diff --git a/compiler/rustc_metadata/src/rmeta/encoder.rs b/compiler/rustc_metadata/src/rmeta/encoder.rs
index 38dbf8d428a20..1299e8d9f3326 100644
--- a/compiler/rustc_metadata/src/rmeta/encoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/encoder.rs
@@ -10,7 +10,7 @@ use std::sync::{Arc, Mutex};
 
 use rustc_attr_ir::{AttributeKind, EncodeCrossCrate, find_attr};
 use rustc_data_structures::fingerprint::Fingerprint;
-use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
+use rustc_data_structures::fx::{FxHashMap, FxIndexMap, FxIndexSet};
 use rustc_data_structures::memmap::{Mmap, MmapMut};
 use rustc_data_structures::owned_slice::slice_owned;
 use rustc_data_structures::stable_hash::{StableHash, StableHasher};
@@ -18,7 +18,7 @@ use rustc_data_structures::sync::{par_for_each_in, par_join};
 use rustc_data_structures::temp_dir::MaybeTempDir;
 use rustc_data_structures::thousands::usize_with_underscores;
 use rustc_hir as hir;
-use rustc_hir::def_id::{CRATE_DEF_ID, LOCAL_CRATE, LocalDefId, LocalDefIdSet};
+use rustc_hir::def_id::{CRATE_DEF_ID, DefId, LOCAL_CRATE, LocalDefId, LocalDefIdSet};
 use rustc_hir::definitions::DefPathData;
 use rustc_hir_pretty::id_to_string;
 use rustc_index::IndexVec;
@@ -425,6 +425,18 @@ impl<'a, 'tcx> TyEncoder<'tcx> for EncodeContext<'a, 'tcx> {
 
         index.encode(self);
     }
+
+    fn encode_def_id_map<V: Encodable<Self>>(&mut self, map: &FxHashMap<DefId, V>) {
+        #[allow(rustc::potential_query_instability)]
+        let mut entries: Vec<(DefId, &V)> = map.iter().map(|(&k, v)| (k, v)).collect();
+        entries.sort_unstable_by_key(|&(def_id, _)| self.tcx.def_path_hash(def_id));
+
+        self.emit_usize(entries.len());
+        for (def_id, value) in entries {
+            def_id.encode(self);
+            value.encode(self);
+        }
+    }
 }
 
 // Shorthand for `$self.$tables.$table.set_some($def_id.index, $self.lazy($value))`, which would
diff --git a/compiler/rustc_middle/src/query/on_disk_cache.rs b/compiler/rustc_middle/src/query/on_disk_cache.rs
index 7383ad23273be..7cf7ef619224b 100644
--- a/compiler/rustc_middle/src/query/on_disk_cache.rs
+++ b/compiler/rustc_middle/src/query/on_disk_cache.rs
@@ -937,6 +937,11 @@ impl<'tcx> TyEncoder<'tcx> for CacheEncoder<'tcx> {
 
         index.encode(self);
     }
+
+    #[inline]
+    fn encode_def_id_map<V: Encodable<Self>>(&mut self, map: &FxHashMap<DefId, V>) {
+        map.encode(self);
+    }
 }
 
 macro_rules! encoder_methods {
diff --git a/compiler/rustc_middle/src/ty/codec.rs b/compiler/rustc_middle/src/ty/codec.rs
index 81462e214b20f..6e121565c8c0b 100644
--- a/compiler/rustc_middle/src/ty/codec.rs
+++ b/compiler/rustc_middle/src/ty/codec.rs
@@ -12,6 +12,7 @@ use std::marker::DiscriminantKind;
 
 use rustc_data_structures::fx::FxHashMap;
 use rustc_serialize::{Decodable, Encodable};
+use rustc_span::def_id::DefId;
 use rustc_span::{SpanDecoder, SpanEncoder};
 
 pub use self::ref_decodable::RefDecodable;
@@ -37,6 +38,10 @@ pub trait TyEncoder<'tcx>: SpanEncoder {
     fn predicate_shorthands(&mut self) -> &mut FxHashMap<ty::PredicateKind<'tcx>, usize>;
 
     fn encode_alloc_id(&mut self, alloc_id: &AllocId);
+
+    fn encode_def_id_map<V: Encodable<Self>>(&mut self, map: &FxHashMap<DefId, V>)
+    where
+        Self: Sized;
 }
 
 pub trait TyDecoder<'tcx>:
diff --git a/compiler/rustc_middle/src/ty/generics.rs b/compiler/rustc_middle/src/ty/generics.rs
index 353ac74bd576f..fdef1c6450957 100644
--- a/compiler/rustc_middle/src/ty/generics.rs
+++ b/compiler/rustc_middle/src/ty/generics.rs
@@ -1,14 +1,16 @@
-use std::ops::ControlFlow;
+use std::ops::{ControlFlow, Deref, DerefMut};
 
 use rustc_ast as ast;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_hir::def_id::DefId;
 use rustc_macros::{StableHash, TyDecodable, TyEncodable};
+use rustc_serialize::{Decodable, Encodable};
 use rustc_span::{Span, Symbol, bug, kw};
 use rustc_type_ir::{TypeSuperVisitable as _, TypeVisitable, TypeVisitor};
 use tracing::instrument;
 
 use super::{Clause, InstantiatedClauses, ParamConst, ParamTy, Ty, TyCtxt, Unnormalized};
+use crate::ty::codec::{TyDecoder, TyEncoder};
 use crate::ty::{self, ClauseKind, EarlyBinder, GenericArgsRef, Region, RegionKind, TyKind};
 
 #[derive(Clone, Debug, TyEncodable, TyDecodable, StableHash)]
@@ -124,7 +126,7 @@ pub struct Generics {
 
     /// Reverse map to the `index` field of each `GenericParamDef`.
     #[stable_hash(ignore)]
-    pub param_def_id_to_index: FxHashMap<DefId, u32>,
+    pub param_def_id_to_index: ParamDefIdToIndex,
 
     pub has_self: bool,
     pub has_late_bound_regions: Option<Span>,
@@ -621,3 +623,37 @@ impl<'tcx> ConstConditions<'tcx> {
         );
     }
 }
+
+#[derive(Clone, Default)]
+pub struct ParamDefIdToIndex(FxHashMap<DefId, u32>);
+
+impl Deref for ParamDefIdToIndex {
+    type Target = FxHashMap<DefId, u32>;
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl DerefMut for ParamDefIdToIndex {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.0
+    }
+}
+
+impl FromIterator<(DefId, u32)> for ParamDefIdToIndex {
+    fn from_iter<I: IntoIterator<Item = (DefId, u32)>>(iter: I) -> Self {
+        Self(iter.into_iter().collect())
+    }
+}
+
+impl<'tcx, E: TyEncoder<'tcx>> Encodable<E> for ParamDefIdToIndex {
+    fn encode(&self, e: &mut E) {
+        e.encode_def_id_map(&self.0);
+    }
+}
+
+impl<'tcx, D: TyDecoder<'tcx>> Decodable<D> for ParamDefIdToIndex {
+    fn decode(d: &mut D) -> Self {
+        Self(Decodable::decode(d))
+    }
+}
diff --git a/compiler/rustc_monomorphize/src/offload/manifest.rs b/compiler/rustc_monomorphize/src/offload/manifest.rs
index a6f5d1228082e..d4e1579e7d83e 100644
--- a/compiler/rustc_monomorphize/src/offload/manifest.rs
+++ b/compiler/rustc_monomorphize/src/offload/manifest.rs
@@ -149,6 +149,10 @@ impl<'a, 'tcx> TyEncoder<'tcx> for OffloadManifestEncoder<'a, 'tcx> {
     fn encode_alloc_id(&mut self, _alloc_id: &rustc_middle::mir::interpret::AllocId) {
         // AllocIds are not expected in the manifest.
     }
+
+    fn encode_def_id_map<V: Encodable<Self>>(&mut self, map: &FxHashMap<DefId, V>) {
+        map.encode(self);
+    }
 }
 
 const UNRESOLVED_DEF_ID: DefId = DefId {
diff --git a/tests/run-make/parallel-reproducible-build/generics-of-issue-163878.rs b/tests/run-make/parallel-reproducible-build/generics-of-issue-163878.rs
new file mode 100644
index 0000000000000..df64b1b2b71f0
--- /dev/null
+++ b/tests/run-make/parallel-reproducible-build/generics-of-issue-163878.rs
@@ -0,0 +1,10 @@
+pub struct A {}
+
+impl A {
+    pub fn f1(&self, _b1: &()) -> impl Iterator<Item = ()> + '_ {
+        std::iter::empty()
+    }
+    pub fn f2(&self, _b2: &()) -> impl Iterator<Item = ()> + '_ {
+        std::iter::empty()
+    }
+}
diff --git a/tests/run-make/parallel-reproducible-build/rmake.rs b/tests/run-make/parallel-reproducible-build/rmake.rs
index caaee1b47c583..b40d93a1a1c86 100644
--- a/tests/run-make/parallel-reproducible-build/rmake.rs
+++ b/tests/run-make/parallel-reproducible-build/rmake.rs
@@ -14,6 +14,17 @@ fn main() {
     const TESTS: &[(&str, &[&str])] = &[
         ("static-muts-issue-140413", &["-Zthreads=50"]),
         ("derives-issue-129094", &["-Zthreads=16", "-Copt-level=3"]),
+        (
+            "generics-of-issue-163878",
+            &[
+                "-Zthreads=30",
+                "--crate-type=lib",
+                "--crate-name=test",
+                "-Ccodegen-units=1",
+                "-Zremap-cwd-prefix=reproducible_dir",
+                "--edition=2024",
+            ],
+        ),
     ];
 
     for (file, args) in TESTS {