# What: make rust_test sharding assign tests by stable name hash and accept
# explicit per-target shard env vars.
# Why: Codex generates separate Bazel test labels for each shard so BuildBuddy
# can report flakiness and timing per shard label, but Bazel reserves TEST_*
# vars in normal test env. This mirrors hermeticbuild/rules_rust#14.

diff --git a/rust/private/rust.bzl b/rust/private/rust.bzl
index 57b2794f7..ffa8ece7d 100644
--- a/rust/private/rust.bzl
+++ b/rust/private/rust.bzl
@@ -928,15 +928,6 @@ _RUST_TEST_ATTRS = {
     "env_inherit": attr.string_list(
         doc = "Specifies additional environment variables to inherit from the external environment when the test is executed by bazel test.",
     ),
-    "use_libtest_harness": attr.bool(
-        mandatory = False,
-        default = True,
-        doc = dedent("""\
-            Whether to use `libtest`. For targets using this flag, individual tests can be run by using the
-            [--test_arg](https://docs.bazel.build/versions/4.0.0/command-line-reference.html#flag--test_arg) flag.
-            E.g. `bazel test //src:rust_test --test_arg=foo::test::test_fn`.
-        """),
-    ),
     "experimental_enable_sharding": attr.bool(
         mandatory = False,
         default = False,
@@ -945,14 +936,25 @@ _RUST_TEST_ATTRS = {
 
             When enabled, tests are executed via a wrapper script that:
             1. Enumerates tests using libtest's --list flag
-            2. Partitions tests across shards based on TEST_SHARD_INDEX/TEST_TOTAL_SHARDS
-            3. Runs only the tests assigned to the current shard
+            2. Sorts tests by name and partitions them across shards by stable name hash
+            3. Uses either Bazel's native TEST_TOTAL_SHARDS/TEST_SHARD_INDEX env
+               or explicit RULES_RUST_TEST_TOTAL_SHARDS/RULES_RUST_TEST_SHARD_INDEX env
+            4. Runs only the tests assigned to the current shard
 
             This attribute only has an effect when use_libtest_harness is True.
 
             This is experimental and may change in future releases.
         """),
     ),
+    "use_libtest_harness": attr.bool(
+        mandatory = False,
+        default = True,
+        doc = dedent("""\
+            Whether to use `libtest`. For targets using this flag, individual tests can be run by using the
+            [--test_arg](https://docs.bazel.build/versions/4.0.0/command-line-reference.html#flag--test_arg) flag.
+            E.g. `bazel test //src:rust_test --test_arg=foo::test::test_fn`.
+        """),
+    ),
     "_test_sharding_wrapper_unix": attr.label(
         default = Label("//rust/private:test_sharding_wrapper.sh"),
         allow_single_file = True,
diff --git a/rust/private/test_sharding_wrapper.bat b/rust/private/test_sharding_wrapper.bat
index 5c90681c8..3e0acbb54 100644
--- a/rust/private/test_sharding_wrapper.bat
+++ b/rust/private/test_sharding_wrapper.bat
@@ -14,7 +14,8 @@
 
 @REM Wrapper script for rust_test that enables Bazel test sharding support.
 @REM This script intercepts test execution, enumerates tests using libtest's
-@REM --list flag, partitions them by shard index, and runs only the relevant subset.
+@REM --list flag, partitions them by stable test-name hash, and runs only the
+@REM relevant subset.
 
 @ECHO OFF
 SETLOCAL EnableDelayedExpansion
@@ -65,6 +66,35 @@ IF !FOUND_BINARY! EQU 0 IF DEFINED RUNFILES_DIR (
     )
 )
 
+@REM Try 4: manifest-based runfile lookup. This covers nested launchers that
+@REM execute the sharding wrapper from another test's runfiles tree.
+IF !FOUND_BINARY! EQU 0 (
+    SET "MANIFEST=!RUNFILES_MANIFEST_FILE!"
+    IF NOT DEFINED MANIFEST IF EXIST "%~f0.runfiles_manifest" SET "MANIFEST=%~f0.runfiles_manifest"
+    IF NOT DEFINED MANIFEST IF EXIST "%~dpn0.runfiles_manifest" SET "MANIFEST=%~dpn0.runfiles_manifest"
+    IF NOT DEFINED MANIFEST IF EXIST "%~f0.exe.runfiles_manifest" SET "MANIFEST=%~f0.exe.runfiles_manifest"
+
+    IF DEFINED MANIFEST IF EXIST "!MANIFEST!" (
+        SET "TEST_BINARY_MANIFEST_PATH=!TEST_BINARY_RAW!"
+        SET "TEST_BINARY_MANIFEST_PATH=!TEST_BINARY_MANIFEST_PATH:\=/!"
+        IF DEFINED TEST_WORKSPACE SET "TEST_BINARY_MANIFEST_WORKSPACE_PATH=!TEST_WORKSPACE!/!TEST_BINARY_MANIFEST_PATH!"
+        FOR /F "usebackq tokens=1,* delims= " %%A IN ("!MANIFEST!") DO (
+            IF "%%A"=="!TEST_BINARY_MANIFEST_PATH!" (
+                SET "TEST_BINARY_PATH=%%B"
+                SET FOUND_BINARY=1
+                GOTO :FOUND_TEST_BINARY
+            )
+            IF DEFINED TEST_BINARY_MANIFEST_WORKSPACE_PATH IF "%%A"=="!TEST_BINARY_MANIFEST_WORKSPACE_PATH!" (
+                SET "TEST_BINARY_PATH=%%B"
+                SET FOUND_BINARY=1
+                GOTO :FOUND_TEST_BINARY
+            )
+        )
+    )
+)
+
+:FOUND_TEST_BINARY
+
 IF !FOUND_BINARY! EQU 0 (
     ECHO ERROR: Could not find test binary at any expected location
     EXIT /B 1
@@ -73,40 +74,84 @@ IF !FOUND_BINARY! EQU 0 (
     EXIT /B 1
 )
 
+@REM Native Bazel test sharding sets TEST_TOTAL_SHARDS/TEST_SHARD_INDEX.
+@REM Explicit shard test targets can set RULES_RUST_TEST_TOTAL_SHARDS/
+@REM RULES_RUST_TEST_SHARD_INDEX instead because Bazel may reserve TEST_*
+@REM variables for its own test runner env.
+SET TOTAL_SHARDS=%RULES_RUST_TEST_TOTAL_SHARDS%
+IF "%TOTAL_SHARDS%"=="" SET TOTAL_SHARDS=%TEST_TOTAL_SHARDS%
+SET SHARD_INDEX=%RULES_RUST_TEST_SHARD_INDEX%
+IF "%SHARD_INDEX%"=="" SET SHARD_INDEX=%TEST_SHARD_INDEX%
+
 @REM If sharding is not enabled, run test binary directly
-IF "%TEST_TOTAL_SHARDS%"=="" (
+IF "%TOTAL_SHARDS%"=="" (
     !TEST_BINARY_PATH! %*
     EXIT /B !ERRORLEVEL!
 )
+IF "%TOTAL_SHARDS%"=="0" (
+    !TEST_BINARY_PATH! %*
+    EXIT /B !ERRORLEVEL!
+)
+
+IF "%SHARD_INDEX%"=="" (
+    ECHO ERROR: TEST_SHARD_INDEX or RULES_RUST_TEST_SHARD_INDEX must be set when sharding is enabled
+    EXIT /B 1
+)
 
 @REM Touch status file to advertise sharding support to Bazel
-IF NOT "%TEST_SHARD_STATUS_FILE%"=="" (
+IF NOT "%TEST_SHARD_STATUS_FILE%"=="" IF NOT "%TEST_TOTAL_SHARDS%"=="" IF NOT "%TEST_TOTAL_SHARDS%"=="0" (
     TYPE NUL > "%TEST_SHARD_STATUS_FILE%"
 )
 
-@REM Create a temporary file for test list
-SET TEMP_LIST=%TEMP%\rust_test_list_%RANDOM%.txt
+@REM Create per-wrapper temporary files. Prefer Bazel's per-test temp directory;
+@REM when falling back to the shared temp directory, avoid %RANDOM%-only file
+@REM names that can collide across concurrently running Windows test shards.
+SET "TEMP_ROOT=%TEST_TMPDIR%"
+IF NOT DEFINED TEMP_ROOT SET "TEMP_ROOT=%TEMP%"
+IF NOT DEFINED TEMP_ROOT SET "TEMP_ROOT=."
+:CREATE_TEMP_DIR
+SET "TEMP_DIR=!TEMP_ROOT!\rust_test_sharding_!RANDOM!_!RANDOM!_!RANDOM!"
+MKDIR "!TEMP_DIR!" 2>NUL
+IF ERRORLEVEL 1 GOTO :CREATE_TEMP_DIR
+SET "TEMP_LIST=!TEMP_DIR!\list.txt"
+SET "TEMP_SHARD_LIST=!TEMP_DIR!\shard.txt"
 
 @REM Enumerate all tests using libtest's --list flag
 !TEST_BINARY_PATH! --list --format terse 2>NUL > "!TEMP_LIST!"
+IF ERRORLEVEL 1 (
+    RMDIR /S /Q "!TEMP_DIR!" 2>NUL
+    EXIT /B 1
+)
 
-@REM Count tests and filter for this shard
-SET INDEX=0
+@REM Sort tests by ordinal name and filter this shard by stable FNV-1a hash so
+@REM adding or removing one test does not move unrelated tests between shards.
+@REM In the PowerShell fragment below, 2166136261 is the 32-bit FNV offset basis,
+@REM 16777619 is the FNV prime, and 4294967295 is the UInt32 mask. Use decimal
+@REM constants because Windows PowerShell can interpret 0xffffffff as -1.
+powershell.exe -NoProfile -ExecutionPolicy Bypass -Command ^
+    "$ErrorActionPreference = 'Stop';" ^
+    "$tests = @(Get-Content -LiteralPath $env:TEMP_LIST | Where-Object { $_.EndsWith(': test') } | ForEach-Object { $_.Substring(0, $_.Length - 6) });" ^
+    "[Array]::Sort($tests, [StringComparer]::Ordinal);" ^
+    "$totalShards = [uint32]$env:TOTAL_SHARDS; $shardIndex = [uint32]$env:SHARD_INDEX;" ^
+    "$fnvPrime = [uint64]16777619; $u32Mask = [uint64]4294967295;" ^
+    "foreach ($test in $tests) { $hash = [uint32]2166136261; foreach ($byte in [Text.Encoding]::UTF8.GetBytes($test)) { $hash = [uint32](([uint64]($hash -bxor $byte) * $fnvPrime) -band $u32Mask) }; if (($hash %% $totalShards) -eq $shardIndex) { $test } }" ^
+    > "!TEMP_SHARD_LIST!"
+IF ERRORLEVEL 1 (
+    RMDIR /S /Q "!TEMP_DIR!" 2>NUL
+    EXIT /B 1
+)
+
 SET SHARD_TESTS=
 
-FOR /F "tokens=1 delims=:" %%T IN ('TYPE "!TEMP_LIST!" ^| FINDSTR /E ": test"') DO (
-    SET /A MOD=!INDEX! %% %TEST_TOTAL_SHARDS%
-    IF !MOD! EQU %TEST_SHARD_INDEX% (
-        IF "!SHARD_TESTS!"=="" (
-            SET SHARD_TESTS=%%T
-        ) ELSE (
-            SET SHARD_TESTS=!SHARD_TESTS! %%T
-        )
+FOR /F "usebackq delims=" %%T IN ("!TEMP_SHARD_LIST!") DO (
+    IF "!SHARD_TESTS!"=="" (
+        SET SHARD_TESTS=%%T
+    ) ELSE (
+        SET SHARD_TESTS=!SHARD_TESTS! %%T
     )
-    SET /A INDEX=!INDEX! + 1
 )
 
-DEL "!TEMP_LIST!" 2>NUL
+RMDIR /S /Q "!TEMP_DIR!" 2>NUL
 
 @REM If no tests for this shard, exit successfully
 IF "!SHARD_TESTS!"=="" (
diff --git a/rust/private/test_sharding_wrapper.sh b/rust/private/test_sharding_wrapper.sh
index e05970ba0..b1f0fb55d 100755
--- a/rust/private/test_sharding_wrapper.sh
+++ b/rust/private/test_sharding_wrapper.sh
@@ -15,40 +15,70 @@
 
 # Wrapper script for rust_test that enables Bazel test sharding support.
 # This script intercepts test execution, enumerates tests using libtest's
-# --list flag, partitions them by shard index, and runs only the relevant subset.
+# --list flag, partitions them by stable test-name hash, and runs only the
+# relevant subset.
 
 set -euo pipefail
 
 TEST_BINARY="{{TEST_BINARY}}"
+# Native Bazel test sharding sets TEST_TOTAL_SHARDS/TEST_SHARD_INDEX. Explicit
+# shard test targets can set RULES_RUST_TEST_TOTAL_SHARDS/RULES_RUST_TEST_SHARD_INDEX
+# instead because Bazel may reserve TEST_* variables for its own test runner env.
+TOTAL_SHARDS="${RULES_RUST_TEST_TOTAL_SHARDS:-${TEST_TOTAL_SHARDS:-}}"
+SHARD_INDEX="${RULES_RUST_TEST_SHARD_INDEX:-${TEST_SHARD_INDEX:-}}"
+
+test_shard_index() {
+    local test_name="$1"
+    # FNV-1a 32-bit hash. The initial value is the FNV offset basis, and
+    # 16777619 is the FNV prime. This gives a stable, cheap string hash without
+    # depending on platform-specific tools being present in the test sandbox.
+    local hash=2166136261
+    local byte
+    local char
+    local i
+    local LC_ALL=C
+
+    for ((i = 0; i < ${#test_name}; i++)); do
+        char="${test_name:i:1}"
+        printf -v byte "%d" "'$char"
+        hash=$(( ((hash ^ byte) * 16777619) & 0xffffffff ))
+    done
+
+    echo $(( hash % TOTAL_SHARDS ))
+}
 
 # If sharding is not enabled, run test binary directly
-if [[ -z "${TEST_TOTAL_SHARDS:-}" ]]; then
+if [[ -z "${TOTAL_SHARDS}" || "${TOTAL_SHARDS}" == "0" ]]; then
     exec "./${TEST_BINARY}" "$@"
 fi
 
+if [[ -z "${SHARD_INDEX}" ]]; then
+    echo "TEST_SHARD_INDEX or RULES_RUST_TEST_SHARD_INDEX must be set when sharding is enabled" >&2
+    exit 1
+fi
+
 # Touch status file to advertise sharding support to Bazel
-if [[ -n "${TEST_SHARD_STATUS_FILE:-}" ]]; then
+if [[ -n "${TEST_SHARD_STATUS_FILE:-}" && "${TEST_TOTAL_SHARDS:-0}" != "0" ]]; then
     touch "${TEST_SHARD_STATUS_FILE}"
 fi
 
-# Enumerate all tests using libtest's --list flag
+# Enumerate all tests using libtest's --list flag. Sort the list so execution
+# order does not depend on libtest's output order.
 # Output format: "test_name: test" - we need to strip the ": test" suffix
-test_list=$("./${TEST_BINARY}" --list --format terse 2>/dev/null | grep ': test$' | sed 's/: test$//' || true)
+test_list=$("./${TEST_BINARY}" --list --format terse 2>/dev/null | grep ': test$' | sed 's/: test$//' | LC_ALL=C sort || true)
 
 # If no tests found, exit successfully
 if [[ -z "$test_list" ]]; then
     exit 0
 fi
 
-# Filter tests for this shard
-# test_index % TEST_TOTAL_SHARDS == TEST_SHARD_INDEX
+# Filter tests for this shard. Use a stable name hash instead of list position
+# so adding or removing one test does not move unrelated tests between shards.
 shard_tests=()
-index=0
 while IFS= read -r test_name; do
-    if (( index % TEST_TOTAL_SHARDS == TEST_SHARD_INDEX )); then
+    if (( $(test_shard_index "$test_name") == SHARD_INDEX )); then
         shard_tests+=("$test_name")
     fi
-    ((index++)) || true
 done <<< "$test_list"
 
 # If no tests for this shard, exit successfully