diff --git a/rust/Cargo.lock b/rust/Cargo.lock
--- a/rust/Cargo.lock
+++ b/rust/Cargo.lock
@@ -42,6 +42,14 @@
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
+name = "bitmaps"
+version = "2.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "typenum 1.12.0 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
 name = "byteorder"
 version = "1.3.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -248,6 +256,7 @@
  "crossbeam 0.7.3 (registry+https://github.com/rust-lang/crates.io-index)",
  "flate2 1.0.17 (registry+https://github.com/rust-lang/crates.io-index)",
  "hex 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)",
+ "im-rc 15.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "lazy_static 1.4.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "log 0.4.11 (registry+https://github.com/rust-lang/crates.io-index)",
  "memchr 2.3.3 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -286,6 +295,19 @@
 ]
 
 [[package]]
+name = "im-rc"
+version = "15.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "bitmaps 2.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "rand_core 0.5.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "rand_xoshiro 0.4.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "sized-chunks 0.6.2 (registry+https://github.com/rust-lang/crates.io-index)",
+ "typenum 1.12.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "version_check 0.9.1 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
 name = "itertools"
 version = "0.9.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -558,6 +580,14 @@
 ]
 
 [[package]]
+name = "rand_xoshiro"
+version = "0.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "rand_core 0.5.1 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
 name = "rayon"
 version = "1.4.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -659,6 +689,15 @@
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
+name = "sized-chunks"
+version = "0.6.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "bitmaps 2.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "typenum 1.12.0 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
 name = "strsim"
 version = "0.8.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -729,6 +768,11 @@
 ]
 
 [[package]]
+name = "typenum"
+version = "1.12.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
 name = "unicode-width"
 version = "0.1.8"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -749,6 +793,11 @@
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
+name = "version_check"
+version = "0.9.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
 name = "wasi"
 version = "0.9.0+wasi-snapshot-preview1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -820,6 +869,7 @@
 "checksum atty 0.2.14 (registry+https://github.com/rust-lang/crates.io-index)" = "d9b39be18770d11421cdb1b9947a45dd3f37e93092cbf377614828a319d5fee8"
 "checksum autocfg 1.0.1 (registry+https://github.com/rust-lang/crates.io-index)" = "cdb031dd78e28731d87d56cc8ffef4a8f36ca26c38fe2de700543e627f8a464a"
 "checksum bitflags 1.2.1 (registry+https://github.com/rust-lang/crates.io-index)" = "cf1de2fe8c75bc145a2f577add951f8134889b4795d47466a54a5c846d691693"
+"checksum bitmaps 2.1.0 (registry+https://github.com/rust-lang/crates.io-index)" = "031043d04099746d8db04daf1fa424b2bc8bd69d92b25962dcde24da39ab64a2"
 "checksum byteorder 1.3.4 (registry+https://github.com/rust-lang/crates.io-index)" = "08c48aae112d48ed9f069b33538ea9e3e90aa263cfa3d1c24309612b1f7472de"
 "checksum cc 1.0.60 (registry+https://github.com/rust-lang/crates.io-index)" = "ef611cc68ff783f18535d77ddd080185275713d852c4f5cbb6122c462a7a825c"
 "checksum cfg-if 0.1.10 (registry+https://github.com/rust-lang/crates.io-index)" = "4785bdd1c96b2a846b2bd7cc02e86b6b3dbf14e7e53446c4f54c92a361040822"
@@ -844,6 +894,7 @@
 "checksum hermit-abi 0.1.16 (registry+https://github.com/rust-lang/crates.io-index)" = "4c30f6d0bc6b00693347368a67d41b58f2fb851215ff1da49e90fe2c5c667151"
 "checksum hex 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)" = "644f9158b2f133fd50f5fb3242878846d9eb792e445c893805ff0e3824006e35"
 "checksum humantime 1.3.0 (registry+https://github.com/rust-lang/crates.io-index)" = "df004cfca50ef23c36850aaaa59ad52cc70d0e90243c3c7737a4dd32dc7a3c4f"
+"checksum im-rc 15.0.0 (registry+https://github.com/rust-lang/crates.io-index)" = "3ca8957e71f04a205cb162508f9326aea04676c8dfd0711220190d6b83664f3f"
 "checksum itertools 0.9.0 (registry+https://github.com/rust-lang/crates.io-index)" = "284f18f85651fe11e8a991b2adb42cb078325c996ed026d994719efcfca1d54b"
 "checksum jobserver 0.1.21 (registry+https://github.com/rust-lang/crates.io-index)" = "5c71313ebb9439f74b00d9d2dcec36440beaf57a6aa0623068441dd7cd81a7f2"
 "checksum lazy_static 1.4.0 (registry+https://github.com/rust-lang/crates.io-index)" = "e2abad23fbc42b3700f2f279844dc832adb2b2eb069b2df918f455c4e18cc646"
@@ -878,6 +929,7 @@
 "checksum rand_distr 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)" = "96977acbdd3a6576fb1d27391900035bf3863d4a16422973a409b488cf29ffb2"
 "checksum rand_hc 0.2.0 (registry+https://github.com/rust-lang/crates.io-index)" = "ca3129af7b92a17112d59ad498c6f81eaf463253766b90396d39ea7a39d6613c"
 "checksum rand_pcg 0.2.1 (registry+https://github.com/rust-lang/crates.io-index)" = "16abd0c1b639e9eb4d7c50c0b8100b0d0f849be2349829c740fe8e6eb4816429"
+"checksum rand_xoshiro 0.4.0 (registry+https://github.com/rust-lang/crates.io-index)" = "a9fcdd2e881d02f1d9390ae47ad8e5696a9e4be7b547a1da2afbc61973217004"
 "checksum rayon 1.4.0 (registry+https://github.com/rust-lang/crates.io-index)" = "cfd016f0c045ad38b5251be2c9c0ab806917f82da4d36b2a327e5166adad9270"
 "checksum rayon-core 1.8.1 (registry+https://github.com/rust-lang/crates.io-index)" = "e8c4fec834fb6e6d2dd5eece3c7b432a52f0ba887cf40e595190c4107edc08bf"
 "checksum rdrand 0.4.0 (registry+https://github.com/rust-lang/crates.io-index)" = "678054eb77286b51581ba43620cc911abf02758c91f93f479767aed0f90458b2"
@@ -889,6 +941,7 @@
 "checksum rustc-serialize 0.3.24 (registry+https://github.com/rust-lang/crates.io-index)" = "dcf128d1287d2ea9d80910b5f1120d0b8eede3fbf1abe91c40d39ea7d51e6fda"
 "checksum same-file 1.0.6 (registry+https://github.com/rust-lang/crates.io-index)" = "93fc1dc3aaa9bfed95e02e6eadabb4baf7e3078b0bd1b4d7b6b0b68378900502"
 "checksum scopeguard 1.1.0 (registry+https://github.com/rust-lang/crates.io-index)" = "d29ab0c6d3fc0ee92fe66e2d99f700eab17a8d57d1c1d3b748380fb20baa78cd"
+"checksum sized-chunks 0.6.2 (registry+https://github.com/rust-lang/crates.io-index)" = "1ec31ceca5644fa6d444cc77548b88b67f46db6f7c71683b0f9336e671830d2f"
 "checksum strsim 0.8.0 (registry+https://github.com/rust-lang/crates.io-index)" = "8ea5119cdb4c55b55d432abb513a0429384878c15dde60cc77b1c99de1a95a6a"
 "checksum syn 1.0.41 (registry+https://github.com/rust-lang/crates.io-index)" = "6690e3e9f692504b941dc6c3b188fd28df054f7fb8469ab40680df52fdcc842b"
 "checksum tempfile 3.1.0 (registry+https://github.com/rust-lang/crates.io-index)" = "7a6e24d9338a0a5be79593e2fa15a648add6138caa803e2d5bc782c371732ca9"
@@ -897,10 +950,12 @@
 "checksum thread_local 1.0.1 (registry+https://github.com/rust-lang/crates.io-index)" = "d40c6d1b69745a6ec6fb1ca717914848da4b44ae29d9b3080cbee91d72a69b14"
 "checksum time 0.1.44 (registry+https://github.com/rust-lang/crates.io-index)" = "6db9e6914ab8b1ae1c260a4ae7a49b6c5611b40328a735b21862567685e73255"
 "checksum twox-hash 1.5.0 (registry+https://github.com/rust-lang/crates.io-index)" = "3bfd5b7557925ce778ff9b9ef90e3ade34c524b5ff10e239c69a42d546d2af56"
+"checksum typenum 1.12.0 (registry+https://github.com/rust-lang/crates.io-index)" = "373c8a200f9e67a0c95e62a4f52fbf80c23b4381c05a17845531982fa99e6b33"
 "checksum unicode-width 0.1.8 (registry+https://github.com/rust-lang/crates.io-index)" = "9337591893a19b88d8d87f2cec1e73fad5cdfd10e5a6f349f498ad6ea2ffb1e3"
 "checksum unicode-xid 0.2.1 (registry+https://github.com/rust-lang/crates.io-index)" = "f7fe0bb3479651439c9112f72b6c505038574c9fbb575ed1bf3b797fa39dd564"
 "checksum vcpkg 0.2.10 (registry+https://github.com/rust-lang/crates.io-index)" = "6454029bf181f092ad1b853286f23e2c507d8e8194d01d92da4a55c274a5508c"
 "checksum vec_map 0.8.2 (registry+https://github.com/rust-lang/crates.io-index)" = "f1bddf1187be692e79c5ffeab891132dfb0f236ed36a43c7ed39f1165ee20191"
+"checksum version_check 0.9.1 (registry+https://github.com/rust-lang/crates.io-index)" = "078775d0255232fb988e6fccf26ddc9d1ac274299aaedcedce21c6f72cc533ce"
 "checksum wasi 0.10.0+wasi-snapshot-preview1 (registry+https://github.com/rust-lang/crates.io-index)" = "1a143597ca7c7793eff794def352d41792a93c481eb1042423ff7ff72ba2c31f"
 "checksum wasi 0.9.0+wasi-snapshot-preview1 (registry+https://github.com/rust-lang/crates.io-index)" = "cccddf32554fecc6acb585f82a32a72e28b48f8c4c1883ddfeeeaa96f7d8e519"
 "checksum winapi 0.3.9 (registry+https://github.com/rust-lang/crates.io-index)" = "5c839a674fcd7a98952e593242ea400abe93992746761e38641405d28b00f419"
diff --git a/rust/hg-core/Cargo.toml b/rust/hg-core/Cargo.toml
--- a/rust/hg-core/Cargo.toml
+++ b/rust/hg-core/Cargo.toml
@@ -11,6 +11,7 @@
 [dependencies]
 byteorder = "1.3.4"
 hex = "0.4.2"
+im-rc = "15.0.*"
 lazy_static = "1.4.0"
 memchr = "2.3.3"
 rand = "0.7.3"
diff --git a/rust/hg-core/src/copy_tracing.rs b/rust/hg-core/src/copy_tracing.rs
--- a/rust/hg-core/src/copy_tracing.rs
+++ b/rust/hg-core/src/copy_tracing.rs
@@ -1,6 +1,8 @@
 use crate::utils::hg_path::HgPathBuf;
 use crate::Revision;
 
+use im_rc::ordmap::OrdMap;
+
 use std::collections::HashMap;
 use std::collections::HashSet;
 
@@ -16,7 +18,7 @@
 }
 
 /// maps CopyDestination to Copy Source (+ a "timestamp" for the operation)
-type TimeStampedPathCopies = HashMap<HgPathBuf, TimeStampedPathCopy>;
+type TimeStampedPathCopies = OrdMap<HgPathBuf, TimeStampedPathCopy>;
 
 /// hold parent 1, parent 2 and relevant files actions.
 pub type RevInfo = (Revision, Revision, ChangedFiles);