Merging upstream version 1.72.1+dfsg1.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1274 insertions, 0 deletions

diff --git a/compiler/rustc_monomorphize/src/partitioning.rs b/compiler/rustc_monomorphize/src/partitioning.rs
new file mode 100644
index 000000000..da76cf223
--- /dev/null
+++ b/compiler/rustc_monomorphize/src/partitioning.rs
@@ -0,0 +1,1274 @@
+//! Partitioning Codegen Units for Incremental Compilation
+//! ======================================================
+//!
+//! The task of this module is to take the complete set of monomorphizations of
+//! a crate and produce a set of codegen units from it, where a codegen unit
+//! is a named set of (mono-item, linkage) pairs. That is, this module
+//! decides which monomorphization appears in which codegen units with which
+//! linkage. The following paragraphs describe some of the background on the
+//! partitioning scheme.
+//!
+//! The most important opportunity for saving on compilation time with
+//! incremental compilation is to avoid re-codegenning and re-optimizing code.
+//! Since the unit of codegen and optimization for LLVM is "modules" or, how
+//! we call them "codegen units", the particulars of how much time can be saved
+//! by incremental compilation are tightly linked to how the output program is
+//! partitioned into these codegen units prior to passing it to LLVM --
+//! especially because we have to treat codegen units as opaque entities once
+//! they are created: There is no way for us to incrementally update an existing
+//! LLVM module and so we have to build any such module from scratch if it was
+//! affected by some change in the source code.
+//!
+//! From that point of view it would make sense to maximize the number of
+//! codegen units by, for example, putting each function into its own module.
+//! That way only those modules would have to be re-compiled that were actually
+//! affected by some change, minimizing the number of functions that could have
+//! been re-used but just happened to be located in a module that is
+//! re-compiled.
+//!
+//! However, since LLVM optimization does not work across module boundaries,
+//! using such a highly granular partitioning would lead to very slow runtime
+//! code since it would effectively prohibit inlining and other inter-procedure
+//! optimizations. We want to avoid that as much as possible.
+//!
+//! Thus we end up with a trade-off: The bigger the codegen units, the better
+//! LLVM's optimizer can do its work, but also the smaller the compilation time
+//! reduction we get from incremental compilation.
+//!
+//! Ideally, we would create a partitioning such that there are few big codegen
+//! units with few interdependencies between them. For now though, we use the
+//! following heuristic to determine the partitioning:
+//!
+//! - There are two codegen units for every source-level module:
+//! - One for "stable", that is non-generic, code
+//! - One for more "volatile" code, i.e., monomorphized instances of functions
+//!   defined in that module
+//!
+//! In order to see why this heuristic makes sense, let's take a look at when a
+//! codegen unit can get invalidated:
+//!
+//! 1. The most straightforward case is when the BODY of a function or global
+//! changes. Then any codegen unit containing the code for that item has to be
+//! re-compiled. Note that this includes all codegen units where the function
+//! has been inlined.
+//!
+//! 2. The next case is when the SIGNATURE of a function or global changes. In
+//! this case, all codegen units containing a REFERENCE to that item have to be
+//! re-compiled. This is a superset of case 1.
+//!
+//! 3. The final and most subtle case is when a REFERENCE to a generic function
+//! is added or removed somewhere. Even though the definition of the function
+//! might be unchanged, a new REFERENCE might introduce a new monomorphized
+//! instance of this function which has to be placed and compiled somewhere.
+//! Conversely, when removing a REFERENCE, it might have been the last one with
+//! that particular set of generic arguments and thus we have to remove it.
+//!
+//! From the above we see that just using one codegen unit per source-level
+//! module is not such a good idea, since just adding a REFERENCE to some
+//! generic item somewhere else would invalidate everything within the module
+//! containing the generic item. The heuristic above reduces this detrimental
+//! side-effect of references a little by at least not touching the non-generic
+//! code of the module.
+//!
+//! A Note on Inlining
+//! ------------------
+//! As briefly mentioned above, in order for LLVM to be able to inline a
+//! function call, the body of the function has to be available in the LLVM
+//! module where the call is made. This has a few consequences for partitioning:
+//!
+//! - The partitioning algorithm has to take care of placing functions into all
+//!   codegen units where they should be available for inlining. It also has to
+//!   decide on the correct linkage for these functions.
+//!
+//! - The partitioning algorithm has to know which functions are likely to get
+//!   inlined, so it can distribute function instantiations accordingly. Since
+//!   there is no way of knowing for sure which functions LLVM will decide to
+//!   inline in the end, we apply a heuristic here: Only functions marked with
+//!   `#[inline]` are considered for inlining by the partitioner. The current
+//!   implementation will not try to determine if a function is likely to be
+//!   inlined by looking at the functions definition.
+//!
+//! Note though that as a side-effect of creating a codegen units per
+//! source-level module, functions from the same module will be available for
+//! inlining, even when they are not marked `#[inline]`.
+
+use std::cmp;
+use std::collections::hash_map::Entry;
+use std::fs::{self, File};
+use std::io::{BufWriter, Write};
+use std::path::{Path, PathBuf};
+
+use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_data_structures::sync;
+use rustc_hir::def::DefKind;
+use rustc_hir::def_id::{DefId, DefIdSet, LOCAL_CRATE};
+use rustc_hir::definitions::DefPathDataName;
+use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
+use rustc_middle::middle::exported_symbols::{SymbolExportInfo, SymbolExportLevel};
+use rustc_middle::mir;
+use rustc_middle::mir::mono::{
+    CodegenUnit, CodegenUnitNameBuilder, InstantiationMode, Linkage, MonoItem, Visibility,
+};
+use rustc_middle::query::Providers;
+use rustc_middle::ty::print::{characteristic_def_id_of_type, with_no_trimmed_paths};
+use rustc_middle::ty::{self, visit::TypeVisitableExt, InstanceDef, TyCtxt};
+use rustc_session::config::{DumpMonoStatsFormat, SwitchWithOptPath};
+use rustc_session::CodegenUnits;
+use rustc_span::symbol::Symbol;
+
+use crate::collector::UsageMap;
+use crate::collector::{self, MonoItemCollectionMode};
+use crate::errors::{CouldntDumpMonoStats, SymbolAlreadyDefined, UnknownCguCollectionMode};
+
+struct PartitioningCx<'a, 'tcx> {
+    tcx: TyCtxt<'tcx>,
+    usage_map: &'a UsageMap<'tcx>,
+}
+
+struct PlacedMonoItems<'tcx> {
+    /// The codegen units, sorted by name to make things deterministic.
+    codegen_units: Vec<CodegenUnit<'tcx>>,
+
+    internalization_candidates: FxHashSet<MonoItem<'tcx>>,
+
+    /// These must be obtained when the iterator in `partition` runs. They
+    /// can't be obtained later because some inlined functions might not be
+    /// reachable.
+    unique_inlined_stats: (usize, usize),
+}
+
+// The output CGUs are sorted by name.
+fn partition<'tcx, I>(
+    tcx: TyCtxt<'tcx>,
+    mono_items: I,
+    usage_map: &UsageMap<'tcx>,
+) -> Vec<CodegenUnit<'tcx>>
+where
+    I: Iterator<Item = MonoItem<'tcx>>,
+{
+    let _prof_timer = tcx.prof.generic_activity("cgu_partitioning");
+
+    let cx = &PartitioningCx { tcx, usage_map };
+
+    // Place all mono items into a codegen unit. `place_mono_items` is
+    // responsible for initializing the CGU size estimates.
+    let PlacedMonoItems { mut codegen_units, internalization_candidates, unique_inlined_stats } = {
+        let _prof_timer = tcx.prof.generic_activity("cgu_partitioning_place_items");
+        let placed = place_mono_items(cx, mono_items);
+
+        debug_dump(tcx, "PLACE", &placed.codegen_units, placed.unique_inlined_stats);
+
+        placed
+    };
+
+    // Merge until we have at most `max_cgu_count` codegen units.
+    // `merge_codegen_units` is responsible for updating the CGU size
+    // estimates.
+    {
+        let _prof_timer = tcx.prof.generic_activity("cgu_partitioning_merge_cgus");
+        merge_codegen_units(cx, &mut codegen_units);
+        debug_dump(tcx, "MERGE", &codegen_units, unique_inlined_stats);
+    }
+
+    // Make as many symbols "internal" as possible, so LLVM has more freedom to
+    // optimize.
+    if !tcx.sess.link_dead_code() {
+        let _prof_timer = tcx.prof.generic_activity("cgu_partitioning_internalize_symbols");
+        internalize_symbols(cx, &mut codegen_units, internalization_candidates);
+
+        debug_dump(tcx, "INTERNALIZE", &codegen_units, unique_inlined_stats);
+    }
+
+    // Mark one CGU for dead code, if necessary.
+    let instrument_dead_code =
+        tcx.sess.instrument_coverage() && !tcx.sess.instrument_coverage_except_unused_functions();
+    if instrument_dead_code {
+        mark_code_coverage_dead_code_cgu(&mut codegen_units);
+    }
+
+    // Ensure CGUs are sorted by name, so that we get deterministic results.
+    if !codegen_units.is_sorted_by(|a, b| Some(a.name().as_str().cmp(b.name().as_str()))) {
+        let mut names = String::new();
+        for cgu in codegen_units.iter() {
+            names += &format!("- {}\n", cgu.name());
+        }
+        bug!("unsorted CGUs:\n{names}");
+    }
+
+    codegen_units
+}
+
+fn place_mono_items<'tcx, I>(cx: &PartitioningCx<'_, 'tcx>, mono_items: I) -> PlacedMonoItems<'tcx>
+where
+    I: Iterator<Item = MonoItem<'tcx>>,
+{
+    let mut codegen_units = FxHashMap::default();
+    let is_incremental_build = cx.tcx.sess.opts.incremental.is_some();
+    let mut internalization_candidates = FxHashSet::default();
+
+    // Determine if monomorphizations instantiated in this crate will be made
+    // available to downstream crates. This depends on whether we are in
+    // share-generics mode and whether the current crate can even have
+    // downstream crates.
+    let export_generics =
+        cx.tcx.sess.opts.share_generics() && cx.tcx.local_crate_exports_generics();
+
+    let cgu_name_builder = &mut CodegenUnitNameBuilder::new(cx.tcx);
+    let cgu_name_cache = &mut FxHashMap::default();
+
+    let mut num_unique_inlined_items = 0;
+    let mut unique_inlined_items_size = 0;
+    for mono_item in mono_items {
+        // Handle only root items directly here. Inlined items are handled at
+        // the bottom of the loop based on reachability.
+        match mono_item.instantiation_mode(cx.tcx) {
+            InstantiationMode::GloballyShared { .. } => {}
+            InstantiationMode::LocalCopy => {
+                num_unique_inlined_items += 1;
+                unique_inlined_items_size += mono_item.size_estimate(cx.tcx);
+                continue;
+            }
+        }
+
+        let characteristic_def_id = characteristic_def_id_of_mono_item(cx.tcx, mono_item);
+        let is_volatile = is_incremental_build && mono_item.is_generic_fn();
+
+        let cgu_name = match characteristic_def_id {
+            Some(def_id) => compute_codegen_unit_name(
+                cx.tcx,
+                cgu_name_builder,
+                def_id,
+                is_volatile,
+                cgu_name_cache,
+            ),
+            None => fallback_cgu_name(cgu_name_builder),
+        };
+
+        let cgu = codegen_units.entry(cgu_name).or_insert_with(|| CodegenUnit::new(cgu_name));
+
+        let mut can_be_internalized = true;
+        let (linkage, visibility) = mono_item_linkage_and_visibility(
+            cx.tcx,
+            &mono_item,
+            &mut can_be_internalized,
+            export_generics,
+        );
+        if visibility == Visibility::Hidden && can_be_internalized {
+            internalization_candidates.insert(mono_item);
+        }
+
+        cgu.items_mut().insert(mono_item, (linkage, visibility));
+
+        // Get all inlined items that are reachable from `mono_item` without
+        // going via another root item. This includes drop-glue, functions from
+        // external crates, and local functions the definition of which is
+        // marked with `#[inline]`.
+        let mut reachable_inlined_items = FxHashSet::default();
+        get_reachable_inlined_items(cx.tcx, mono_item, cx.usage_map, &mut reachable_inlined_items);
+
+        // Add those inlined items. It's possible an inlined item is reachable
+        // from multiple root items within a CGU, which is fine, it just means
+        // the `insert` will be a no-op.
+        for inlined_item in reachable_inlined_items {
+            // This is a CGU-private copy.
+            cgu.items_mut().insert(inlined_item, (Linkage::Internal, Visibility::Default));
+        }
+    }
+
+    // Always ensure we have at least one CGU; otherwise, if we have a
+    // crate with just types (for example), we could wind up with no CGU.
+    if codegen_units.is_empty() {
+        let cgu_name = fallback_cgu_name(cgu_name_builder);
+        codegen_units.insert(cgu_name, CodegenUnit::new(cgu_name));
+    }
+
+    let mut codegen_units: Vec<_> = codegen_units.into_values().collect();
+    codegen_units.sort_by(|a, b| a.name().as_str().cmp(b.name().as_str()));
+
+    for cgu in codegen_units.iter_mut() {
+        cgu.compute_size_estimate(cx.tcx);
+    }
+
+    return PlacedMonoItems {
+        codegen_units,
+        internalization_candidates,
+        unique_inlined_stats: (num_unique_inlined_items, unique_inlined_items_size),
+    };
+
+    fn get_reachable_inlined_items<'tcx>(
+        tcx: TyCtxt<'tcx>,
+        item: MonoItem<'tcx>,
+        usage_map: &UsageMap<'tcx>,
+        visited: &mut FxHashSet<MonoItem<'tcx>>,
+    ) {
+        usage_map.for_each_inlined_used_item(tcx, item, |inlined_item| {
+            let is_new = visited.insert(inlined_item);
+            if is_new {
+                get_reachable_inlined_items(tcx, inlined_item, usage_map, visited);
+            }
+        });
+    }
+}
+
+// This function requires the CGUs to be sorted by name on input, and ensures
+// they are sorted by name on return, for deterministic behaviour.
+fn merge_codegen_units<'tcx>(
+    cx: &PartitioningCx<'_, 'tcx>,
+    codegen_units: &mut Vec<CodegenUnit<'tcx>>,
+) {
+    assert!(cx.tcx.sess.codegen_units().as_usize() >= 1);
+
+    // A sorted order here ensures merging is deterministic.
+    assert!(codegen_units.is_sorted_by(|a, b| Some(a.name().as_str().cmp(b.name().as_str()))));
+
+    // This map keeps track of what got merged into what.
+    let mut cgu_contents: FxHashMap<Symbol, Vec<Symbol>> =
+        codegen_units.iter().map(|cgu| (cgu.name(), vec![cgu.name()])).collect();
+
+    // Having multiple CGUs can drastically speed up compilation. But for
+    // non-incremental builds, tiny CGUs slow down compilation *and* result in
+    // worse generated code. So we don't allow CGUs smaller than this (unless
+    // there is just one CGU, of course). Note that CGU sizes of 100,000+ are
+    // common in larger programs, so this isn't all that large.
+    const NON_INCR_MIN_CGU_SIZE: usize = 1800;
+
+    // Repeatedly merge the two smallest codegen units as long as:
+    // - we have more CGUs than the upper limit, or
+    // - (Non-incremental builds only) the user didn't specify a CGU count, and
+    //   there are multiple CGUs, and some are below the minimum size.
+    //
+    // The "didn't specify a CGU count" condition is because when an explicit
+    // count is requested we observe it as closely as possible. For example,
+    // the `compiler_builtins` crate sets `codegen-units = 10000` and it's
+    // critical they aren't merged. Also, some tests use explicit small values
+    // and likewise won't work if small CGUs are merged.
+    while codegen_units.len() > cx.tcx.sess.codegen_units().as_usize()
+        || (cx.tcx.sess.opts.incremental.is_none()
+            && matches!(cx.tcx.sess.codegen_units(), CodegenUnits::Default(_))
+            && codegen_units.len() > 1
+            && codegen_units.iter().any(|cgu| cgu.size_estimate() < NON_INCR_MIN_CGU_SIZE))
+    {
+        // Sort small cgus to the back.
+        codegen_units.sort_by_cached_key(|cgu| cmp::Reverse(cgu.size_estimate()));
+
+        let mut smallest = codegen_units.pop().unwrap();
+        let second_smallest = codegen_units.last_mut().unwrap();
+
+        // Move the items from `smallest` to `second_smallest`. Some of them
+        // may be duplicate inlined items, in which case the destination CGU is
+        // unaffected. Recalculate size estimates afterwards.
+        second_smallest.items_mut().extend(smallest.items_mut().drain());
+        second_smallest.compute_size_estimate(cx.tcx);
+
+        // Record that `second_smallest` now contains all the stuff that was
+        // in `smallest` before.
+        let mut consumed_cgu_names = cgu_contents.remove(&smallest.name()).unwrap();
+        cgu_contents.get_mut(&second_smallest.name()).unwrap().append(&mut consumed_cgu_names);
+
+        debug!(
+            "CodegenUnit {} merged into CodegenUnit {}",
+            smallest.name(),
+            second_smallest.name()
+        );
+    }
+
+    let cgu_name_builder = &mut CodegenUnitNameBuilder::new(cx.tcx);
+
+    // Rename the newly merged CGUs.
+    if cx.tcx.sess.opts.incremental.is_some() {
+        // If we are doing incremental compilation, we want CGU names to
+        // reflect the path of the source level module they correspond to.
+        // For CGUs that contain the code of multiple modules because of the
+        // merging done above, we use a concatenation of the names of all
+        // contained CGUs.
+        let new_cgu_names: FxHashMap<Symbol, String> = cgu_contents
+            .into_iter()
+            // This `filter` makes sure we only update the name of CGUs that
+            // were actually modified by merging.
+            .filter(|(_, cgu_contents)| cgu_contents.len() > 1)
+            .map(|(current_cgu_name, cgu_contents)| {
+                let mut cgu_contents: Vec<&str> = cgu_contents.iter().map(|s| s.as_str()).collect();
+
+                // Sort the names, so things are deterministic and easy to
+                // predict. We are sorting primitive `&str`s here so we can
+                // use unstable sort.
+                cgu_contents.sort_unstable();
+
+                (current_cgu_name, cgu_contents.join("--"))
+            })
+            .collect();
+
+        for cgu in codegen_units.iter_mut() {
+            if let Some(new_cgu_name) = new_cgu_names.get(&cgu.name()) {
+                if cx.tcx.sess.opts.unstable_opts.human_readable_cgu_names {
+                    cgu.set_name(Symbol::intern(&new_cgu_name));
+                } else {
+                    // If we don't require CGU names to be human-readable,
+                    // we use a fixed length hash of the composite CGU name
+                    // instead.
+                    let new_cgu_name = CodegenUnit::mangle_name(&new_cgu_name);
+                    cgu.set_name(Symbol::intern(&new_cgu_name));
+                }
+            }
+        }
+
+        // A sorted order here ensures what follows can be deterministic.
+        codegen_units.sort_by(|a, b| a.name().as_str().cmp(b.name().as_str()));
+    } else {
+        // When compiling non-incrementally, we rename the CGUS so they have
+        // identical names except for the numeric suffix, something like
+        // `regex.f10ba03eb5ec7975-cgu.N`, where `N` varies.
+        //
+        // It is useful for debugging and profiling purposes if the resulting
+        // CGUs are sorted by name *and* reverse sorted by size. (CGU 0 is the
+        // biggest, CGU 1 is the second biggest, etc.)
+        //
+        // So first we reverse sort by size. Then we generate the names with
+        // zero-padded suffixes, which means they are automatically sorted by
+        // names. The numeric suffix width depends on the number of CGUs, which
+        // is always greater than zero:
+        // - [1,9]     CGUs: `0`, `1`, `2`, ...
+        // - [10,99]   CGUs: `00`, `01`, `02`, ...
+        // - [100,999] CGUs: `000`, `001`, `002`, ...
+        // - etc.
+        //
+        // If we didn't zero-pad the sorted-by-name order would be `XYZ-cgu.0`,
+        // `XYZ-cgu.1`, `XYZ-cgu.10`, `XYZ-cgu.11`, ..., `XYZ-cgu.2`, etc.
+        codegen_units.sort_by_key(|cgu| cmp::Reverse(cgu.size_estimate()));
+        let num_digits = codegen_units.len().ilog10() as usize + 1;
+        for (index, cgu) in codegen_units.iter_mut().enumerate() {
+            // Note: `WorkItem::short_description` depends on this name ending
+            // with `-cgu.` followed by a numeric suffix. Please keep it in
+            // sync with this code.
+            let suffix = format!("{index:0num_digits$}");
+            let numbered_codegen_unit_name =
+                cgu_name_builder.build_cgu_name_no_mangle(LOCAL_CRATE, &["cgu"], Some(suffix));
+            cgu.set_name(numbered_codegen_unit_name);
+        }
+    }
+}
+
+fn internalize_symbols<'tcx>(
+    cx: &PartitioningCx<'_, 'tcx>,
+    codegen_units: &mut [CodegenUnit<'tcx>],
+    internalization_candidates: FxHashSet<MonoItem<'tcx>>,
+) {
+    /// For symbol internalization, we need to know whether a symbol/mono-item
+    /// is used from outside the codegen unit it is defined in. This type is
+    /// used to keep track of that.
+    #[derive(Clone, PartialEq, Eq, Debug)]
+    enum MonoItemPlacement {
+        SingleCgu(Symbol),
+        MultipleCgus,
+    }
+
+    let mut mono_item_placements = FxHashMap::default();
+    let single_codegen_unit = codegen_units.len() == 1;
+
+    if !single_codegen_unit {
+        for cgu in codegen_units.iter() {
+            for item in cgu.items().keys() {
+                // If there is more than one codegen unit, we need to keep track
+                // in which codegen units each monomorphization is placed.
+                match mono_item_placements.entry(*item) {
+                    Entry::Occupied(e) => {
+                        let placement = e.into_mut();
+                        debug_assert!(match *placement {
+                            MonoItemPlacement::SingleCgu(cgu_name) => cgu_name != cgu.name(),
+                            MonoItemPlacement::MultipleCgus => true,
+                        });
+                        *placement = MonoItemPlacement::MultipleCgus;
+                    }
+                    Entry::Vacant(e) => {
+                        e.insert(MonoItemPlacement::SingleCgu(cgu.name()));
+                    }
+                }
+            }
+        }
+    }
+
+    // For each internalization candidates in each codegen unit, check if it is
+    // used from outside its defining codegen unit.
+    for cgu in codegen_units {
+        let home_cgu = MonoItemPlacement::SingleCgu(cgu.name());
+
+        for (item, linkage_and_visibility) in cgu.items_mut() {
+            if !internalization_candidates.contains(item) {
+                // This item is no candidate for internalizing, so skip it.
+                continue;
+            }
+
+            if !single_codegen_unit {
+                debug_assert_eq!(mono_item_placements[item], home_cgu);
+
+                if cx
+                    .usage_map
+                    .get_user_items(*item)
+                    .iter()
+                    .filter_map(|user_item| {
+                        // Some user mono items might not have been
+                        // instantiated. We can safely ignore those.
+                        mono_item_placements.get(user_item)
+                    })
+                    .any(|placement| *placement != home_cgu)
+                {
+                    // Found a user from another CGU, so skip to the next item
+                    // without marking this one as internal.
+                    continue;
+                }
+            }
+
+            // If we got here, we did not find any uses from other CGUs, so
+            // it's fine to make this monomorphization internal.
+            *linkage_and_visibility = (Linkage::Internal, Visibility::Default);
+        }
+    }
+}
+
+fn mark_code_coverage_dead_code_cgu<'tcx>(codegen_units: &mut [CodegenUnit<'tcx>]) {
+    assert!(!codegen_units.is_empty());
+
+    // Find the smallest CGU that has exported symbols and put the dead
+    // function stubs in that CGU. We look for exported symbols to increase
+    // the likelihood the linker won't throw away the dead functions.
+    // FIXME(#92165): In order to truly resolve this, we need to make sure
+    // the object file (CGU) containing the dead function stubs is included
+    // in the final binary. This will probably require forcing these
+    // function symbols to be included via `-u` or `/include` linker args.
+    let dead_code_cgu = codegen_units
+        .iter_mut()
+        .filter(|cgu| cgu.items().iter().any(|(_, (linkage, _))| *linkage == Linkage::External))
+        .min_by_key(|cgu| cgu.size_estimate());
+
+    // If there are no CGUs that have externally linked items, then we just
+    // pick the first CGU as a fallback.
+    let dead_code_cgu = if let Some(cgu) = dead_code_cgu { cgu } else { &mut codegen_units[0] };
+
+    dead_code_cgu.make_code_coverage_dead_code_cgu();
+}
+
+fn characteristic_def_id_of_mono_item<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    mono_item: MonoItem<'tcx>,
+) -> Option<DefId> {
+    match mono_item {
+        MonoItem::Fn(instance) => {
+            let def_id = match instance.def {
+                ty::InstanceDef::Item(def) => def,
+                ty::InstanceDef::VTableShim(..)
+                | ty::InstanceDef::ReifyShim(..)
+                | ty::InstanceDef::FnPtrShim(..)
+                | ty::InstanceDef::ClosureOnceShim { .. }
+                | ty::InstanceDef::Intrinsic(..)
+                | ty::InstanceDef::DropGlue(..)
+                | ty::InstanceDef::Virtual(..)
+                | ty::InstanceDef::CloneShim(..)
+                | ty::InstanceDef::ThreadLocalShim(..)
+                | ty::InstanceDef::FnPtrAddrShim(..) => return None,
+            };
+
+            // If this is a method, we want to put it into the same module as
+            // its self-type. If the self-type does not provide a characteristic
+            // DefId, we use the location of the impl after all.
+
+            if tcx.trait_of_item(def_id).is_some() {
+                let self_ty = instance.substs.type_at(0);
+                // This is a default implementation of a trait method.
+                return characteristic_def_id_of_type(self_ty).or(Some(def_id));
+            }
+
+            if let Some(impl_def_id) = tcx.impl_of_method(def_id) {
+                if tcx.sess.opts.incremental.is_some()
+                    && tcx.trait_id_of_impl(impl_def_id) == tcx.lang_items().drop_trait()
+                {
+                    // Put `Drop::drop` into the same cgu as `drop_in_place`
+                    // since `drop_in_place` is the only thing that can
+                    // call it.
+                    return None;
+                }
+
+                // When polymorphization is enabled, methods which do not depend on their generic
+                // parameters, but the self-type of their impl block do will fail to normalize.
+                if !tcx.sess.opts.unstable_opts.polymorphize || !instance.has_param() {
+                    // This is a method within an impl, find out what the self-type is:
+                    let impl_self_ty = tcx.subst_and_normalize_erasing_regions(
+                        instance.substs,
+                        ty::ParamEnv::reveal_all(),
+                        tcx.type_of(impl_def_id),
+                    );
+                    if let Some(def_id) = characteristic_def_id_of_type(impl_self_ty) {
+                        return Some(def_id);
+                    }
+                }
+            }
+
+            Some(def_id)
+        }
+        MonoItem::Static(def_id) => Some(def_id),
+        MonoItem::GlobalAsm(item_id) => Some(item_id.owner_id.to_def_id()),
+    }
+}
+
+fn compute_codegen_unit_name(
+    tcx: TyCtxt<'_>,
+    name_builder: &mut CodegenUnitNameBuilder<'_>,
+    def_id: DefId,
+    volatile: bool,
+    cache: &mut CguNameCache,
+) -> Symbol {
+    // Find the innermost module that is not nested within a function.
+    let mut current_def_id = def_id;
+    let mut cgu_def_id = None;
+    // Walk backwards from the item we want to find the module for.
+    loop {
+        if current_def_id.is_crate_root() {
+            if cgu_def_id.is_none() {
+                // If we have not found a module yet, take the crate root.
+                cgu_def_id = Some(def_id.krate.as_def_id());
+            }
+            break;
+        } else if tcx.def_kind(current_def_id) == DefKind::Mod {
+            if cgu_def_id.is_none() {
+                cgu_def_id = Some(current_def_id);
+            }
+        } else {
+            // If we encounter something that is not a module, throw away
+            // any module that we've found so far because we now know that
+            // it is nested within something else.
+            cgu_def_id = None;
+        }
+
+        current_def_id = tcx.parent(current_def_id);
+    }
+
+    let cgu_def_id = cgu_def_id.unwrap();
+
+    *cache.entry((cgu_def_id, volatile)).or_insert_with(|| {
+        let def_path = tcx.def_path(cgu_def_id);
+
+        let components = def_path.data.iter().map(|part| match part.data.name() {
+            DefPathDataName::Named(name) => name,
+            DefPathDataName::Anon { .. } => unreachable!(),
+        });
+
+        let volatile_suffix = volatile.then_some("volatile");
+
+        name_builder.build_cgu_name(def_path.krate, components, volatile_suffix)
+    })
+}
+
+// Anything we can't find a proper codegen unit for goes into this.
+fn fallback_cgu_name(name_builder: &mut CodegenUnitNameBuilder<'_>) -> Symbol {
+    name_builder.build_cgu_name(LOCAL_CRATE, &["fallback"], Some("cgu"))
+}
+
+fn mono_item_linkage_and_visibility<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    mono_item: &MonoItem<'tcx>,
+    can_be_internalized: &mut bool,
+    export_generics: bool,
+) -> (Linkage, Visibility) {
+    if let Some(explicit_linkage) = mono_item.explicit_linkage(tcx) {
+        return (explicit_linkage, Visibility::Default);
+    }
+    let vis = mono_item_visibility(tcx, mono_item, can_be_internalized, export_generics);
+    (Linkage::External, vis)
+}
+
+type CguNameCache = FxHashMap<(DefId, bool), Symbol>;
+
+fn static_visibility<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    can_be_internalized: &mut bool,
+    def_id: DefId,
+) -> Visibility {
+    if tcx.is_reachable_non_generic(def_id) {
+        *can_be_internalized = false;
+        default_visibility(tcx, def_id, false)
+    } else {
+        Visibility::Hidden
+    }
+}
+
+fn mono_item_visibility<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    mono_item: &MonoItem<'tcx>,
+    can_be_internalized: &mut bool,
+    export_generics: bool,
+) -> Visibility {
+    let instance = match mono_item {
+        // This is pretty complicated; see below.
+        MonoItem::Fn(instance) => instance,
+
+        // Misc handling for generics and such, but otherwise:
+        MonoItem::Static(def_id) => return static_visibility(tcx, can_be_internalized, *def_id),
+        MonoItem::GlobalAsm(item_id) => {
+            return static_visibility(tcx, can_be_internalized, item_id.owner_id.to_def_id());
+        }
+    };
+
+    let def_id = match instance.def {
+        InstanceDef::Item(def_id) | InstanceDef::DropGlue(def_id, Some(_)) => def_id,
+
+        // We match the visibility of statics here
+        InstanceDef::ThreadLocalShim(def_id) => {
+            return static_visibility(tcx, can_be_internalized, def_id);
+        }
+
+        // These are all compiler glue and such, never exported, always hidden.
+        InstanceDef::VTableShim(..)
+        | InstanceDef::ReifyShim(..)
+        | InstanceDef::FnPtrShim(..)
+        | InstanceDef::Virtual(..)
+        | InstanceDef::Intrinsic(..)
+        | InstanceDef::ClosureOnceShim { .. }
+        | InstanceDef::DropGlue(..)
+        | InstanceDef::CloneShim(..)
+        | InstanceDef::FnPtrAddrShim(..) => return Visibility::Hidden,
+    };
+
+    // The `start_fn` lang item is actually a monomorphized instance of a
+    // function in the standard library, used for the `main` function. We don't
+    // want to export it so we tag it with `Hidden` visibility but this symbol
+    // is only referenced from the actual `main` symbol which we unfortunately
+    // don't know anything about during partitioning/collection. As a result we
+    // forcibly keep this symbol out of the `internalization_candidates` set.
+    //
+    // FIXME: eventually we don't want to always force this symbol to have
+    //        hidden visibility, it should indeed be a candidate for
+    //        internalization, but we have to understand that it's referenced
+    //        from the `main` symbol we'll generate later.
+    //
+    //        This may be fixable with a new `InstanceDef` perhaps? Unsure!
+    if tcx.lang_items().start_fn() == Some(def_id) {
+        *can_be_internalized = false;
+        return Visibility::Hidden;
+    }
+
+    let is_generic = instance.substs.non_erasable_generics().next().is_some();
+
+    // Upstream `DefId` instances get different handling than local ones.
+    let Some(def_id) = def_id.as_local() else {
+        return if export_generics && is_generic {
+            // If it is an upstream monomorphization and we export generics, we must make
+            // it available to downstream crates.
+            *can_be_internalized = false;
+            default_visibility(tcx, def_id, true)
+        } else {
+            Visibility::Hidden
+        };
+    };
+
+    if is_generic {
+        if export_generics {
+            if tcx.is_unreachable_local_definition(def_id) {
+                // This instance cannot be used from another crate.
+                Visibility::Hidden
+            } else {
+                // This instance might be useful in a downstream crate.
+                *can_be_internalized = false;
+                default_visibility(tcx, def_id.to_def_id(), true)
+            }
+        } else {
+            // We are not exporting generics or the definition is not reachable
+            // for downstream crates, we can internalize its instantiations.
+            Visibility::Hidden
+        }
+    } else {
+        // If this isn't a generic function then we mark this a `Default` if
+        // this is a reachable item, meaning that it's a symbol other crates may
+        // use when they link to us.
+        if tcx.is_reachable_non_generic(def_id.to_def_id()) {
+            *can_be_internalized = false;
+            debug_assert!(!is_generic);
+            return default_visibility(tcx, def_id.to_def_id(), false);
+        }
+
+        // If this isn't reachable then we're gonna tag this with `Hidden`
+        // visibility. In some situations though we'll want to prevent this
+        // symbol from being internalized.
+        //
+        // There's two categories of items here:
+        //
+        // * First is weak lang items. These are basically mechanisms for
+        //   libcore to forward-reference symbols defined later in crates like
+        //   the standard library or `#[panic_handler]` definitions. The
+        //   definition of these weak lang items needs to be referencable by
+        //   libcore, so we're no longer a candidate for internalization.
+        //   Removal of these functions can't be done by LLVM but rather must be
+        //   done by the linker as it's a non-local decision.
+        //
+        // * Second is "std internal symbols". Currently this is primarily used
+        //   for allocator symbols. Allocators are a little weird in their
+        //   implementation, but the idea is that the compiler, at the last
+        //   minute, defines an allocator with an injected object file. The
+        //   `alloc` crate references these symbols (`__rust_alloc`) and the
+        //   definition doesn't get hooked up until a linked crate artifact is
+        //   generated.
+        //
+        //   The symbols synthesized by the compiler (`__rust_alloc`) are thin
+        //   veneers around the actual implementation, some other symbol which
+        //   implements the same ABI. These symbols (things like `__rg_alloc`,
+        //   `__rdl_alloc`, `__rde_alloc`, etc), are all tagged with "std
+        //   internal symbols".
+        //
+        //   The std-internal symbols here **should not show up in a dll as an
+        //   exported interface**, so they return `false` from
+        //   `is_reachable_non_generic` above and we'll give them `Hidden`
+        //   visibility below. Like the weak lang items, though, we can't let
+        //   LLVM internalize them as this decision is left up to the linker to
+        //   omit them, so prevent them from being internalized.
+        let attrs = tcx.codegen_fn_attrs(def_id);
+        if attrs.flags.contains(CodegenFnAttrFlags::RUSTC_STD_INTERNAL_SYMBOL) {
+            *can_be_internalized = false;
+        }
+
+        Visibility::Hidden
+    }
+}
+
+fn default_visibility(tcx: TyCtxt<'_>, id: DefId, is_generic: bool) -> Visibility {
+    if !tcx.sess.target.default_hidden_visibility {
+        return Visibility::Default;
+    }
+
+    // Generic functions never have export-level C.
+    if is_generic {
+        return Visibility::Hidden;
+    }
+
+    // Things with export level C don't get instantiated in
+    // downstream crates.
+    if !id.is_local() {
+        return Visibility::Hidden;
+    }
+
+    // C-export level items remain at `Default`, all other internal
+    // items become `Hidden`.
+    match tcx.reachable_non_generics(id.krate).get(&id) {
+        Some(SymbolExportInfo { level: SymbolExportLevel::C, .. }) => Visibility::Default,
+        _ => Visibility::Hidden,
+    }
+}
+
+fn debug_dump<'a, 'tcx: 'a>(
+    tcx: TyCtxt<'tcx>,
+    label: &str,
+    cgus: &[CodegenUnit<'tcx>],
+    (unique_inlined_items, unique_inlined_size): (usize, usize),
+) {
+    let dump = move || {
+        use std::fmt::Write;
+
+        let mut num_cgus = 0;
+        let mut all_cgu_sizes = Vec::new();
+
+        // Note: every unique root item is placed exactly once, so the number
+        // of unique root items always equals the number of placed root items.
+
+        let mut root_items = 0;
+        // unique_inlined_items is passed in above.
+        let mut placed_inlined_items = 0;
+
+        let mut root_size = 0;
+        // unique_inlined_size is passed in above.
+        let mut placed_inlined_size = 0;
+
+        for cgu in cgus.iter() {
+            num_cgus += 1;
+            all_cgu_sizes.push(cgu.size_estimate());
+
+            for (item, _) in cgu.items() {
+                match item.instantiation_mode(tcx) {
+                    InstantiationMode::GloballyShared { .. } => {
+                        root_items += 1;
+                        root_size += item.size_estimate(tcx);
+                    }
+                    InstantiationMode::LocalCopy => {
+                        placed_inlined_items += 1;
+                        placed_inlined_size += item.size_estimate(tcx);
+                    }
+                }
+            }
+        }
+
+        all_cgu_sizes.sort_unstable_by_key(|&n| cmp::Reverse(n));
+
+        let unique_items = root_items + unique_inlined_items;
+        let placed_items = root_items + placed_inlined_items;
+        let items_ratio = placed_items as f64 / unique_items as f64;
+
+        let unique_size = root_size + unique_inlined_size;
+        let placed_size = root_size + placed_inlined_size;
+        let size_ratio = placed_size as f64 / unique_size as f64;
+
+        let mean_cgu_size = placed_size as f64 / num_cgus as f64;
+
+        assert_eq!(placed_size, all_cgu_sizes.iter().sum::<usize>());
+
+        let s = &mut String::new();
+        let _ = writeln!(s, "{label}");
+        let _ = writeln!(
+            s,
+            "- unique items: {unique_items} ({root_items} root + {unique_inlined_items} inlined), \
+               unique size: {unique_size} ({root_size} root + {unique_inlined_size} inlined)\n\
+             - placed items: {placed_items} ({root_items} root + {placed_inlined_items} inlined), \
+               placed size: {placed_size} ({root_size} root + {placed_inlined_size} inlined)\n\
+             - placed/unique items ratio: {items_ratio:.2}, \
+               placed/unique size ratio: {size_ratio:.2}\n\
+             - CGUs: {num_cgus}, mean size: {mean_cgu_size:.1}, sizes: {}",
+            list(&all_cgu_sizes),
+        );
+        let _ = writeln!(s);
+
+        for (i, cgu) in cgus.iter().enumerate() {
+            let name = cgu.name();
+            let size = cgu.size_estimate();
+            let num_items = cgu.items().len();
+            let mean_size = size as f64 / num_items as f64;
+
+            let mut placed_item_sizes: Vec<_> =
+                cgu.items().iter().map(|(item, _)| item.size_estimate(tcx)).collect();
+            placed_item_sizes.sort_unstable_by_key(|&n| cmp::Reverse(n));
+            let sizes = list(&placed_item_sizes);
+
+            let _ = writeln!(s, "- CGU[{i}]");
+            let _ = writeln!(s, "  - {name}, size: {size}");
+            let _ =
+                writeln!(s, "  - items: {num_items}, mean size: {mean_size:.1}, sizes: {sizes}",);
+
+            for (item, linkage) in cgu.items_in_deterministic_order(tcx) {
+                let symbol_name = item.symbol_name(tcx).name;
+                let symbol_hash_start = symbol_name.rfind('h');
+                let symbol_hash = symbol_hash_start.map_or("<no hash>", |i| &symbol_name[i..]);
+                let size = item.size_estimate(tcx);
+                let kind = match item.instantiation_mode(tcx) {
+                    InstantiationMode::GloballyShared { .. } => "root",
+                    InstantiationMode::LocalCopy => "inlined",
+                };
+                let _ = with_no_trimmed_paths!(writeln!(
+                    s,
+                    "  - {item} [{linkage:?}] [{symbol_hash}] ({kind}, size: {size})"
+                ));
+            }
+
+            let _ = writeln!(s);
+        }
+
+        return std::mem::take(s);
+
+        // Converts a slice to a string, capturing repetitions to save space.
+        // E.g. `[4, 4, 4, 3, 2, 1, 1, 1, 1, 1]` -> "[4 (x3), 3, 2, 1 (x5)]".
+        fn list(ns: &[usize]) -> String {
+            let mut v = Vec::new();
+            if ns.is_empty() {
+                return "[]".to_string();
+            }
+
+            let mut elem = |curr, curr_count| {
+                if curr_count == 1 {
+                    v.push(format!("{curr}"));
+                } else {
+                    v.push(format!("{curr} (x{curr_count})"));
+                }
+            };
+
+            let mut curr = ns[0];
+            let mut curr_count = 1;
+
+            for &n in &ns[1..] {
+                if n != curr {
+                    elem(curr, curr_count);
+                    curr = n;
+                    curr_count = 1;
+                } else {
+                    curr_count += 1;
+                }
+            }
+            elem(curr, curr_count);
+
+            let mut s = "[".to_string();
+            s.push_str(&v.join(", "));
+            s.push_str("]");
+            s
+        }
+    };
+
+    debug!("{}", dump());
+}
+
+#[inline(never)] // give this a place in the profiler
+fn assert_symbols_are_distinct<'a, 'tcx, I>(tcx: TyCtxt<'tcx>, mono_items: I)
+where
+    I: Iterator<Item = &'a MonoItem<'tcx>>,
+    'tcx: 'a,
+{
+    let _prof_timer = tcx.prof.generic_activity("assert_symbols_are_distinct");
+
+    let mut symbols: Vec<_> =
+        mono_items.map(|mono_item| (mono_item, mono_item.symbol_name(tcx))).collect();
+
+    symbols.sort_by_key(|sym| sym.1);
+
+    for &[(mono_item1, ref sym1), (mono_item2, ref sym2)] in symbols.array_windows() {
+        if sym1 == sym2 {
+            let span1 = mono_item1.local_span(tcx);
+            let span2 = mono_item2.local_span(tcx);
+
+            // Deterministically select one of the spans for error reporting
+            let span = match (span1, span2) {
+                (Some(span1), Some(span2)) => {
+                    Some(if span1.lo().0 > span2.lo().0 { span1 } else { span2 })
+                }
+                (span1, span2) => span1.or(span2),
+            };
+
+            tcx.sess.emit_fatal(SymbolAlreadyDefined { span, symbol: sym1.to_string() });
+        }
+    }
+}
+
+fn collect_and_partition_mono_items(tcx: TyCtxt<'_>, (): ()) -> (&DefIdSet, &[CodegenUnit<'_>]) {
+    let collection_mode = match tcx.sess.opts.unstable_opts.print_mono_items {
+        Some(ref s) => {
+            let mode = s.to_lowercase();
+            let mode = mode.trim();
+            if mode == "eager" {
+                MonoItemCollectionMode::Eager
+            } else {
+                if mode != "lazy" {
+                    tcx.sess.emit_warning(UnknownCguCollectionMode { mode });
+                }
+
+                MonoItemCollectionMode::Lazy
+            }
+        }
+        None => {
+            if tcx.sess.link_dead_code() {
+                MonoItemCollectionMode::Eager
+            } else {
+                MonoItemCollectionMode::Lazy
+            }
+        }
+    };
+
+    let (items, usage_map) = collector::collect_crate_mono_items(tcx, collection_mode);
+
+    tcx.sess.abort_if_errors();
+
+    let (codegen_units, _) = tcx.sess.time("partition_and_assert_distinct_symbols", || {
+        sync::join(
+            || {
+                let mut codegen_units = partition(tcx, items.iter().copied(), &usage_map);
+                codegen_units[0].make_primary();
+                &*tcx.arena.alloc_from_iter(codegen_units)
+            },
+            || assert_symbols_are_distinct(tcx, items.iter()),
+        )
+    });
+
+    if tcx.prof.enabled() {
+        // Record CGU size estimates for self-profiling.
+        for cgu in codegen_units {
+            tcx.prof.artifact_size(
+                "codegen_unit_size_estimate",
+                cgu.name().as_str(),
+                cgu.size_estimate() as u64,
+            );
+        }
+    }
+
+    let mono_items: DefIdSet = items
+        .iter()
+        .filter_map(|mono_item| match *mono_item {
+            MonoItem::Fn(ref instance) => Some(instance.def_id()),
+            MonoItem::Static(def_id) => Some(def_id),
+            _ => None,
+        })
+        .collect();
+
+    // Output monomorphization stats per def_id
+    if let SwitchWithOptPath::Enabled(ref path) = tcx.sess.opts.unstable_opts.dump_mono_stats {
+        if let Err(err) =
+            dump_mono_items_stats(tcx, &codegen_units, path, tcx.crate_name(LOCAL_CRATE))
+        {
+            tcx.sess.emit_fatal(CouldntDumpMonoStats { error: err.to_string() });
+        }
+    }
+
+    if tcx.sess.opts.unstable_opts.print_mono_items.is_some() {
+        let mut item_to_cgus: FxHashMap<_, Vec<_>> = Default::default();
+
+        for cgu in codegen_units {
+            for (&mono_item, &linkage) in cgu.items() {
+                item_to_cgus.entry(mono_item).or_default().push((cgu.name(), linkage));
+            }
+        }
+
+        let mut item_keys: Vec<_> = items
+            .iter()
+            .map(|i| {
+                let mut output = with_no_trimmed_paths!(i.to_string());
+                output.push_str(" @@");
+                let mut empty = Vec::new();
+                let cgus = item_to_cgus.get_mut(i).unwrap_or(&mut empty);
+                cgus.sort_by_key(|(name, _)| *name);
+                cgus.dedup();
+                for &(ref cgu_name, (linkage, _)) in cgus.iter() {
+                    output.push(' ');
+                    output.push_str(cgu_name.as_str());
+
+                    let linkage_abbrev = match linkage {
+                        Linkage::External => "External",
+                        Linkage::AvailableExternally => "Available",
+                        Linkage::LinkOnceAny => "OnceAny",
+                        Linkage::LinkOnceODR => "OnceODR",
+                        Linkage::WeakAny => "WeakAny",
+                        Linkage::WeakODR => "WeakODR",
+                        Linkage::Appending => "Appending",
+                        Linkage::Internal => "Internal",
+                        Linkage::Private => "Private",
+                        Linkage::ExternalWeak => "ExternalWeak",
+                        Linkage::Common => "Common",
+                    };
+
+                    output.push('[');
+                    output.push_str(linkage_abbrev);
+                    output.push(']');
+                }
+                output
+            })
+            .collect();
+
+        item_keys.sort();
+
+        for item in item_keys {
+            println!("MONO_ITEM {item}");
+        }
+    }
+
+    (tcx.arena.alloc(mono_items), codegen_units)
+}
+
+/// Outputs stats about instantiation counts and estimated size, per `MonoItem`'s
+/// def, to a file in the given output directory.
+fn dump_mono_items_stats<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    codegen_units: &[CodegenUnit<'tcx>],
+    output_directory: &Option<PathBuf>,
+    crate_name: Symbol,
+) -> Result<(), Box<dyn std::error::Error>> {
+    let output_directory = if let Some(ref directory) = output_directory {
+        fs::create_dir_all(directory)?;
+        directory
+    } else {
+        Path::new(".")
+    };
+
+    let format = tcx.sess.opts.unstable_opts.dump_mono_stats_format;
+    let ext = format.extension();
+    let filename = format!("{crate_name}.mono_items.{ext}");
+    let output_path = output_directory.join(&filename);
+    let file = File::create(&output_path)?;
+    let mut file = BufWriter::new(file);
+
+    // Gather instantiated mono items grouped by def_id
+    let mut items_per_def_id: FxHashMap<_, Vec<_>> = Default::default();
+    for cgu in codegen_units {
+        for (&mono_item, _) in cgu.items() {
+            // Avoid variable-sized compiler-generated shims
+            if mono_item.is_user_defined() {
+                items_per_def_id.entry(mono_item.def_id()).or_default().push(mono_item);
+            }
+        }
+    }
+
+    #[derive(serde::Serialize)]
+    struct MonoItem {
+        name: String,
+        instantiation_count: usize,
+        size_estimate: usize,
+        total_estimate: usize,
+    }
+
+    // Output stats sorted by total instantiated size, from heaviest to lightest
+    let mut stats: Vec<_> = items_per_def_id
+        .into_iter()
+        .map(|(def_id, items)| {
+            let name = with_no_trimmed_paths!(tcx.def_path_str(def_id));
+            let instantiation_count = items.len();
+            let size_estimate = items[0].size_estimate(tcx);
+            let total_estimate = instantiation_count * size_estimate;
+            MonoItem { name, instantiation_count, size_estimate, total_estimate }
+        })
+        .collect();
+    stats.sort_unstable_by_key(|item| cmp::Reverse(item.total_estimate));
+
+    if !stats.is_empty() {
+        match format {
+            DumpMonoStatsFormat::Json => serde_json::to_writer(file, &stats)?,
+            DumpMonoStatsFormat::Markdown => {
+                writeln!(
+                    file,
+                    "| Item | Instantiation count | Estimated Cost Per Instantiation | Total Estimated Cost |"
+                )?;
+                writeln!(file, "| --- | ---: | ---: | ---: |")?;
+
+                for MonoItem { name, instantiation_count, size_estimate, total_estimate } in stats {
+                    writeln!(
+                        file,
+                        "| `{name}` | {instantiation_count} | {size_estimate} | {total_estimate} |"
+                    )?;
+                }
+            }
+        }
+    }
+
+    Ok(())
+}
+
+fn codegened_and_inlined_items(tcx: TyCtxt<'_>, (): ()) -> &DefIdSet {
+    let (items, cgus) = tcx.collect_and_partition_mono_items(());
+    let mut visited = DefIdSet::default();
+    let mut result = items.clone();
+
+    for cgu in cgus {
+        for (item, _) in cgu.items() {
+            if let MonoItem::Fn(ref instance) = item {
+                let did = instance.def_id();
+                if !visited.insert(did) {
+                    continue;
+                }
+                let body = tcx.instance_mir(instance.def);
+                for block in body.basic_blocks.iter() {
+                    for statement in &block.statements {
+                        let mir::StatementKind::Coverage(_) = statement.kind else { continue };
+                        let scope = statement.source_info.scope;
+                        if let Some(inlined) = scope.inlined_instance(&body.source_scopes) {
+                            result.insert(inlined.def_id());
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    tcx.arena.alloc(result)
+}
+
+pub fn provide(providers: &mut Providers) {
+    providers.collect_and_partition_mono_items = collect_and_partition_mono_items;
+    providers.codegened_and_inlined_items = codegened_and_inlined_items;
+
+    providers.is_codegened_item = |tcx, def_id| {
+        let (all_mono_items, _) = tcx.collect_and_partition_mono_items(());
+        all_mono_items.contains(&def_id)
+    };
+
+    providers.codegen_unit = |tcx, name| {
+        let (_, all) = tcx.collect_and_partition_mono_items(());
+        all.iter()
+            .find(|cgu| cgu.name() == name)
+            .unwrap_or_else(|| panic!("failed to find cgu with name {name:?}"))
+    };
+}