fix(rust): Ensure ASCII ellipsis fits in column width (#21275)

crates/polars-core/src/fmt.rs

@@ -747,8 +747,12 @@ impl Display for DataFrame {
                     str_truncate + ellipsis_len + padding,
                     std::cmp::max(name_lengths[idx], *elem_len),
                 );
-                if mx <= min_col_width {
+                if (mx <= min_col_width) && !(max_n_rows > 0 && height > max_n_rows) {
+                    // col width is less than min width + table is not truncated
                     constraints.push(col_width_exact(mx));
+                } else if mx <= min_col_width {
+                    // col width is less than min width + table is truncated (w/ ellipsis)
+                    constraints.push(col_width_bounds(mx, min_col_width));
                 } else {
                     constraints.push(col_width_bounds(min_col_width, mx));
                 }

py-polars/tests/unit/test_format.py

@@ -473,3 +473,37 @@ def test_fmt_decimal_max_scale() -> None:
 def test_simple_project_format(lf: pl.LazyFrame, expected: str) -> None:
     result = lf.explain()
     assert expected in result
+
+
+@pytest.mark.parametrize(
+    ("df", "expected"),
+    [
+        pytest.param(
+            pl.DataFrame({"A": range(4)}),
+            """shape: (4, 1)
++-----+
+| A   |
++=====+
+| 0   |
+| 1   |
+| ... |
+| 3   |
++-----+""",
+            id="Ellipsis correctly aligned",
+        ),
+        pytest.param(
+            pl.DataFrame({"A": range(2)}),
+            """shape: (2, 1)
++---+
+| A |
++===+
+| 0 |
+| 1 |
++---+""",
+            id="No ellipsis needed",
+        ),
+    ],
+)
+def test_format_ascii_table_truncation(df: pl.DataFrame, expected: str) -> None:
+    with pl.Config(tbl_rows=3, tbl_hide_column_data_types=True, ascii_tables=True):
+        assert str(df) == expected