numfmt: fix rounding for small scaled values

src/uu/numfmt/src/format.rs

@@ -592,10 +592,15 @@ fn transform_to(
         }
     };
     Ok(match s {
-        None => localize(format!(
+        None if opts.to == Unit::None => localize(format!(
             "{:.precision$}",
             round_with_precision(i2, round_method, precision),
         )),
+        None if is_precision_specified => {
+            let i2 = round_with_precision(i2, round_method, 0);
+            localize(format!("{i2:.precision$}"))
+        }
+        None => localize(format!("{i2:.0}")),
         Some(s) if precision > 0 => localize(format!(
             "{i2:.precision$}{unit_separator}{}",
             DisplayableSuffix(s, opts.to),

tests/by-util/test_numfmt.rs

@@ -2,7 +2,8 @@
 //
 // For the full copyright and license information, please view the LICENSE
 // file that was distributed with this source code.
-// spell-checker:ignore (paths) gnutest ronna quetta
+
+// spell-checker:ignore (paths) gnutest ronna quetta unitless
 
 use uutests::new_ucmd;
 
@@ -633,6 +634,16 @@ fn test_round() {
     }
 }
 
+#[test]
+fn test_to_unitless_small_values_use_display_rounding() {
+    new_ucmd!()
+        .args(&[
+            "--to=si", "--", "0.4", "0.5", "0.6", "1.4", "3.14", "-0.4", "-0.5", "-0.6", "-1.4",
+        ])
+        .succeeds()
+        .stdout_only("0\n0\n1\n1\n3\n-0\n-0\n-1\n-1\n");
+}
+
 #[test]
 fn test_round_with_to_unit() {
     for (method, exp) in [
@@ -659,6 +670,24 @@ fn test_round_with_to_unit() {
     }
 }
 
+#[test]
+fn test_to_unit_with_unitless_small_value_uses_display_rounding() {
+    new_ucmd!()
+        .args(&["--to=iec", "--to-unit=689", "701"])
+        .succeeds()
+        .stdout_only("1\n");
+
+    new_ucmd!()
+        .args(&["--to=si", "--to-unit=689", "701"])
+        .succeeds()
+        .stdout_only("1\n");
+
+    new_ucmd!()
+        .args(&["--to=none", "--to-unit=689", "701"])
+        .succeeds()
+        .stdout_only("2\n");
+}
+
 #[test]
 fn test_suffix_is_added_if_not_supplied() {
     new_ucmd!()
@@ -1024,6 +1053,19 @@ fn test_format_with_precision_and_to_arg() {
     }
 }
 
+#[test]
+fn test_format_with_precision_and_unitless_to_arg() {
+    new_ucmd!()
+        .args(&["--to=si", "--format=%.1f", "3.14"])
+        .succeeds()
+        .stdout_is("4.0\n");
+
+    new_ucmd!()
+        .args(&["--to=si", "--format=%.1f", "--round=down", "3.14"])
+        .succeeds()
+        .stdout_is("3.0\n");
+}
+
 #[test]
 fn test_format_preserve_trailing_zeros_if_no_precision_is_specified() {
     let values = vec!["10.0", "0.0100"];