fix: filter empty segments in colon-separated env var parsing

e2e/config/test_override_config_filenames_empty

@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+# Regression test: MISE_OVERRIDE_CONFIG_FILENAMES set to empty string
+# should not cause a panic. An empty value should be treated the same
+# as the variable being unset (i.e., use default config filenames).
+#
+# Background: split(':') on "" produces [""], a set with one empty-string
+# entry. This non-empty set bypasses the default config filename list and
+# injects "" as a config filename, which eventually causes a panic in
+# config_root() during Config::load.
+#
+# See: https://github.com/jdx/mise/pull/9076
+
+echo 'tools.dummy = "1"' >mise.toml
+
+# Empty string — should behave like unset (use mise.toml)
+MISE_OVERRIDE_CONFIG_FILENAMES="" assert_succeed "mise ls"
+
+# Colon-only — split produces ["", ""], both empty, should behave like unset
+MISE_OVERRIDE_CONFIG_FILENAMES=":" assert_succeed "mise ls"
+
+# Trailing/leading colons — should ignore empty segments
+echo 'tools.dummy = "2"' >mise.ci.toml
+MISE_OVERRIDE_CONFIG_FILENAMES=":mise.ci.toml:" assert "mise ls dummy" "dummy  2.0.0 (missing)  ~/workdir/mise.ci.toml  2"

src/env.rs

@@ -244,14 +244,14 @@ pub static MISE_DEFAULT_CONFIG_FILENAME: Lazy<String> = Lazy::new(|| {
 pub static MISE_OVERRIDE_TOOL_VERSIONS_FILENAMES: Lazy<Option<IndexSet<String>>> =
     Lazy::new(|| match var("MISE_OVERRIDE_TOOL_VERSIONS_FILENAMES") {
         Ok(v) if v == "none" => Some([].into()),
-        Ok(v) => Some(v.split(':').map(|s| s.to_string()).collect()),
+        Ok(v) => Some(split_colon_list(&v)),
         Err(_) => {
             miserc::get_override_tool_versions_filenames().map(|v| v.iter().cloned().collect())
         }
     });
 pub static MISE_OVERRIDE_CONFIG_FILENAMES: Lazy<IndexSet<String>> =
     Lazy::new(|| match var("MISE_OVERRIDE_CONFIG_FILENAMES") {
-        Ok(v) => v.split(':').map(|s| s.to_string()).collect(),
+        Ok(v) => split_colon_list(&v),
         Err(_) => miserc::get_override_config_filenames()
             .map(|v| v.iter().cloned().collect())
             .unwrap_or_default(),
@@ -738,6 +738,18 @@ fn linux_glibc_version() -> Option<(u32, u32)> {
     None
 }
 
+/// Split a colon-separated string into a set, filtering empty segments.
+/// Empty segments arise from empty strings, leading/trailing colons, or
+/// consecutive colons — all of which should be ignored rather than
+/// injected as empty paths into config discovery.
+fn split_colon_list(value: &str) -> IndexSet<String> {
+    value
+        .split(':')
+        .filter(|s| !s.is_empty())
+        .map(|s| s.to_string())
+        .collect()
+}
+
 fn filename(path: &str) -> &str {
     path.rsplit_once(path::MAIN_SEPARATOR_STR)
         .map(|(_, file)| file)
@@ -834,6 +846,24 @@ mod tests {
         remove_var("MISE_TEST_PATH");
     }
 
+    #[test]
+    fn test_split_colon_list() {
+        let cases: Vec<(&str, Vec<&str>)> = vec![
+            ("", vec![]),    // empty string — was causing panic
+            (":", vec![]),   // colon only
+            (":::", vec![]), // multiple colons
+            ("mise.toml", vec!["mise.toml"]),
+            ("a:b", vec!["a", "b"]),
+            (":a:b:", vec!["a", "b"]), // leading/trailing colons
+            ("a::b", vec!["a", "b"]),  // consecutive colons
+        ];
+        for (input, expected) in cases {
+            let result = split_colon_list(input);
+            let expected: IndexSet<String> = expected.into_iter().map(|s| s.to_string()).collect();
+            assert_eq!(result, expected, "input: {input:?}");
+        }
+    }
+
     #[test]
     fn test_token_overwrite() {
         // Clean up any existing environment variables that might interfere