Formulas: keep _xludf. so user-defined calls stay user-defined (fixes #726)

The parser stripped _xludf. like _xlfn., but the two mean opposite things:
_xludf. marks a name Excel must not treat as a built-in. Keeping it in the
call name makes the call an unknown function (#NAME?, as in Excel) and the
writer emits it unchanged, so _xludf.EOMONTH() no longer saves as an
EOMONTH() call Excel repairs away.

crates/calc/src/tests.rs

@@ -167,6 +167,11 @@ fn errors_and_cycles() {
     assert!(t.get("B1").is_error());
     t.set("C1", "=NOSUCHFN(1)");
     assert_eq!(t.get("C1"), Value::Error(CellError::Name));
+    // `_xludf.` marks a user-defined function, never the built-in of the same name (#726).
+    for f in ["=_xludf.CONCAT(\"a\",\"b\")", "=_xludf.SUM(1,2)", "=_xludf.EOMONTH()"] {
+        t.set("C3", f);
+        assert_eq!(t.get("C3"), Value::Error(CellError::Name), "{f}");
+    }
     t.set("C2", "=1+");
     assert_eq!(t.get("C2"), Value::Error(CellError::Name));
 }

crates/formula/src/parser.rs

@@ -88,15 +88,19 @@ fn word_is_cell(w: &str) -> Option<Anchor> {
     Some(Anchor { row, col, row_abs, col_abs })
 }
 
-/// `_xlfn.XLOOKUP` → `XLOOKUP`.
+/// `_xlfn.XLOOKUP` → `XLOOKUP`. `_xludf.` is kept (`_xludf.SUM`): it marks a user-defined
+/// function, never the built-in of the same name, and files need it back.
 pub fn normalize_function_name(name: &str) -> String {
     let u = name.to_ascii_uppercase();
-    for p in ["_XLFN._XLWS.", "_XLFN.", "_XLWS.", "_XLUDF."] {
+    for p in ["_XLFN._XLWS.", "_XLFN.", "_XLWS."] {
         if let Some(r) = u.strip_prefix(p) {
             return r.to_string();
         }
     }
-    u
+    match u.strip_prefix("_XLUDF.") {
+        Some(r) => format!("_xludf.{r}"),
+        None => u,
+    }
 }
 
 impl Parser {

crates/formula/src/tests.rs

@@ -48,6 +48,7 @@ fn normalises() {
     assert_eq!(rt("=sum(a1:b2)"), "SUM(A1:B2)");
     assert_eq!(rt("= 1 + 2"), "1+2");
     assert_eq!(rt("_xlfn.XLOOKUP(1,A:A,B:B)"), "XLOOKUP(1,A:A,B:B)");
+    assert_eq!(rt("_XLUDF.concat(\"a\",\"b\")"), "_xludf.CONCAT(\"a\",\"b\")");
     assert_eq!(rt("true"), "TRUE");
     assert_eq!(rt("'Sheet1'!A1"), "Sheet1!A1");
 }

crates/functions/src/lib.rs

@@ -197,7 +197,8 @@ fn registry() -> &'static (Vec<FnSpec>, HashMap<&'static str, usize>) {
     })
 }
 
-/// Looks a function up by name (case-insensitive; `_xlfn.`, `_xlws.` and `_xludf.`-free names).
+/// Looks a function up by name (case-insensitive; `_xlfn.`- and `_xlws.`-free names). A
+/// `_xludf.` name is user-defined and never found.
 pub fn lookup(name: &str) -> Option<&'static FnSpec> {
     let mut n = name.trim().to_ascii_uppercase();
     loop {

crates/xlsx/src/tests/fixtures.rs

@@ -495,3 +495,28 @@ fn text_box_formatting_is_read_and_written() {
     assert_eq!(rgb(g.text_outline.map(|o| o.0)), rgb(f.text_outline.map(|o| o.0)));
     assert_eq!(g.text_outline.map(|o| o.1), Some(1.5));
 }
+
+/// `_xludf.` marks a user-defined call even when a built-in has the same name; dropping it turns
+/// the call into the built-in, and `_xludf.EOMONTH()` into a call Excel repairs away (#726).
+#[test]
+fn user_defined_function_prefix_round_trips() {
+    let cases =
+        [("A1", "_xludf.EOMONTH()"), ("A2", "_xludf.CONCAT(\"a\",\"b\")"), ("A3", "_xludf.SUM(1,2)"), ("A4", "MYUDF(1)"), ("A5", "_xludf.MYUDF(1)")];
+    let rows: String = cases
+        .iter()
+        .enumerate()
+        .map(|(i, (a, f))| format!(r#"<row r="{}"><c r="{a}"><f>{}</f></c></row>"#, i + 1, f.replace('"', "&quot;")))
+        .collect();
+    let (wb, _) = read_xlsx(&minimal(&format!("<sheetData>{rows}</sheetData>"), &[], "", "")).unwrap();
+    let s = wb.sheet(0).unwrap();
+    for (a, f) in cases {
+        assert_eq!(s.cell(at(a)).unwrap().formula.as_ref().unwrap().text, f, "read {a}");
+    }
+    let bytes = crate::write_xlsx(&wb).unwrap();
+    let mut pkg = crate::package::Package::open(&bytes).unwrap();
+    let sheet = pkg.read_xml("xl/worksheets/sheet1.xml").unwrap().unwrap();
+    for (a, f) in cases {
+        let cell = sheet.child("sheetData").unwrap().kids("row").flat_map(|r| r.kids("c")).find(|c| c.attr("r") == Some(a)).unwrap();
+        assert_eq!(cell.child("f").unwrap().text, f, "written {a}");
+    }
+}