diff --git a/Cargo.lock b/Cargo.lock
index 57e7079dfb2f0..2685de2244e94 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1382,9 +1382,13 @@ dependencies = [
 
 [[package]]
 name = "indexmap"
-version = "1.0.2"
+version = "1.5.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "7e81a7c05f79578dbc15793d8b619db9ba32b4577003ef3af1a91c416798c58d"
+checksum = "86b45e59b16c76b11bf9738fd5d38879d3bd28ad292d7b313608becb17ae2df9"
+dependencies = [
+ "autocfg",
+ "hashbrown",
+]
 
 [[package]]
 name = "installer"
diff --git a/src/librustc_codegen_llvm/coverageinfo/mapgen.rs b/src/librustc_codegen_llvm/coverageinfo/mapgen.rs
index 9d2383abeed22..b50b3b6d975c5 100644
--- a/src/librustc_codegen_llvm/coverageinfo/mapgen.rs
+++ b/src/librustc_codegen_llvm/coverageinfo/mapgen.rs
@@ -6,7 +6,7 @@ use llvm::coverageinfo::CounterMappingRegion;
 use log::debug;
 use rustc_codegen_ssa::coverageinfo::map::{Counter, CounterExpression, Region};
 use rustc_codegen_ssa::traits::{BaseTypeMethods, ConstMethods};
-use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::fx::FxIndexSet;
 use rustc_llvm::RustString;
 
 use std::ffi::CString;
@@ -76,13 +76,12 @@ pub fn finalize<'ll, 'tcx>(cx: &CodegenCx<'ll, 'tcx>) {
 }
 
 struct CoverageMapGenerator {
-    filenames: Vec<CString>,
-    filename_to_index: FxHashMap<CString, u32>,
+    filenames: FxIndexSet<CString>,
 }
 
 impl CoverageMapGenerator {
     fn new() -> Self {
-        Self { filenames: Vec::new(), filename_to_index: FxHashMap::default() }
+        Self { filenames: FxIndexSet::default() }
     }
 
     /// Using the `expressions` and `counter_regions` collected for the current function, generate
@@ -122,16 +121,8 @@ impl CoverageMapGenerator {
                 let c_filename =
                     CString::new(file_name).expect("null error converting filename to C string");
                 debug!("  file_id: {} = '{:?}'", current_file_id, c_filename);
-                let filenames_index = match self.filename_to_index.get(&c_filename) {
-                    Some(index) => *index,
-                    None => {
-                        let index = self.filenames.len() as u32;
-                        self.filenames.push(c_filename.clone());
-                        self.filename_to_index.insert(c_filename.clone(), index);
-                        index
-                    }
-                };
-                virtual_file_mapping.push(filenames_index);
+                let (filenames_index, _) = self.filenames.insert_full(c_filename);
+                virtual_file_mapping.push(filenames_index as u32);
             }
             mapping_regions.push(CounterMappingRegion::code_region(
                 counter,
diff --git a/src/librustc_codegen_llvm/coverageinfo/mod.rs b/src/librustc_codegen_llvm/coverageinfo/mod.rs
index 7b864e499d1de..90831f0bcfbee 100644
--- a/src/librustc_codegen_llvm/coverageinfo/mod.rs
+++ b/src/librustc_codegen_llvm/coverageinfo/mod.rs
@@ -97,8 +97,11 @@ impl CoverageInfoBuilderMethods<'tcx> for Builder<'a, 'll, 'tcx> {
     }
 }
 
-pub(crate) fn write_filenames_section_to_buffer(filenames: &Vec<CString>, buffer: &RustString) {
-    let c_str_vec = filenames.iter().map(|cstring| cstring.as_ptr()).collect::<Vec<_>>();
+pub(crate) fn write_filenames_section_to_buffer<'a>(
+    filenames: impl IntoIterator<Item = &'a CString>,
+    buffer: &RustString,
+) {
+    let c_str_vec = filenames.into_iter().map(|cstring| cstring.as_ptr()).collect::<Vec<_>>();
     unsafe {
         llvm::LLVMRustCoverageWriteFilenamesSectionToBuffer(
             c_str_vec.as_ptr(),
diff --git a/src/librustc_data_structures/Cargo.toml b/src/librustc_data_structures/Cargo.toml
index 811d1e49626c4..d0dbad5d2afe5 100644
--- a/src/librustc_data_structures/Cargo.toml
+++ b/src/librustc_data_structures/Cargo.toml
@@ -11,7 +11,7 @@ doctest = false
 
 [dependencies]
 ena = "0.14"
-indexmap = "1"
+indexmap = "1.5.1"
 log = { package = "tracing", version = "0.1" }
 jobserver_crate = { version = "0.1.13", package = "jobserver" }
 lazy_static = "1"
diff --git a/src/librustc_data_structures/transitive_relation.rs b/src/librustc_data_structures/transitive_relation.rs
index 189da3395ad1b..7d137a55033f7 100644
--- a/src/librustc_data_structures/transitive_relation.rs
+++ b/src/librustc_data_structures/transitive_relation.rs
@@ -1,4 +1,4 @@
-use crate::fx::FxHashMap;
+use crate::fx::FxIndexSet;
 use crate::stable_hasher::{HashStable, StableHasher};
 use crate::sync::Lock;
 use rustc_index::bit_set::BitMatrix;
@@ -13,10 +13,7 @@ mod tests;
 #[derive(Clone, Debug)]
 pub struct TransitiveRelation<T: Eq + Hash> {
     // List of elements. This is used to map from a T to a usize.
-    elements: Vec<T>,
-
-    // Maps each element to an index.
-    map: FxHashMap<T, Index>,
+    elements: FxIndexSet<T>,
 
     // List of base edges in the graph. Require to compute transitive
     // closure.
@@ -39,7 +36,6 @@ impl<T: Eq + Hash> Default for TransitiveRelation<T> {
     fn default() -> Self {
         TransitiveRelation {
             elements: Default::default(),
-            map: Default::default(),
             edges: Default::default(),
             closure: Default::default(),
         }
@@ -65,20 +61,16 @@ impl<T: Clone + Debug + Eq + Hash> TransitiveRelation<T> {
     }
 
     fn index(&self, a: &T) -> Option<Index> {
-        self.map.get(a).cloned()
+        self.elements.get_index_of(a).map(Index)
     }
 
     fn add_index(&mut self, a: T) -> Index {
-        let &mut TransitiveRelation { ref mut elements, ref mut closure, ref mut map, .. } = self;
-
-        *map.entry(a.clone()).or_insert_with(|| {
-            elements.push(a);
-
+        let (index, added) = self.elements.insert_full(a);
+        if added {
             // if we changed the dimensions, clear the cache
-            *closure.get_mut() = None;
-
-            Index(elements.len() - 1)
-        })
+            *self.closure.get_mut() = None;
+        }
+        Index(index)
     }
 
     /// Applies the (partial) function to each edge and returns a new
@@ -430,14 +422,11 @@ where
 {
     fn decode<D: Decoder>(d: &mut D) -> Result<Self, D::Error> {
         d.read_struct("TransitiveRelation", 2, |d| {
-            let elements: Vec<T> = d.read_struct_field("elements", 0, |d| Decodable::decode(d))?;
-            let edges = d.read_struct_field("edges", 1, |d| Decodable::decode(d))?;
-            let map = elements
-                .iter()
-                .enumerate()
-                .map(|(index, elem)| (elem.clone(), Index(index)))
-                .collect();
-            Ok(TransitiveRelation { elements, edges, map, closure: Lock::new(None) })
+            Ok(TransitiveRelation {
+                elements: d.read_struct_field("elements", 0, |d| Decodable::decode(d))?,
+                edges: d.read_struct_field("edges", 1, |d| Decodable::decode(d))?,
+                closure: Lock::new(None),
+            })
         })
     }
 }
@@ -452,8 +441,6 @@ where
         let TransitiveRelation {
             ref elements,
             ref edges,
-            // "map" is just a copy of elements vec
-            map: _,
             // "closure" is just a copy of the data above
             closure: _,
         } = *self;
diff --git a/src/librustc_metadata/rmeta/encoder.rs b/src/librustc_metadata/rmeta/encoder.rs
index f75f0b74a0e1e..6723e236a1fe9 100644
--- a/src/librustc_metadata/rmeta/encoder.rs
+++ b/src/librustc_metadata/rmeta/encoder.rs
@@ -4,7 +4,7 @@ use crate::rmeta::*;
 use log::{debug, trace};
 use rustc_ast::ast;
 use rustc_data_structures::fingerprint::Fingerprint;
-use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexSet};
 use rustc_data_structures::stable_hasher::StableHasher;
 use rustc_data_structures::sync::{join, Lrc};
 use rustc_hir as hir;
@@ -48,8 +48,7 @@ struct EncodeContext<'a, 'tcx> {
     type_shorthands: FxHashMap<Ty<'tcx>, usize>,
     predicate_shorthands: FxHashMap<ty::Predicate<'tcx>, usize>,
 
-    interpret_allocs: FxHashMap<interpret::AllocId, usize>,
-    interpret_allocs_inverse: Vec<interpret::AllocId>,
+    interpret_allocs: FxIndexSet<interpret::AllocId>,
 
     // This is used to speed up Span encoding.
     // The `usize` is an index into the `MonotonicVec`
@@ -331,17 +330,7 @@ impl<'a, 'b, 'tcx> SpecializedEncoder<ty::Predicate<'b>> for EncodeContext<'a, '
 
 impl<'a, 'tcx> SpecializedEncoder<interpret::AllocId> for EncodeContext<'a, 'tcx> {
     fn specialized_encode(&mut self, alloc_id: &interpret::AllocId) -> Result<(), Self::Error> {
-        use std::collections::hash_map::Entry;
-        let index = match self.interpret_allocs.entry(*alloc_id) {
-            Entry::Occupied(e) => *e.get(),
-            Entry::Vacant(e) => {
-                let idx = self.interpret_allocs_inverse.len();
-                self.interpret_allocs_inverse.push(*alloc_id);
-                e.insert(idx);
-                idx
-            }
-        };
-
+        let (index, _) = self.interpret_allocs.insert_full(*alloc_id);
         index.encode(self)
     }
 }
@@ -583,7 +572,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
             let mut n = 0;
             trace!("beginning to encode alloc ids");
             loop {
-                let new_n = self.interpret_allocs_inverse.len();
+                let new_n = self.interpret_allocs.len();
                 // if we have found new ids, serialize those, too
                 if n == new_n {
                     // otherwise, abort
@@ -591,7 +580,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
                 }
                 trace!("encoding {} further alloc ids", new_n - n);
                 for idx in n..new_n {
-                    let id = self.interpret_allocs_inverse[idx];
+                    let id = self.interpret_allocs[idx];
                     let pos = self.position() as u32;
                     interpret_alloc_index.push(pos);
                     interpret::specialized_encode_alloc_id(self, tcx, id).unwrap();
@@ -2019,7 +2008,6 @@ fn encode_metadata_impl(tcx: TyCtxt<'_>) -> EncodedMetadata {
         predicate_shorthands: Default::default(),
         source_file_cache: (source_map_files[0].clone(), 0),
         interpret_allocs: Default::default(),
-        interpret_allocs_inverse: Default::default(),
         required_source_files: Some(GrowableBitSet::with_capacity(source_map_files.len())),
         is_proc_macro: tcx.sess.crate_types().contains(&CrateType::ProcMacro),
         hygiene_ctxt: &hygiene_ctxt,
diff --git a/src/librustc_middle/ty/query/on_disk_cache.rs b/src/librustc_middle/ty/query/on_disk_cache.rs
index 643fbe793ab80..08b0bfecf4909 100644
--- a/src/librustc_middle/ty/query/on_disk_cache.rs
+++ b/src/librustc_middle/ty/query/on_disk_cache.rs
@@ -5,7 +5,7 @@ use crate::ty::codec::{self as ty_codec, TyDecoder, TyEncoder};
 use crate::ty::context::TyCtxt;
 use crate::ty::{self, Ty};
 use rustc_data_structures::fingerprint::Fingerprint;
-use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::fx::{FxHashMap, FxIndexSet};
 use rustc_data_structures::sync::{HashMapExt, Lock, Lrc, OnceCell};
 use rustc_data_structures::thin_vec::ThinVec;
 use rustc_errors::Diagnostic;
@@ -212,7 +212,6 @@ impl<'sess> OnDiskCache<'sess> {
                 type_shorthands: Default::default(),
                 predicate_shorthands: Default::default(),
                 interpret_allocs: Default::default(),
-                interpret_allocs_inverse: Vec::new(),
                 source_map: CachingSourceMapView::new(tcx.sess.source_map()),
                 file_to_file_index,
                 hygiene_context: &hygiene_encode_context,
@@ -267,7 +266,7 @@ impl<'sess> OnDiskCache<'sess> {
                 let mut interpret_alloc_index = Vec::new();
                 let mut n = 0;
                 loop {
-                    let new_n = encoder.interpret_allocs_inverse.len();
+                    let new_n = encoder.interpret_allocs.len();
                     // If we have found new IDs, serialize those too.
                     if n == new_n {
                         // Otherwise, abort.
@@ -275,7 +274,7 @@ impl<'sess> OnDiskCache<'sess> {
                     }
                     interpret_alloc_index.reserve(new_n - n);
                     for idx in n..new_n {
-                        let id = encoder.interpret_allocs_inverse[idx];
+                        let id = encoder.interpret_allocs[idx];
                         let pos = encoder.position() as u32;
                         interpret_alloc_index.push(pos);
                         interpret::specialized_encode_alloc_id(&mut encoder, tcx, id)?;
@@ -767,8 +766,7 @@ struct CacheEncoder<'a, 'tcx, E: ty_codec::TyEncoder> {
     encoder: &'a mut E,
     type_shorthands: FxHashMap<Ty<'tcx>, usize>,
     predicate_shorthands: FxHashMap<ty::Predicate<'tcx>, usize>,
-    interpret_allocs: FxHashMap<interpret::AllocId, usize>,
-    interpret_allocs_inverse: Vec<interpret::AllocId>,
+    interpret_allocs: FxIndexSet<interpret::AllocId>,
     source_map: CachingSourceMapView<'tcx>,
     file_to_file_index: FxHashMap<*const SourceFile, SourceFileIndex>,
     hygiene_context: &'a HygieneEncodeContext,
@@ -807,17 +805,7 @@ where
     E: 'a + TyEncoder,
 {
     fn specialized_encode(&mut self, alloc_id: &interpret::AllocId) -> Result<(), Self::Error> {
-        use std::collections::hash_map::Entry;
-        let index = match self.interpret_allocs.entry(*alloc_id) {
-            Entry::Occupied(e) => *e.get(),
-            Entry::Vacant(e) => {
-                let idx = self.interpret_allocs_inverse.len();
-                self.interpret_allocs_inverse.push(*alloc_id);
-                e.insert(idx);
-                idx
-            }
-        };
-
+        let (index, _) = self.interpret_allocs.insert_full(*alloc_id);
         index.encode(self)
     }
 }
diff --git a/src/librustc_mir/borrow_check/borrow_set.rs b/src/librustc_mir/borrow_check/borrow_set.rs
index ef9af7bace96f..b4299fbc5a1fe 100644
--- a/src/librustc_mir/borrow_check/borrow_set.rs
+++ b/src/librustc_mir/borrow_check/borrow_set.rs
@@ -3,9 +3,8 @@ use crate::borrow_check::path_utils::allow_two_phase_borrow;
 use crate::borrow_check::place_ext::PlaceExt;
 use crate::dataflow::indexes::BorrowIndex;
 use crate::dataflow::move_paths::MoveData;
-use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexMap};
 use rustc_index::bit_set::BitSet;
-use rustc_index::vec::IndexVec;
 use rustc_middle::mir::traversal;
 use rustc_middle::mir::visit::{MutatingUseContext, NonUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::{self, Body, Local, Location};
@@ -15,14 +14,11 @@ use std::ops::Index;
 
 crate struct BorrowSet<'tcx> {
     /// The fundamental map relating bitvector indexes to the borrows
-    /// in the MIR.
-    crate borrows: IndexVec<BorrowIndex, BorrowData<'tcx>>,
-
-    /// Each borrow is also uniquely identified in the MIR by the
-    /// `Location` of the assignment statement in which it appears on
-    /// the right hand side; we map each such location to the
-    /// corresponding `BorrowIndex`.
-    crate location_map: FxHashMap<Location, BorrowIndex>,
+    /// in the MIR. Each borrow is also uniquely identified in the MIR
+    /// by the `Location` of the assignment statement in which it
+    /// appears on the right hand side. Thus the location is the map
+    /// key, and its position in the map corresponds to `BorrowIndex`.
+    crate location_map: FxIndexMap<Location, BorrowData<'tcx>>,
 
     /// Locations which activate borrows.
     /// NOTE: a given location may activate more than one borrow in the future
@@ -40,7 +36,7 @@ impl<'tcx> Index<BorrowIndex> for BorrowSet<'tcx> {
     type Output = BorrowData<'tcx>;
 
     fn index(&self, index: BorrowIndex) -> &BorrowData<'tcx> {
-        &self.borrows[index]
+        &self.location_map[index.as_usize()]
     }
 }
 
@@ -129,7 +125,6 @@ impl<'tcx> BorrowSet<'tcx> {
         let mut visitor = GatherBorrows {
             tcx,
             body: &body,
-            idx_vec: IndexVec::new(),
             location_map: Default::default(),
             activation_map: Default::default(),
             local_map: Default::default(),
@@ -146,7 +141,6 @@ impl<'tcx> BorrowSet<'tcx> {
         }
 
         BorrowSet {
-            borrows: visitor.idx_vec,
             location_map: visitor.location_map,
             activation_map: visitor.activation_map,
             local_map: visitor.local_map,
@@ -157,13 +151,32 @@ impl<'tcx> BorrowSet<'tcx> {
     crate fn activations_at_location(&self, location: Location) -> &[BorrowIndex] {
         self.activation_map.get(&location).map(|activations| &activations[..]).unwrap_or(&[])
     }
+
+    crate fn len(&self) -> usize {
+        self.location_map.len()
+    }
+
+    crate fn indices(&self) -> impl Iterator<Item = BorrowIndex> {
+        BorrowIndex::from_usize(0)..BorrowIndex::from_usize(self.len())
+    }
+
+    crate fn iter_enumerated(&self) -> impl Iterator<Item = (BorrowIndex, &BorrowData<'tcx>)> {
+        self.indices().zip(self.location_map.values())
+    }
+
+    crate fn get_index_of(&self, location: &Location) -> Option<BorrowIndex> {
+        self.location_map.get_index_of(location).map(BorrowIndex::from)
+    }
+
+    crate fn contains(&self, location: &Location) -> bool {
+        self.location_map.contains_key(location)
+    }
 }
 
 struct GatherBorrows<'a, 'tcx> {
     tcx: TyCtxt<'tcx>,
     body: &'a Body<'tcx>,
-    idx_vec: IndexVec<BorrowIndex, BorrowData<'tcx>>,
-    location_map: FxHashMap<Location, BorrowIndex>,
+    location_map: FxIndexMap<Location, BorrowData<'tcx>>,
     activation_map: FxHashMap<Location, Vec<BorrowIndex>>,
     local_map: FxHashMap<mir::Local, FxHashSet<BorrowIndex>>,
 
@@ -203,8 +216,8 @@ impl<'a, 'tcx> Visitor<'tcx> for GatherBorrows<'a, 'tcx> {
                 borrowed_place: *borrowed_place,
                 assigned_place: *assigned_place,
             };
-            let idx = self.idx_vec.push(borrow);
-            self.location_map.insert(location, idx);
+            let (idx, _) = self.location_map.insert_full(location, borrow);
+            let idx = BorrowIndex::from(idx);
 
             self.insert_as_pending_if_two_phase(location, assigned_place, kind, idx);
 
@@ -224,7 +237,7 @@ impl<'a, 'tcx> Visitor<'tcx> for GatherBorrows<'a, 'tcx> {
         //
         //     TMP = &mut place
         if let Some(&borrow_index) = self.pending_activations.get(temp) {
-            let borrow_data = &mut self.idx_vec[borrow_index];
+            let borrow_data = &mut self.location_map[borrow_index.as_usize()];
 
             // Watch out: the use of TMP in the borrow itself
             // doesn't count as an activation. =)
@@ -265,8 +278,7 @@ impl<'a, 'tcx> Visitor<'tcx> for GatherBorrows<'a, 'tcx> {
         if let mir::Rvalue::Ref(region, kind, ref place) = *rvalue {
             // double-check that we already registered a BorrowData for this
 
-            let borrow_index = self.location_map[&location];
-            let borrow_data = &self.idx_vec[borrow_index];
+            let borrow_data = &self.location_map[&location];
             assert_eq!(borrow_data.reserve_location, location);
             assert_eq!(borrow_data.kind, kind);
             assert_eq!(borrow_data.region, region.to_region_vid());
@@ -316,7 +328,7 @@ impl<'a, 'tcx> GatherBorrows<'a, 'tcx> {
         // Consider the borrow not activated to start. When we find an activation, we'll update
         // this field.
         {
-            let borrow_data = &mut self.idx_vec[borrow_index];
+            let borrow_data = &mut self.location_map[borrow_index.as_usize()];
             borrow_data.activation_location = TwoPhaseActivation::NotActivated;
         }
 
@@ -332,7 +344,7 @@ impl<'a, 'tcx> GatherBorrows<'a, 'tcx> {
                        at borrow_index: {:?} with associated data {:?}",
                 temp,
                 old_index,
-                self.idx_vec[old_index]
+                self.location_map[old_index.as_usize()]
             );
         }
     }
diff --git a/src/librustc_mir/borrow_check/constraint_generation.rs b/src/librustc_mir/borrow_check/constraint_generation.rs
index e0420d974fbdf..33b09dcb888ed 100644
--- a/src/librustc_mir/borrow_check/constraint_generation.rs
+++ b/src/librustc_mir/borrow_check/constraint_generation.rs
@@ -217,7 +217,7 @@ impl<'cx, 'cg, 'tcx> ConstraintGeneration<'cx, 'cg, 'tcx> {
                             let places_conflict = places_conflict::places_conflict(
                                 self.infcx.tcx,
                                 self.body,
-                                self.borrow_set.borrows[borrow_index].borrowed_place,
+                                self.borrow_set[borrow_index].borrowed_place,
                                 place,
                                 places_conflict::PlaceConflictBias::NoOverlap,
                             );
diff --git a/src/librustc_mir/borrow_check/invalidation.rs b/src/librustc_mir/borrow_check/invalidation.rs
index fd8f17718e795..2de2124dc5e0e 100644
--- a/src/librustc_mir/borrow_check/invalidation.rs
+++ b/src/librustc_mir/borrow_check/invalidation.rs
@@ -166,8 +166,8 @@ impl<'cx, 'tcx> Visitor<'tcx> for InvalidationGenerator<'cx, 'tcx> {
                 // Invalidate all borrows of local places
                 let borrow_set = self.borrow_set.clone();
                 let resume = self.location_table.start_index(resume.start_location());
-                for i in borrow_set.borrows.indices() {
-                    if borrow_of_local_data(borrow_set.borrows[i].borrowed_place) {
+                for (i, data) in borrow_set.iter_enumerated() {
+                    if borrow_of_local_data(data.borrowed_place) {
                         self.all_facts.invalidates.push((resume, i));
                     }
                 }
@@ -178,8 +178,8 @@ impl<'cx, 'tcx> Visitor<'tcx> for InvalidationGenerator<'cx, 'tcx> {
                 // Invalidate all borrows of local places
                 let borrow_set = self.borrow_set.clone();
                 let start = self.location_table.start_index(location);
-                for i in borrow_set.borrows.indices() {
-                    if borrow_of_local_data(borrow_set.borrows[i].borrowed_place) {
+                for (i, data) in borrow_set.iter_enumerated() {
+                    if borrow_of_local_data(data.borrowed_place) {
                         self.all_facts.invalidates.push((start, i));
                     }
                 }
@@ -369,7 +369,7 @@ impl<'cx, 'tcx> InvalidationGenerator<'cx, 'tcx> {
         let tcx = self.tcx;
         let body = self.body;
         let borrow_set = self.borrow_set.clone();
-        let indices = self.borrow_set.borrows.indices();
+        let indices = self.borrow_set.indices();
         each_borrow_involving_path(
             self,
             tcx,
diff --git a/src/librustc_mir/borrow_check/mod.rs b/src/librustc_mir/borrow_check/mod.rs
index 76cc03fa60909..6e211b42a052e 100644
--- a/src/librustc_mir/borrow_check/mod.rs
+++ b/src/librustc_mir/borrow_check/mod.rs
@@ -1131,11 +1131,8 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 (
                     Reservation(WriteKind::MutableBorrow(bk)),
                     BorrowKind::Shallow | BorrowKind::Shared,
-                ) if {
-                    tcx.migrate_borrowck() && this.borrow_set.location_map.contains_key(&location)
-                } =>
-                {
-                    let bi = this.borrow_set.location_map[&location];
+                ) if { tcx.migrate_borrowck() && this.borrow_set.contains(&location) } => {
+                    let bi = this.borrow_set.get_index_of(&location).unwrap();
                     debug!(
                         "recording invalid reservation of place: {:?} with \
                          borrow index {:?} as warning",
diff --git a/src/librustc_mir/borrow_check/nll.rs b/src/librustc_mir/borrow_check/nll.rs
index f6b3be59d9576..66a17cba6bb01 100644
--- a/src/librustc_mir/borrow_check/nll.rs
+++ b/src/librustc_mir/borrow_check/nll.rs
@@ -206,7 +206,7 @@ pub(in crate::borrow_check) fn compute_regions<'cx, 'tcx>(
         //   the `borrow_set`, their `BorrowIndex` are synthesized as the universal region index
         //   added to the existing number of loans, as if they succeeded them in the set.
         //
-        let borrow_count = borrow_set.borrows.len();
+        let borrow_count = borrow_set.len();
         debug!(
             "compute_regions: polonius placeholders, num_universals={}, borrow_count={}",
             universal_regions.len(),
diff --git a/src/librustc_mir/borrow_check/region_infer/values.rs b/src/librustc_mir/borrow_check/region_infer/values.rs
index 6cd814962c613..8a5a600cfdd8a 100644
--- a/src/librustc_mir/borrow_check/region_infer/values.rs
+++ b/src/librustc_mir/borrow_check/region_infer/values.rs
@@ -1,4 +1,4 @@
-use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::fx::FxIndexSet;
 use rustc_index::bit_set::{HybridBitSet, SparseBitMatrix};
 use rustc_index::vec::Idx;
 use rustc_index::vec::IndexVec;
@@ -193,26 +193,25 @@ impl<N: Idx> LivenessValues<N> {
 /// NLL.
 #[derive(Default)]
 crate struct PlaceholderIndices {
-    to_index: FxHashMap<ty::PlaceholderRegion, PlaceholderIndex>,
-    from_index: IndexVec<PlaceholderIndex, ty::PlaceholderRegion>,
+    indices: FxIndexSet<ty::PlaceholderRegion>,
 }
 
 impl PlaceholderIndices {
     crate fn insert(&mut self, placeholder: ty::PlaceholderRegion) -> PlaceholderIndex {
-        let PlaceholderIndices { to_index, from_index } = self;
-        *to_index.entry(placeholder).or_insert_with(|| from_index.push(placeholder))
+        let (index, _) = self.indices.insert_full(placeholder);
+        index.into()
     }
 
     crate fn lookup_index(&self, placeholder: ty::PlaceholderRegion) -> PlaceholderIndex {
-        self.to_index[&placeholder]
+        self.indices.get_index_of(&placeholder).unwrap().into()
     }
 
     crate fn lookup_placeholder(&self, placeholder: PlaceholderIndex) -> ty::PlaceholderRegion {
-        self.from_index[placeholder]
+        self.indices[placeholder.index()]
     }
 
     crate fn len(&self) -> usize {
-        self.from_index.len()
+        self.indices.len()
     }
 }
 
diff --git a/src/librustc_mir/borrow_check/type_check/mod.rs b/src/librustc_mir/borrow_check/type_check/mod.rs
index bc5c144cd742c..ff98de5475ecf 100644
--- a/src/librustc_mir/borrow_check/type_check/mod.rs
+++ b/src/librustc_mir/borrow_check/type_check/mod.rs
@@ -2469,11 +2469,11 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
         // example).
         if let Some(all_facts) = all_facts {
             let _prof_timer = self.infcx.tcx.prof.generic_activity("polonius_fact_generation");
-            if let Some(borrow_index) = borrow_set.location_map.get(&location) {
+            if let Some(borrow_index) = borrow_set.get_index_of(&location) {
                 let region_vid = borrow_region.to_region_vid();
                 all_facts.borrow_region.push((
                     region_vid,
-                    *borrow_index,
+                    borrow_index,
                     location_table.mid_index(location),
                 ));
             }
diff --git a/src/librustc_mir/dataflow/impls/borrows.rs b/src/librustc_mir/dataflow/impls/borrows.rs
index dfca270396de9..7e7b7f2cc76fa 100644
--- a/src/librustc_mir/dataflow/impls/borrows.rs
+++ b/src/librustc_mir/dataflow/impls/borrows.rs
@@ -136,9 +136,9 @@ impl<'a, 'tcx> Borrows<'a, 'tcx> {
         borrow_set: &Rc<BorrowSet<'tcx>>,
     ) -> Self {
         let mut borrows_out_of_scope_at_location = FxHashMap::default();
-        for (borrow_index, borrow_data) in borrow_set.borrows.iter_enumerated() {
+        for (borrow_index, borrow_data) in borrow_set.iter_enumerated() {
             let borrow_region = borrow_data.region.to_region_vid();
-            let location = borrow_set.borrows[borrow_index].reserve_location;
+            let location = borrow_data.reserve_location;
 
             precompute_borrows_out_of_scope(
                 body,
@@ -160,7 +160,7 @@ impl<'a, 'tcx> Borrows<'a, 'tcx> {
     }
 
     pub fn location(&self, idx: BorrowIndex) -> &Location {
-        &self.borrow_set.borrows[idx].reserve_location
+        &self.borrow_set[idx].reserve_location
     }
 
     /// Add all borrows to the kill set, if those borrows are out of scope at `location`.
@@ -216,7 +216,7 @@ impl<'a, 'tcx> Borrows<'a, 'tcx> {
             places_conflict(
                 self.tcx,
                 self.body,
-                self.borrow_set.borrows[i].borrowed_place,
+                self.borrow_set[i].borrowed_place,
                 place,
                 PlaceConflictBias::NoOverlap,
             )
@@ -232,7 +232,7 @@ impl<'tcx> dataflow::AnalysisDomain<'tcx> for Borrows<'_, 'tcx> {
     const NAME: &'static str = "borrows";
 
     fn bits_per_block(&self, _: &mir::Body<'tcx>) -> usize {
-        self.borrow_set.borrows.len() * 2
+        self.borrow_set.len() * 2
     }
 
     fn initialize_start_block(&self, _: &mir::Body<'tcx>, _: &mut BitSet<Self::Idx>) {
@@ -271,11 +271,11 @@ impl<'tcx> dataflow::GenKillAnalysis<'tcx> for Borrows<'_, 'tcx> {
                     ) {
                         return;
                     }
-                    let index = self.borrow_set.location_map.get(&location).unwrap_or_else(|| {
+                    let index = self.borrow_set.get_index_of(&location).unwrap_or_else(|| {
                         panic!("could not find BorrowIndex for location {:?}", location);
                     });
 
-                    trans.gen(*index);
+                    trans.gen(index);
                 }
 
                 // Make sure there are no remaining borrows for variables
diff --git a/src/librustc_mir_build/build/matches/mod.rs b/src/librustc_mir_build/build/matches/mod.rs
index 77c0fe8dda534..5f87cb364b891 100644
--- a/src/librustc_mir_build/build/matches/mod.rs
+++ b/src/librustc_mir_build/build/matches/mod.rs
@@ -11,7 +11,7 @@ use crate::build::{BlockAnd, BlockAndExtension, Builder};
 use crate::build::{GuardFrame, GuardFrameLocal, LocalsForNode};
 use crate::thir::{self, *};
 use rustc_data_structures::{
-    fx::{FxHashMap, FxHashSet},
+    fx::{FxHashSet, FxIndexMap},
     stack::ensure_sufficient_stack,
 };
 use rustc_hir::HirId;
@@ -817,9 +817,7 @@ enum TestKind<'tcx> {
         ///
         /// For `bool` we always generate two edges, one for `true` and one for
         /// `false`.
-        options: Vec<u128>,
-        /// Reverse map used to ensure that the values in `options` are unique.
-        indices: FxHashMap<&'tcx ty::Const<'tcx>, usize>,
+        options: FxIndexMap<&'tcx ty::Const<'tcx>, u128>,
     },
 
     /// Test for equality with value, possibly after an unsizing coercion to
@@ -1396,14 +1394,13 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         // may want to add cases based on the candidates that are
         // available
         match test.kind {
-            TestKind::SwitchInt { switch_ty, ref mut options, ref mut indices } => {
+            TestKind::SwitchInt { switch_ty, ref mut options } => {
                 for candidate in candidates.iter() {
                     if !self.add_cases_to_switch(
                         &match_place,
                         candidate,
                         switch_ty,
                         options,
-                        indices,
                     ) {
                         break;
                     }
diff --git a/src/librustc_mir_build/build/matches/test.rs b/src/librustc_mir_build/build/matches/test.rs
index 158ad78a1bf36..87977d6fe8904 100644
--- a/src/librustc_mir_build/build/matches/test.rs
+++ b/src/librustc_mir_build/build/matches/test.rs
@@ -9,7 +9,7 @@ use crate::build::matches::{Candidate, MatchPair, Test, TestKind};
 use crate::build::Builder;
 use crate::thir::pattern::compare_const_vals;
 use crate::thir::*;
-use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::fx::FxIndexMap;
 use rustc_hir::RangeEnd;
 use rustc_index::bit_set::BitSet;
 use rustc_middle::mir::*;
@@ -44,8 +44,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
 
                         // these maps are empty to start; cases are
                         // added below in add_cases_to_switch
-                        options: vec![],
-                        indices: Default::default(),
+                        options: Default::default(),
                     },
                 }
             }
@@ -83,8 +82,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         test_place: &Place<'tcx>,
         candidate: &Candidate<'pat, 'tcx>,
         switch_ty: Ty<'tcx>,
-        options: &mut Vec<u128>,
-        indices: &mut FxHashMap<&'tcx ty::Const<'tcx>, usize>,
+        options: &mut FxIndexMap<&'tcx ty::Const<'tcx>, u128>,
     ) -> bool {
         let match_pair = match candidate.match_pairs.iter().find(|mp| mp.place == *test_place) {
             Some(match_pair) => match_pair,
@@ -95,9 +93,8 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
 
         match *match_pair.pattern.kind {
             PatKind::Constant { value } => {
-                indices.entry(value).or_insert_with(|| {
-                    options.push(value.eval_bits(self.hir.tcx(), self.hir.param_env, switch_ty));
-                    options.len() - 1
+                options.entry(value).or_insert_with(|| {
+                    value.eval_bits(self.hir.tcx(), self.hir.param_env, switch_ty)
                 });
                 true
             }
@@ -106,7 +103,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             }
             PatKind::Range(range) => {
                 // Check that none of the switch values are in the range.
-                self.values_not_contained_in_range(range, indices).unwrap_or(false)
+                self.values_not_contained_in_range(range, options).unwrap_or(false)
             }
             PatKind::Slice { .. }
             | PatKind::Array { .. }
@@ -216,7 +213,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 );
             }
 
-            TestKind::SwitchInt { switch_ty, ref options, indices: _ } => {
+            TestKind::SwitchInt { switch_ty, ref options } => {
                 let target_blocks = make_target_blocks(self);
                 let terminator = if switch_ty.kind == ty::Bool {
                     assert!(!options.is_empty() && options.len() <= 2);
@@ -236,7 +233,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     TerminatorKind::SwitchInt {
                         discr: Operand::Copy(place),
                         switch_ty,
-                        values: options.clone().into(),
+                        values: options.values().copied().collect(),
                         targets: target_blocks,
                     }
                 };
@@ -532,20 +529,20 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             // FIXME(#29623) we could use PatKind::Range to rule
             // things out here, in some cases.
             (
-                &TestKind::SwitchInt { switch_ty: _, options: _, ref indices },
+                &TestKind::SwitchInt { switch_ty: _, ref options },
                 &PatKind::Constant { ref value },
             ) if is_switch_ty(match_pair.pattern.ty) => {
-                let index = indices[value];
+                let index = options.get_index_of(value).unwrap();
                 self.candidate_without_match_pair(match_pair_index, candidate);
                 Some(index)
             }
 
             (
-                &TestKind::SwitchInt { switch_ty: _, ref options, ref indices },
+                &TestKind::SwitchInt { switch_ty: _, ref options },
                 &PatKind::Range(range),
             ) => {
                 let not_contained =
-                    self.values_not_contained_in_range(range, indices).unwrap_or(false);
+                    self.values_not_contained_in_range(range, options).unwrap_or(false);
 
                 if not_contained {
                     // No switch values are contained in the pattern range,
@@ -777,9 +774,9 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     fn values_not_contained_in_range(
         &self,
         range: PatRange<'tcx>,
-        indices: &FxHashMap<&'tcx ty::Const<'tcx>, usize>,
+        options: &FxIndexMap<&'tcx ty::Const<'tcx>, u128>,
     ) -> Option<bool> {
-        for &val in indices.keys() {
+        for &val in options.keys() {
             if self.const_range_contains(range, val)? {
                 return Some(false);
             }
diff --git a/src/librustc_span/span_encoding.rs b/src/librustc_span/span_encoding.rs
index 6b672d344fa51..b05e01d666bd6 100644
--- a/src/librustc_span/span_encoding.rs
+++ b/src/librustc_span/span_encoding.rs
@@ -8,7 +8,7 @@ use crate::hygiene::SyntaxContext;
 use crate::SESSION_GLOBALS;
 use crate::{BytePos, SpanData};
 
-use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::fx::FxIndexSet;
 
 /// A compressed span.
 ///
@@ -111,25 +111,18 @@ impl Span {
 
 #[derive(Default)]
 pub struct SpanInterner {
-    spans: FxHashMap<SpanData, u32>,
-    span_data: Vec<SpanData>,
+    spans: FxIndexSet<SpanData>,
 }
 
 impl SpanInterner {
     fn intern(&mut self, span_data: &SpanData) -> u32 {
-        if let Some(index) = self.spans.get(span_data) {
-            return *index;
-        }
-
-        let index = self.spans.len() as u32;
-        self.span_data.push(*span_data);
-        self.spans.insert(*span_data, index);
-        index
+        let (index, _) = self.spans.insert_full(*span_data);
+        index as u32
     }
 
     #[inline]
     fn get(&self, index: u32) -> &SpanData {
-        &self.span_data[index as usize]
+        &self.spans[index as usize]
     }
 }
 
diff --git a/src/librustc_span/symbol.rs b/src/librustc_span/symbol.rs
index 5203bfdb3b7ce..ca9702784a268 100644
--- a/src/librustc_span/symbol.rs
+++ b/src/librustc_span/symbol.rs
@@ -1481,6 +1481,10 @@ impl<CTX> ToStableHashKey<CTX> for Symbol {
 }
 
 // The `&'static str`s in this type actually point into the arena.
+//
+// The `FxHashMap`+`Vec` pair could be replaced by `FxIndexSet`, but #75278
+// found that to regress performance up to 2% in some cases. This might be
+// revisited after further improvements to `indexmap`.
 #[derive(Default)]
 pub struct Interner {
     arena: DroplessArena,
diff --git a/src/librustc_typeck/check/generator_interior.rs b/src/librustc_typeck/check/generator_interior.rs
index c5004e4ce119b..93fdf93e9e394 100644
--- a/src/librustc_typeck/check/generator_interior.rs
+++ b/src/librustc_typeck/check/generator_interior.rs
@@ -4,7 +4,7 @@
 //! types computed here.
 
 use super::FnCtxt;
-use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_data_structures::fx::{FxHashSet, FxIndexSet};
 use rustc_hir as hir;
 use rustc_hir::def::{CtorKind, DefKind, Res};
 use rustc_hir::def_id::DefId;
@@ -16,7 +16,7 @@ use rustc_span::Span;
 
 struct InteriorVisitor<'a, 'tcx> {
     fcx: &'a FnCtxt<'a, 'tcx>,
-    types: FxHashMap<ty::GeneratorInteriorTypeCause<'tcx>, usize>,
+    types: FxIndexSet<ty::GeneratorInteriorTypeCause<'tcx>>,
     region_scope_tree: &'tcx region::ScopeTree,
     expr_count: usize,
     kind: hir::GeneratorKind,
@@ -88,18 +88,15 @@ impl<'a, 'tcx> InteriorVisitor<'a, 'tcx> {
                     .span_note(yield_data.span, &*note)
                     .emit();
             } else {
-                // Map the type to the number of types added before it
-                let entries = self.types.len();
+                // Insert the type into the ordered set.
                 let scope_span = scope.map(|s| s.span(self.fcx.tcx, self.region_scope_tree));
-                self.types
-                    .entry(ty::GeneratorInteriorTypeCause {
-                        span: source_span,
-                        ty: &ty,
-                        scope_span,
-                        yield_span: yield_data.span,
-                        expr: expr.map(|e| e.hir_id),
-                    })
-                    .or_insert(entries);
+                self.types.insert(ty::GeneratorInteriorTypeCause {
+                    span: source_span,
+                    ty: &ty,
+                    scope_span,
+                    yield_span: yield_data.span,
+                    expr: expr.map(|e| e.hir_id),
+                });
             }
         } else {
             debug!(
@@ -132,7 +129,7 @@ pub fn resolve_interior<'a, 'tcx>(
     let body = fcx.tcx.hir().body(body_id);
     let mut visitor = InteriorVisitor {
         fcx,
-        types: FxHashMap::default(),
+        types: FxIndexSet::default(),
         region_scope_tree: fcx.tcx.region_scope_tree(def_id),
         expr_count: 0,
         kind,
@@ -144,10 +141,8 @@ pub fn resolve_interior<'a, 'tcx>(
     let region_expr_count = visitor.region_scope_tree.body_expr_count(body_id).unwrap();
     assert_eq!(region_expr_count, visitor.expr_count);
 
-    let mut types: Vec<_> = visitor.types.drain().collect();
-
-    // Sort types by insertion order
-    types.sort_by_key(|t| t.1);
+    // The types are already kept in insertion order.
+    let types = visitor.types;
 
     // The types in the generator interior contain lifetimes local to the generator itself,
     // which should not be exposed outside of the generator. Therefore, we replace these
@@ -164,7 +159,7 @@ pub fn resolve_interior<'a, 'tcx>(
     let mut captured_tys = FxHashSet::default();
     let type_causes: Vec<_> = types
         .into_iter()
-        .filter_map(|(mut cause, _)| {
+        .filter_map(|mut cause| {
             // Erase regions and canonicalize late-bound regions to deduplicate as many types as we
             // can.
             let erased = fcx.tcx.erase_regions(&cause.ty);
diff --git a/src/librustc_typeck/collect.rs b/src/librustc_typeck/collect.rs
index c0f56a2817282..b47ef34600404 100644
--- a/src/librustc_typeck/collect.rs
+++ b/src/librustc_typeck/collect.rs
@@ -22,7 +22,7 @@ use rustc_ast::ast;
 use rustc_ast::ast::MetaItemKind;
 use rustc_attr::{list_contains_name, InlineAttr, OptimizeAttr};
 use rustc_data_structures::captures::Captures;
-use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexSet};
 use rustc_errors::{struct_span_err, Applicability};
 use rustc_hir as hir;
 use rustc_hir::def::{CtorKind, DefKind, Res};
@@ -1718,21 +1718,17 @@ fn explicit_predicates_of(tcx: TyCtxt<'_>, def_id: DefId) -> ty::GenericPredicat
     /// A data structure with unique elements, which preserves order of insertion.
     /// Preserving the order of insertion is important here so as not to break
     /// compile-fail UI tests.
-    // FIXME(eddyb) just use `IndexSet` from `indexmap`.
     struct UniquePredicates<'tcx> {
-        predicates: Vec<(ty::Predicate<'tcx>, Span)>,
-        uniques: FxHashSet<(ty::Predicate<'tcx>, Span)>,
+        predicates: FxIndexSet<(ty::Predicate<'tcx>, Span)>,
     }
 
     impl<'tcx> UniquePredicates<'tcx> {
         fn new() -> Self {
-            UniquePredicates { predicates: vec![], uniques: FxHashSet::default() }
+            UniquePredicates { predicates: FxIndexSet::default() }
         }
 
         fn push(&mut self, value: (ty::Predicate<'tcx>, Span)) {
-            if self.uniques.insert(value) {
-                self.predicates.push(value);
-            }
+            self.predicates.insert(value);
         }
 
         fn extend<I: IntoIterator<Item = (ty::Predicate<'tcx>, Span)>>(&mut self, iter: I) {
@@ -2014,7 +2010,7 @@ fn explicit_predicates_of(tcx: TyCtxt<'_>, def_id: DefId) -> ty::GenericPredicat
         }))
     }
 
-    let mut predicates = predicates.predicates;
+    let mut predicates: Vec<_> = predicates.predicates.into_iter().collect();
 
     // Subtle: before we store the predicates into the tcx, we
     // sort them so that predicates like `T: Foo<Item=U>` come