit_fatal(errors::StartNotFound);
1559        };
1560        let main_ret_ty = self.tcx.fn_sig(main_def_id).no_bound_vars().unwrap().output();
1561
1562        // Given that `main()` has no arguments,
1563        // then its return type cannot have
1564        // late-bound regions, since late-bound
1565        // regions must appear in the argument
1566        // listing.
1567        let main_ret_ty = self.tcx.normalize_erasing_regions(
1568            ty::TypingEnv::fully_monomorphized(),
1569            main_ret_ty.no_bound_vars().unwrap(),
1570        );
1571
1572        let start_instance = Instance::expect_resolve(
1573            self.tcx,
1574            ty::TypingEnv::fully_monomorphized(),
1575            start_def_id,
1576            self.tcx.mk_args(&[main_ret_ty.into()]),
1577            DUMMY_SP,
1578        );
1579
1580        self.output.push(create_fn_mono_item(self.tcx, start_instance, DUMMY_SP));
1581    }
1582}
1583
1584#[instrument(level = "debug", skip(tcx, output))]
1585fn create_mono_items_for_default_impls<'tcx>(
1586    tcx: TyCtxt<'tcx>,
1587    item: hir::ItemId,
1588    output: &mut MonoItems<'tcx>,
1589) {
1590    let Some(impl_) = tcx.impl_trait_header(item.owner_id) else {
1591        return;
1592    };
1593
1594    if matches!(impl_.polarity, ty::ImplPolarity::Negative) {
1595        return;
1596    }
1597
1598    if tcx.generics_of(item.owner_id).own_requires_monomorphization() {
1599        return;
1600    }
1601
1602    // Lifetimes never affect trait selection, so we are allowed to eagerly
1603    // instantiate an instance of an impl method if the impl (and method,
1604    // which we check below) is only parameterized over lifetime. In that case,
1605    // we use the ReErased, which has no lifetime information associated with
1606    // it, to validate whether or not the impl is legal to instantiate at all.
1607    let only_region_params = |param: &ty::GenericParamDef, _: &_| match param.kind {
1608        GenericParamDefKind::Lifetime => tcx.lifetimes.re_erased.into(),
1609        GenericParamDefKind::Type { .. } | GenericParamDefKind::Const { .. } => {
1610            unreachable!(
1611                "`own_requires_monomorphization` check means that \
1612                we should have no type/const params"
1613            )
1614        }
1615    };
1616    let impl_args = GenericArgs::for_item(tcx, item.owner_id.to_def_id(), only_region_params);
1617    let trait_ref = impl_.trait_ref.instantiate(tcx, impl_args);
1618
1619    // Unlike 'lazy' monomorphization that begins by collecting items transitively
1620    // called by `main` or other global items, when eagerly monomorphizing impl
1621    // items, we never actually check that the predicates of this impl are satisfied
1622    // in a empty param env (i.e. with no assumptions).
1623    //
1624    // Even though this impl has no type or const generic parameters, because we don't
1625    // consider higher-ranked predicates such as `for<'a> &'a mut [u8]: Copy` to
1626    // be trivially false. We must now check that the impl has no impossible-to-satisfy
1627    // predicates.
1628    if tcx.instantiate_and_check_impossible_predicates((item.owner_id.to_def_id(), impl_args)) {
1629        return;
1630    }
1631
1632    let typing_env = ty::TypingEnv::fully_monomorphized();
1633    let trait_ref = tcx.normalize_erasing_regions(typing_env, trait_ref);
1634    let overridden_methods = tcx.impl_item_implementor_ids(item.owner_id);
1635    for method in tcx.provided_trait_methods(trait_ref.def_id) {
1636        if overridden_methods.contains_key(&method.def_id) {
1637            continue;
1638        }
1639
1640        if tcx.generics_of(method.def_id).own_requires_monomorphization() {
1641            continue;
1642        }
1643
1644        // As mentioned above, the method is legal to eagerly instantiate if it
1645        // only has lifetime generic parameters. This is validated by calling
1646        // `own_requires_monomorphization` on both the impl and method.
1647        let args = trait_ref.args.extend_to(tcx, method.def_id, only_region_params);
1648        let instance = ty::Instance::expect_resolve(tcx, typing_env, method.def_id, args, DUMMY_SP);
1649
1650        let mono_item = create_fn_mono_item(tcx, instance, DUMMY_SP);
1651        if mono_item.node.is_instantiable(tcx) && tcx.should_codegen_locally(instance) {
1652            output.push(mono_item);
1653        }
1654    }
1655}
1656
1657//=-----------------------------------------------------------------------------
1658// Top-level entry point, tying it all together
1659//=-----------------------------------------------------------------------------
1660
1661#[instrument(skip(tcx, strategy), level = "debug")]
1662pub(crate) fn collect_crate_mono_items<'tcx>(
1663    tcx: TyCtxt<'tcx>,
1664    strategy: MonoItemCollectionStrategy,
1665) -> (Vec<MonoItem<'tcx>>, UsageMap<'tcx>) {
1666    let _prof_timer = tcx.prof.generic_activity("monomorphization_collector");
1667
1668    let roots = tcx
1669        .sess
1670        .time("monomorphization_collector_root_collections", || collect_roots(tcx, strategy));
1671
1672    debug!("building mono item graph, beginning at roots");
1673
1674    let state = SharedState {
1675        visited: MTLock::new(UnordSet::default()),
1676        mentioned: MTLock::new(UnordSet::default()),
1677        usage_map: MTLock::new(UsageMap::new()),
1678    };
1679    let recursion_limit = tcx.recursion_limit();
1680
1681    tcx.sess.time("monomorphization_collector_graph_walk", || {
1682        par_for_each_in(roots, |root| {
1683            let mut recursion_depths = DefIdMap::default();
1684            collect_items_rec(
1685                tcx,
1686                dummy_spanned(root),
1687                &state,
1688                &mut recursion_depths,
1689                recursion_limit,
1690                CollectionMode::UsedItems,
1691            );
1692        });
1693    });
1694
1695    // The set of MonoItems was created in an inherently indeterministic order because
1696    // of parallelism. We sort it here to ensure that the output is deterministic.
1697    let mono_items = tcx.with_stable_hashing_context(move |ref hcx| {
1698        state.visited.into_inner().into_sorted(hcx, true)
1699    });
1700
1701    (mono_items, state.usage_map.into_inner())
1702}
1703
1704pub(crate) fn provide(providers: &mut Providers) {
1705    providers.hooks.should_codegen_locally = should_codegen_locally;
1706    providers.items_of_instance = items_of_instance;
1707}