diff --git a/Cargo.lock b/Cargo.lock
index 0ac5280c1..24994225e 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -65,6 +65,12 @@ dependencies = [
  "windows-targets",
 ]
 
+[[package]]
+name = "base64"
+version = "0.21.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9d297deb1925b89f2ccc13d7635fa0714f12c87adce1c75356b39ca9b7178567"
+
 [[package]]
 name = "base64"
 version = "0.22.1"
@@ -144,6 +150,15 @@ version = "0.8.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "773648b94d0e5d620f64f280777445740e61fe701025087ec8b57f45c791888b"
 
+[[package]]
+name = "core_maths"
+version = "0.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "77745e017f5edba1a9c1d854f6f3a52dac8a12dd5af5d2f54aecf61e43d80d30"
+dependencies = [
+ "libm",
+]
+
 [[package]]
 name = "cpufeatures"
 version = "0.2.12"
@@ -365,7 +380,7 @@ version = "0.4.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b3314d5adb5d94bcdf56771f2e50dbbc80bb4bdf88967526706205ac9eff24eb"
 dependencies = [
- "base64",
+ "base64 0.22.1",
  "bytes",
  "headers-core",
  "http",
@@ -485,7 +500,7 @@ version = "0.1.14"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "dc2fdfdbff08affe55bb779f33b053aa1fe5dd5b54c257343c17edfa55711bdb"
 dependencies = [
- "base64",
+ "base64 0.22.1",
  "bytes",
  "futures-channel",
  "futures-core",
@@ -510,11 +525,59 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "db2fa452206ebee18c4b5c2274dbf1de17008e874b4dc4f0aea9d01ca79e4526"
 dependencies = [
  "displaydoc",
- "yoke",
+ "yoke 0.7.5",
  "zerofrom",
- "zerovec",
+ "zerovec 0.10.4",
 ]
 
+[[package]]
+name = "icu_collections"
+version = "2.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "200072f5d0e3614556f94a9930d5dc3e0662a652823904c3a75dc3b0af7fee47"
+dependencies = [
+ "displaydoc",
+ "potential_utf",
+ "yoke 0.8.0",
+ "zerofrom",
+ "zerovec 0.11.2",
+]
+
+[[package]]
+name = "icu_locale"
+version = "2.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6ae5921528335e91da1b6c695dbf1ec37df5ac13faa3f91e5640be93aa2fbefd"
+dependencies = [
+ "displaydoc",
+ "icu_collections 2.0.0",
+ "icu_locale_core",
+ "icu_locale_data",
+ "icu_provider 2.0.0",
+ "potential_utf",
+ "tinystr 0.8.1",
+ "zerovec 0.11.2",
+]
+
+[[package]]
+name = "icu_locale_core"
+version = "2.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0cde2700ccaed3872079a65fb1a78f6c0a36c91570f28755dda67bc8f7d9f00a"
+dependencies = [
+ "displaydoc",
+ "litemap 0.8.0",
+ "tinystr 0.8.1",
+ "writeable 0.6.1",
+ "zerovec 0.11.2",
+]
+
+[[package]]
+name = "icu_locale_data"
+version = "2.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4fdef0c124749d06a743c69e938350816554eb63ac979166590e2b4ee4252765"
+
 [[package]]
 name = "icu_locid"
 version = "1.5.0"
@@ -522,10 +585,10 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "13acbb8371917fc971be86fc8057c41a64b521c184808a698c02acc242dbf637"
 dependencies = [
  "displaydoc",
- "litemap",
- "tinystr",
- "writeable",
- "zerovec",
+ "litemap 0.7.5",
+ "tinystr 0.7.6",
+ "writeable 0.5.5",
+ "zerovec 0.10.4",
 ]
 
 [[package]]
@@ -537,9 +600,9 @@ dependencies = [
  "displaydoc",
  "icu_locid",
  "icu_locid_transform_data",
- "icu_provider",
- "tinystr",
- "zerovec",
+ "icu_provider 1.5.0",
+ "tinystr 0.7.6",
+ "zerovec 0.10.4",
 ]
 
 [[package]]
@@ -555,15 +618,15 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "19ce3e0da2ec68599d193c93d088142efd7f9c5d6fc9b803774855747dc6a84f"
 dependencies = [
  "displaydoc",
- "icu_collections",
+ "icu_collections 1.5.0",
  "icu_normalizer_data",
  "icu_properties",
- "icu_provider",
+ "icu_provider 1.5.0",
  "smallvec",
  "utf16_iter",
  "utf8_iter",
  "write16",
- "zerovec",
+ "zerovec 0.10.4",
 ]
 
 [[package]]
@@ -579,12 +642,12 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "93d6020766cfc6302c15dbbc9c8778c37e62c14427cb7f6e601d849e092aeef5"
 dependencies = [
  "displaydoc",
- "icu_collections",
+ "icu_collections 1.5.0",
  "icu_locid_transform",
  "icu_properties_data",
- "icu_provider",
- "tinystr",
- "zerovec",
+ "icu_provider 1.5.0",
+ "tinystr 0.7.6",
+ "zerovec 0.10.4",
 ]
 
 [[package]]
@@ -603,11 +666,28 @@ dependencies = [
  "icu_locid",
  "icu_provider_macros",
  "stable_deref_trait",
- "tinystr",
- "writeable",
- "yoke",
+ "tinystr 0.7.6",
+ "writeable 0.5.5",
+ "yoke 0.7.5",
  "zerofrom",
- "zerovec",
+ "zerovec 0.10.4",
+]
+
+[[package]]
+name = "icu_provider"
+version = "2.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "03c80da27b5f4187909049ee2d72f276f0d9f99a42c306bd0131ecfe04d8e5af"
+dependencies = [
+ "displaydoc",
+ "icu_locale_core",
+ "stable_deref_trait",
+ "tinystr 0.8.1",
+ "writeable 0.6.1",
+ "yoke 0.8.0",
+ "zerofrom",
+ "zerotrie",
+ "zerovec 0.11.2",
 ]
 
 [[package]]
@@ -621,6 +701,30 @@ dependencies = [
  "syn",
 ]
 
+[[package]]
+name = "icu_segmenter"
+version = "2.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e185fc13b6401c138cf40db12b863b35f5edf31b88192a545857b41aeaf7d3d3"
+dependencies = [
+ "core_maths",
+ "displaydoc",
+ "icu_collections 2.0.0",
+ "icu_locale",
+ "icu_locale_core",
+ "icu_provider 2.0.0",
+ "icu_segmenter_data",
+ "potential_utf",
+ "utf8_iter",
+ "zerovec 0.11.2",
+]
+
+[[package]]
+name = "icu_segmenter_data"
+version = "2.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5360a2fbe97f617c4f8b944356dedb36d423f7da7f13c070995cf89e59f01220"
+
 [[package]]
 name = "idna"
 version = "1.0.3"
@@ -713,12 +817,24 @@ version = "0.2.172"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d750af042f7ef4f724306de029d18836c26c1765a54a6a3f094cbd23a7267ffa"
 
+[[package]]
+name = "libm"
+version = "0.2.15"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f9fbbcab51052fe104eb5e5d351cf728d30a5be1fe14d9be8a3b097481fb97de"
+
 [[package]]
 name = "litemap"
 version = "0.7.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "23fb14cb19457329c82206317a5663005a4d404783dc74f4252769b0d5f42856"
 
+[[package]]
+name = "litemap"
+version = "0.8.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "241eaef5fd12c88705a01fc1066c48c4b36e0dd4377dcdc7ec3942cea7a69956"
+
 [[package]]
 name = "log"
 version = "0.4.27"
@@ -811,6 +927,16 @@ version = "1.6.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7170ef9988bc169ba16dd36a7fa041e5c4cbeb6a35b76d4c03daded371eae7c0"
 
+[[package]]
+name = "potential_utf"
+version = "0.1.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e5a7c30837279ca13e7c867e9e40053bc68740f988cb07f7ca6df43cc734b585"
+dependencies = [
+ "serde",
+ "zerovec 0.11.2",
+]
+
 [[package]]
 name = "ppv-lite86"
 version = "0.2.17"
@@ -1064,7 +1190,7 @@ version = "0.12.22"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "cbc931937e6ca3a06e3b6c0aa7841849b160a90351d6ab467a8b9b9959767531"
 dependencies = [
- "base64",
+ "base64 0.22.1",
  "bytes",
  "futures-core",
  "futures-util",
@@ -1338,7 +1464,7 @@ name = "synapse"
 version = "0.1.0"
 dependencies = [
  "anyhow",
- "base64",
+ "base64 0.21.7",
  "blake2",
  "bytes",
  "futures",
@@ -1346,6 +1472,7 @@ dependencies = [
  "hex",
  "http",
  "http-body-util",
+ "icu_segmenter",
  "lazy_static",
  "log",
  "mime",
@@ -1414,7 +1541,17 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9117f5d4db391c1cf6927e7bea3db74b9a1c1add8f7eda9ffd5364f40f57b82f"
 dependencies = [
  "displaydoc",
- "zerovec",
+ "zerovec 0.10.4",
+]
+
+[[package]]
+name = "tinystr"
+version = "0.8.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5d4f6d1145dcb577acf783d4e601bc1d76a13337bb54e6233add580b07344c8b"
+dependencies = [
+ "displaydoc",
+ "zerovec 0.11.2",
 ]
 
 [[package]]
@@ -1836,6 +1973,12 @@ version = "0.5.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1e9df38ee2d2c3c5948ea468a8406ff0db0b29ae1ffde1bcf20ef305bcc95c51"
 
+[[package]]
+name = "writeable"
+version = "0.6.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ea2f10b9bb0928dfb1b42b65e1f9e36f7f54dbdf08457afefb38afcdec4fa2bb"
+
 [[package]]
 name = "yoke"
 version = "0.7.5"
@@ -1844,7 +1987,19 @@ checksum = "120e6aef9aa629e3d4f52dc8cc43a015c7724194c97dfaf45180d2daf2b77f40"
 dependencies = [
  "serde",
  "stable_deref_trait",
- "yoke-derive",
+ "yoke-derive 0.7.5",
+ "zerofrom",
+]
+
+[[package]]
+name = "yoke"
+version = "0.8.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5f41bb01b8226ef4bfd589436a297c53d118f65921786300e427be8d487695cc"
+dependencies = [
+ "serde",
+ "stable_deref_trait",
+ "yoke-derive 0.8.0",
  "zerofrom",
 ]
 
@@ -1860,6 +2015,18 @@ dependencies = [
  "synstructure",
 ]
 
+[[package]]
+name = "yoke-derive"
+version = "0.8.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "38da3c9736e16c5d3c8c597a9aaa5d1fa565d0532ae05e27c24aa62fb32c0ab6"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+ "synstructure",
+]
+
 [[package]]
 name = "zerocopy"
 version = "0.8.17"
@@ -1907,15 +2074,35 @@ version = "1.8.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ced3678a2879b30306d323f4542626697a464a97c0a07c9aebf7ebca65cd4dde"
 
+[[package]]
+name = "zerotrie"
+version = "0.2.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "36f0bbd478583f79edad978b407914f61b2972f5af6fa089686016be8f9af595"
+dependencies = [
+ "displaydoc",
+]
+
 [[package]]
 name = "zerovec"
 version = "0.10.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "aa2b893d79df23bfb12d5461018d408ea19dfafe76c2c7ef6d4eba614f8ff079"
 dependencies = [
- "yoke",
+ "yoke 0.7.5",
  "zerofrom",
- "zerovec-derive",
+ "zerovec-derive 0.10.3",
+]
+
+[[package]]
+name = "zerovec"
+version = "0.11.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4a05eb080e015ba39cc9e23bbe5e7fb04d5fb040350f99f34e338d5fdd294428"
+dependencies = [
+ "yoke 0.8.0",
+ "zerofrom",
+ "zerovec-derive 0.11.1",
 ]
 
 [[package]]
@@ -1928,3 +2115,14 @@ dependencies = [
  "quote",
  "syn",
 ]
+
+[[package]]
+name = "zerovec-derive"
+version = "0.11.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5b96237efa0c878c64bd89c436f661be4e46b2f3eff1ebb976f7ef2321d2f58f"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
diff --git a/changelog.d/18646.misc b/changelog.d/18646.misc
new file mode 100644
index 000000000..bb143aacf
--- /dev/null
+++ b/changelog.d/18646.misc
@@ -0,0 +1 @@
+Replace `PyICU` crate with equivalent `icu_segmenter` Rust crate.
\ No newline at end of file