diff --git a/src/tools/run-make-support/src/run.rs b/src/tools/run-make-support/src/run/mod.rs
similarity index 90%
rename from src/tools/run-make-support/src/run.rs
rename to src/tools/run-make-support/src/run/mod.rs
index 43f0473ddf4a1..ea5d16248bcf9 100644
--- a/src/tools/run-make-support/src/run.rs
+++ b/src/tools/run-make-support/src/run/mod.rs
@@ -89,11 +89,13 @@ pub fn cmd<S: AsRef<OsStr>>(program: S) -> Command {
     command
 }
 
-fn split_maybe_args(s: &str) -> Vec<OsString> {
-    // FIXME(132599): implement proper env var/shell argument splitting.
-    s.split(' ')
-        .filter_map(|s| {
-            if s.chars().all(|c| c.is_whitespace()) { None } else { Some(OsString::from(s)) }
-        })
+pub(crate) fn split_maybe_args(s: &str) -> Vec<OsString> {
+    // Split on default shell IFS (space, tab, newline).
+    s.split(|c: char| matches!(c, ' ' | '\t' | '\n'))
+        .filter(|s| !s.is_empty())
+        .map(OsString::from)
         .collect()
 }
+
+#[cfg(test)]
+mod tests;
diff --git a/src/tools/run-make-support/src/run/tests.rs b/src/tools/run-make-support/src/run/tests.rs
new file mode 100644
index 0000000000000..690a0c4e68bc7
--- /dev/null
+++ b/src/tools/run-make-support/src/run/tests.rs
@@ -0,0 +1,51 @@
+use std::ffi::OsString;
+
+use super::split_maybe_args;
+
+fn os(s: &str) -> OsString {
+    OsString::from(s)
+}
+
+#[test]
+fn split_on_space() {
+    assert_eq!(
+        split_maybe_args("valgrind --tool=memcheck"),
+        vec![os("valgrind"), os("--tool=memcheck")]
+    );
+}
+
+#[test]
+fn single_arg_no_whitespace() {
+    assert_eq!(split_maybe_args("valgrind"), vec![os("valgrind")]);
+}
+
+#[test]
+fn empty_string() {
+    assert_eq!(split_maybe_args(""), Vec::<OsString>::new());
+}
+
+#[test]
+fn split_on_tab() {
+    assert_eq!(
+        split_maybe_args("valgrind\t--tool=memcheck"),
+        vec![os("valgrind"), os("--tool=memcheck")]
+    );
+}
+
+#[test]
+fn split_on_newline() {
+    assert_eq!(
+        split_maybe_args("valgrind\n--tool=memcheck"),
+        vec![os("valgrind"), os("--tool=memcheck")]
+    );
+}
+
+#[test]
+fn multiple_ifs_separators() {
+    assert_eq!(split_maybe_args("a  b\t\tc\n\nd"), vec![os("a"), os("b"), os("c"), os("d")]);
+}
+
+#[test]
+fn leading_and_trailing_whitespace() {
+    assert_eq!(split_maybe_args("  valgrind\t"), vec![os("valgrind")]);
+}