diff --git a/compiler/rustc_monomorphize/src/collector.rs b/compiler/rustc_monomorphize/src/collector.rs
index cf7226a..ec1de30 100644
--- a/compiler/rustc_monomorphize/src/collector.rs
+++ b/compiler/rustc_monomorphize/src/collector.rs
@@ -526,7 +526,7 @@
         let formatted_item = with_no_trimmed_paths!(starting_point.node.to_string());
         tcx.sess.span_note_without_error(
             starting_point.span,
-            &format!("the above error was encountered while instantiating `{}`", formatted_item),
+            &format!("the above error was encountered while instantiating `{formatted_item}`"),
         );
     }
     inlining_map.lock_mut().record_accesses(starting_point.node, &neighbors.items);
@@ -595,8 +595,8 @@
         let def_path_str = tcx.def_path_str(def_id);
         let (shrunk, written_to_path) = shrunk_instance_name(tcx, &instance);
         let mut path = PathBuf::new();
-        let was_written = if written_to_path.is_some() {
-            path = written_to_path.unwrap();
+        let was_written = if let Some(written_to_path) = written_to_path {
+            path = written_to_path;
             Some(())
         } else {
             None
@@ -843,7 +843,7 @@
             mir::TerminatorKind::Abort { .. } => {
                 let instance = Instance::mono(
                     tcx,
-                    tcx.require_lang_item(LangItem::PanicNoUnwind, Some(source)),
+                    tcx.require_lang_item(LangItem::PanicCannotUnwind, Some(source)),
                 );
                 if should_codegen_locally(tcx, &instance) {
                     self.output.push(create_fn_mono_item(tcx, instance, source));
@@ -931,10 +931,13 @@
 ) {
     if let ty::FnDef(def_id, substs) = *ty.kind() {
         let instance = if is_direct_call {
-            ty::Instance::resolve(tcx, ty::ParamEnv::reveal_all(), def_id, substs).unwrap().unwrap()
+            ty::Instance::expect_resolve(tcx, ty::ParamEnv::reveal_all(), def_id, substs)
         } else {
-            ty::Instance::resolve_for_fn_ptr(tcx, ty::ParamEnv::reveal_all(), def_id, substs)
-                .unwrap()
+            match ty::Instance::resolve_for_fn_ptr(tcx, ty::ParamEnv::reveal_all(), def_id, substs)
+            {
+                Some(instance) => instance,
+                _ => bug!("failed to resolve instance for {ty}"),
+            }
         };
         visit_instance_use(tcx, instance, is_direct_call, source, output);
     }
@@ -1349,6 +1352,8 @@
             );
 
             if let Some(trait_ref) = tcx.impl_trait_ref(item.owner_id) {
+                let trait_ref = trait_ref.subst_identity();
+
                 let param_env = ty::ParamEnv::reveal_all();
                 let trait_ref = tcx.normalize_erasing_regions(param_env, trait_ref);
                 let overridden_methods = tcx.impl_item_implementor_ids(item.owner_id);
@@ -1369,9 +1374,8 @@
                                 trait_ref.substs[param.index as usize]
                             }
                         });
-                    let instance = ty::Instance::resolve(tcx, param_env, method.def_id, substs)
-                        .unwrap()
-                        .unwrap();
+                    let instance =
+                        ty::Instance::expect_resolve(tcx, param_env, method.def_id, substs);
 
                     let mono_item = create_fn_mono_item(tcx, instance, DUMMY_SP);
                     if mono_item.node.is_instantiable(tcx) && should_codegen_locally(tcx, &instance)
diff --git a/compiler/rustc_monomorphize/src/errors.rs b/compiler/rustc_monomorphize/src/errors.rs
index f1ca72d..5233cfb 100644
--- a/compiler/rustc_monomorphize/src/errors.rs
+++ b/compiler/rustc_monomorphize/src/errors.rs
@@ -32,13 +32,13 @@
     pub type_length: usize,
 }
 
-pub struct UnusedGenericParams {
+pub struct UnusedGenericParamsHint {
     pub span: Span,
     pub param_spans: Vec<Span>,
     pub param_names: Vec<String>,
 }
 
-impl IntoDiagnostic<'_> for UnusedGenericParams {
+impl IntoDiagnostic<'_> for UnusedGenericParamsHint {
     #[track_caller]
     fn into_diagnostic(
         self,
@@ -50,7 +50,7 @@
             // FIXME: I can figure out how to do a label with a fluent string with a fixed message,
             // or a label with a dynamic value in a hard-coded string, but I haven't figured out
             // how to combine the two. 😢
-            diag.span_label(span, format!("generic parameter `{}` is unused", name));
+            diag.span_label(span, format!("generic parameter `{name}` is unused"));
         }
         diag
     }
@@ -77,3 +77,9 @@
     pub span: Option<Span>,
     pub symbol: String,
 }
+
+#[derive(Diagnostic)]
+#[diag(monomorphize_couldnt_dump_mono_stats)]
+pub struct CouldntDumpMonoStats {
+    pub error: String,
+}
diff --git a/compiler/rustc_monomorphize/src/lib.rs b/compiler/rustc_monomorphize/src/lib.rs
index b616ed3..f88155e 100644
--- a/compiler/rustc_monomorphize/src/lib.rs
+++ b/compiler/rustc_monomorphize/src/lib.rs
@@ -1,5 +1,4 @@
 #![feature(array_windows)]
-#![feature(control_flow_enum)]
 #![recursion_limit = "256"]
 #![allow(rustc::potential_query_instability)]
 #![deny(rustc::untranslatable_diagnostic)]
diff --git a/compiler/rustc_monomorphize/src/partitioning/mod.rs b/compiler/rustc_monomorphize/src/partitioning/mod.rs
index 932edc6..fd6bcad 100644
--- a/compiler/rustc_monomorphize/src/partitioning/mod.rs
+++ b/compiler/rustc_monomorphize/src/partitioning/mod.rs
@@ -95,20 +95,26 @@
 mod default;
 mod merging;
 
+use std::cmp;
+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_id::DefIdSet;
+use rustc_hir::def_id::{DefIdSet, LOCAL_CRATE};
 use rustc_middle::mir;
 use rustc_middle::mir::mono::MonoItem;
 use rustc_middle::mir::mono::{CodegenUnit, Linkage};
 use rustc_middle::ty::print::with_no_trimmed_paths;
 use rustc_middle::ty::query::Providers;
 use rustc_middle::ty::TyCtxt;
+use rustc_session::config::{DumpMonoStatsFormat, SwitchWithOptPath};
 use rustc_span::symbol::Symbol;
 
 use crate::collector::InliningMap;
 use crate::collector::{self, MonoItemCollectionMode};
-use crate::errors::{SymbolAlreadyDefined, UnknownPartitionStrategy};
+use crate::errors::{CouldntDumpMonoStats, SymbolAlreadyDefined, UnknownPartitionStrategy};
 
 pub struct PartitioningCx<'a, 'tcx> {
     tcx: TyCtxt<'tcx>,
@@ -279,7 +285,7 @@
         use std::fmt::Write;
 
         let s = &mut String::new();
-        let _ = writeln!(s, "{}", label);
+        let _ = writeln!(s, "{label}");
         for cgu in cgus {
             let _ =
                 writeln!(s, "CodegenUnit {} estimated size {} :", cgu.name(), cgu.size_estimate());
@@ -299,7 +305,7 @@
                 );
             }
 
-            let _ = writeln!(s, "");
+            let _ = writeln!(s);
         }
 
         std::mem::take(s)
@@ -339,10 +345,7 @@
     }
 }
 
-fn collect_and_partition_mono_items<'tcx>(
-    tcx: TyCtxt<'tcx>,
-    (): (),
-) -> (&'tcx DefIdSet, &'tcx [CodegenUnit<'tcx>]) {
+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_string = s.to_lowercase();
@@ -352,9 +355,8 @@
             } else {
                 if mode_string != "lazy" {
                     let message = format!(
-                        "Unknown codegen-item collection mode '{}'. \
-                                           Falling back to 'lazy' mode.",
-                        mode_string
+                        "Unknown codegen-item collection mode '{mode_string}'. \
+                                           Falling back to 'lazy' mode."
                     );
                     tcx.sess.warn(&message);
                 }
@@ -411,6 +413,15 @@
         })
         .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();
 
@@ -458,14 +469,91 @@
         item_keys.sort();
 
         for item in item_keys {
-            println!("MONO_ITEM {}", item);
+            println!("MONO_ITEM {item}");
         }
     }
 
     (tcx.arena.alloc(mono_items), codegen_units)
 }
 
-fn codegened_and_inlined_items<'tcx>(tcx: TyCtxt<'tcx>, (): ()) -> &'tcx DefIdSet {
+/// Outputs stats about instantation 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();
@@ -507,6 +595,6 @@
         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))
+            .unwrap_or_else(|| panic!("failed to find cgu with name {name:?}"))
     };
 }
diff --git a/compiler/rustc_monomorphize/src/polymorphize.rs b/compiler/rustc_monomorphize/src/polymorphize.rs
index 650076c..cf13d45 100644
--- a/compiler/rustc_monomorphize/src/polymorphize.rs
+++ b/compiler/rustc_monomorphize/src/polymorphize.rs
@@ -6,7 +6,6 @@
 //! for their size, offset of a field, etc.).
 
 use rustc_hir::{def::DefKind, def_id::DefId, ConstContext};
-use rustc_index::bit_set::FiniteBitSet;
 use rustc_middle::mir::{
     self,
     visit::{TyContext, Visitor},
@@ -17,13 +16,12 @@
     query::Providers,
     subst::SubstsRef,
     visit::{TypeSuperVisitable, TypeVisitable, TypeVisitor},
-    Const, Ty, TyCtxt,
+    Const, Ty, TyCtxt, UnusedGenericParams,
 };
 use rustc_span::symbol::sym;
-use std::convert::TryInto;
 use std::ops::ControlFlow;
 
-use crate::errors::UnusedGenericParams;
+use crate::errors::UnusedGenericParamsHint;
 
 /// Provide implementations of queries relating to polymorphization analysis.
 pub fn provide(providers: &mut Providers) {
@@ -37,16 +35,16 @@
 fn unused_generic_params<'tcx>(
     tcx: TyCtxt<'tcx>,
     instance: ty::InstanceDef<'tcx>,
-) -> FiniteBitSet<u32> {
+) -> UnusedGenericParams {
     if !tcx.sess.opts.unstable_opts.polymorphize {
         // If polymorphization disabled, then all parameters are used.
-        return FiniteBitSet::new_empty();
+        return UnusedGenericParams::new_all_used();
     }
 
     let def_id = instance.def_id();
     // Exit early if this instance should not be polymorphized.
     if !should_polymorphize(tcx, def_id, instance) {
-        return FiniteBitSet::new_empty();
+        return UnusedGenericParams::new_all_used();
     }
 
     let generics = tcx.generics_of(def_id);
@@ -54,14 +52,13 @@
 
     // Exit early when there are no parameters to be unused.
     if generics.count() == 0 {
-        return FiniteBitSet::new_empty();
+        return UnusedGenericParams::new_all_used();
     }
 
     // Create a bitset with N rightmost ones for each parameter.
     let generics_count: u32 =
         generics.count().try_into().expect("more generic parameters than can fit into a `u32`");
-    let mut unused_parameters = FiniteBitSet::<u32>::new_empty();
-    unused_parameters.set_range(0..generics_count);
+    let mut unused_parameters = UnusedGenericParams::new_all_unused(generics_count);
     debug!(?unused_parameters, "(start)");
 
     mark_used_by_default_parameters(tcx, def_id, generics, &mut unused_parameters);
@@ -79,7 +76,7 @@
     debug!(?unused_parameters, "(end)");
 
     // Emit errors for debugging and testing if enabled.
-    if !unused_parameters.is_empty() {
+    if !unused_parameters.all_used() {
         emit_unused_generic_params_error(tcx, def_id, generics, &unused_parameters);
     }
 
@@ -137,13 +134,13 @@
     tcx: TyCtxt<'tcx>,
     def_id: DefId,
     generics: &'tcx ty::Generics,
-    unused_parameters: &mut FiniteBitSet<u32>,
+    unused_parameters: &mut UnusedGenericParams,
 ) {
     match tcx.def_kind(def_id) {
         DefKind::Closure | DefKind::Generator => {
             for param in &generics.params {
                 debug!(?param, "(closure/gen)");
-                unused_parameters.clear(param.index);
+                unused_parameters.mark_used(param.index);
             }
         }
         DefKind::Mod
@@ -179,7 +176,7 @@
             for param in &generics.params {
                 debug!(?param, "(other)");
                 if let ty::GenericParamDefKind::Lifetime = param.kind {
-                    unused_parameters.clear(param.index);
+                    unused_parameters.mark_used(param.index);
                 }
             }
         }
@@ -197,7 +194,7 @@
     tcx: TyCtxt<'tcx>,
     def_id: DefId,
     generics: &'tcx ty::Generics,
-    unused_parameters: &FiniteBitSet<u32>,
+    unused_parameters: &UnusedGenericParams,
 ) {
     let base_def_id = tcx.typeck_root_def_id(def_id);
     if !tcx.has_attr(base_def_id, sym::rustc_polymorphize_error) {
@@ -214,7 +211,7 @@
     let mut next_generics = Some(generics);
     while let Some(generics) = next_generics {
         for param in &generics.params {
-            if unused_parameters.contains(param.index).unwrap_or(false) {
+            if unused_parameters.is_unused(param.index) {
                 debug!(?param);
                 let def_span = tcx.def_span(param.def_id);
                 param_spans.push(def_span);
@@ -225,14 +222,14 @@
         next_generics = generics.parent.map(|did| tcx.generics_of(did));
     }
 
-    tcx.sess.emit_err(UnusedGenericParams { span: fn_span, param_spans, param_names });
+    tcx.sess.emit_err(UnusedGenericParamsHint { span: fn_span, param_spans, param_names });
 }
 
 /// Visitor used to aggregate generic parameter uses.
 struct MarkUsedGenericParams<'a, 'tcx> {
     tcx: TyCtxt<'tcx>,
     def_id: DefId,
-    unused_parameters: &'a mut FiniteBitSet<u32>,
+    unused_parameters: &'a mut UnusedGenericParams,
 }
 
 impl<'a, 'tcx> MarkUsedGenericParams<'a, 'tcx> {
@@ -245,7 +242,7 @@
         debug!(?self.unused_parameters, ?unused);
         for (i, arg) in substs.iter().enumerate() {
             let i = i.try_into().unwrap();
-            if !unused.contains(i).unwrap_or(false) {
+            if unused.is_used(i) {
                 arg.visit_with(self);
             }
         }
@@ -303,20 +300,20 @@
     #[instrument(level = "debug", skip(self))]
     fn visit_const(&mut self, c: Const<'tcx>) -> ControlFlow<Self::BreakTy> {
         if !c.has_non_region_param() {
-            return ControlFlow::CONTINUE;
+            return ControlFlow::Continue(());
         }
 
         match c.kind() {
             ty::ConstKind::Param(param) => {
                 debug!(?param);
-                self.unused_parameters.clear(param.index);
-                ControlFlow::CONTINUE
+                self.unused_parameters.mark_used(param.index);
+                ControlFlow::Continue(())
             }
             ty::ConstKind::Unevaluated(ty::UnevaluatedConst { def, substs })
                 if matches!(self.tcx.def_kind(def.did), DefKind::AnonConst) =>
             {
                 self.visit_child_body(def.did, substs);
-                ControlFlow::CONTINUE
+                ControlFlow::Continue(())
             }
             _ => c.super_visit_with(self),
         }
@@ -325,7 +322,7 @@
     #[instrument(level = "debug", skip(self))]
     fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
         if !ty.has_non_region_param() {
-            return ControlFlow::CONTINUE;
+            return ControlFlow::Continue(());
         }
 
         match *ty.kind() {
@@ -333,63 +330,18 @@
                 debug!(?def_id);
                 // Avoid cycle errors with generators.
                 if def_id == self.def_id {
-                    return ControlFlow::CONTINUE;
+                    return ControlFlow::Continue(());
                 }
 
                 // Consider any generic parameters used by any closures/generators as used in the
                 // parent.
                 self.visit_child_body(def_id, substs);
-                ControlFlow::CONTINUE
+                ControlFlow::Continue(())
             }
             ty::Param(param) => {
                 debug!(?param);
-                self.unused_parameters.clear(param.index);
-                ControlFlow::CONTINUE
-            }
-            _ => ty.super_visit_with(self),
-        }
-    }
-}
-
-/// Visitor used to check if a generic parameter is used.
-struct HasUsedGenericParams<'a> {
-    unused_parameters: &'a FiniteBitSet<u32>,
-}
-
-impl<'a, 'tcx> TypeVisitor<'tcx> for HasUsedGenericParams<'a> {
-    type BreakTy = ();
-
-    #[instrument(level = "debug", skip(self))]
-    fn visit_const(&mut self, c: Const<'tcx>) -> ControlFlow<Self::BreakTy> {
-        if !c.has_non_region_param() {
-            return ControlFlow::CONTINUE;
-        }
-
-        match c.kind() {
-            ty::ConstKind::Param(param) => {
-                if self.unused_parameters.contains(param.index).unwrap_or(false) {
-                    ControlFlow::CONTINUE
-                } else {
-                    ControlFlow::BREAK
-                }
-            }
-            _ => c.super_visit_with(self),
-        }
-    }
-
-    #[instrument(level = "debug", skip(self))]
-    fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
-        if !ty.has_non_region_param() {
-            return ControlFlow::CONTINUE;
-        }
-
-        match ty.kind() {
-            ty::Param(param) => {
-                if self.unused_parameters.contains(param.index).unwrap_or(false) {
-                    ControlFlow::CONTINUE
-                } else {
-                    ControlFlow::BREAK
-                }
+                self.unused_parameters.mark_used(param.index);
+                ControlFlow::Continue(())
             }
             _ => ty.super_visit_with(self),
         }
diff --git a/compiler/rustc_monomorphize/src/util.rs b/compiler/rustc_monomorphize/src/util.rs
index 6a4d2df..33e1f6c 100644
--- a/compiler/rustc_monomorphize/src/util.rs
+++ b/compiler/rustc_monomorphize/src/util.rs
@@ -40,12 +40,12 @@
         let new_size = tcx
             .layout_of(param_env.and(after_feature_tys))
             .map(|l| format!("{:?}", l.size.bytes()))
-            .unwrap_or_else(|e| format!("Failed {:?}", e));
+            .unwrap_or_else(|e| format!("Failed {e:?}"));
 
         let old_size = tcx
             .layout_of(param_env.and(before_feature_tys))
             .map(|l| format!("{:?}", l.size.bytes()))
-            .unwrap_or_else(|e| format!("Failed {:?}", e));
+            .unwrap_or_else(|e| format!("Failed {e:?}"));
 
         let closure_span = tcx.def_span(closure_def_id);
         let src_file = tcx.sess.source_map().span_to_filename(closure_span);
@@ -54,7 +54,7 @@
             .source_map()
             .span_to_lines(closure_span)
             .map(|l| format!("{:?} {:?}", l.lines.first(), l.lines.last()))
-            .unwrap_or_else(|e| format!("{:?}", e));
+            .unwrap_or_else(|e| format!("{e:?}"));
 
         if let Err(e) = writeln!(
             file,
@@ -64,7 +64,7 @@
             src_file.prefer_local(),
             line_nos
         ) {
-            eprintln!("Error writing to file {}", e)
+            eprintln!("Error writing to file {e}")
         }
     }
 }