Fix inconsistent rounding of 0.5 when formatted to 0 decimal places

Author: ajtribick

library/core/src/num/flt2dec/strategy/dragon.rs

@@ -366,7 +366,7 @@ pub fn format_exact<'a>(
     if order == Ordering::Greater
         || (order == Ordering::Equal
             // SAFETY: `buf[len-1]` is initialized.
-            && (len == 0 || unsafe { buf[len - 1].assume_init() } & 1 == 1))
+            && len > 0 && unsafe { buf[len - 1].assume_init() } & 1 == 1)
     {
         // if rounding up changes the length, the exponent should also change.
         // but we've been requested a fixed number of digits, so do not alter the buffer...

library/core/tests/fmt/float.rs

@@ -5,7 +5,7 @@ fn test_format_f64() {
     assert_eq!("10", format!("{:.0}", 9.9f64));
     assert_eq!("9.8", format!("{:.1}", 9.849f64));
     assert_eq!("9.9", format!("{:.1}", 9.851f64));
-    assert_eq!("1", format!("{:.0}", 0.5f64));
+    assert_eq!("0", format!("{:.0}", 0.5f64));
     assert_eq!("1.23456789e6", format!("{:e}", 1234567.89f64));
     assert_eq!("1.23456789e3", format!("{:e}", 1234.56789f64));
     assert_eq!("1.23456789E6", format!("{:E}", 1234567.89f64));
@@ -31,7 +31,7 @@ fn test_format_f32() {
     assert_eq!("10", format!("{:.0}", 9.9f32));
     assert_eq!("9.8", format!("{:.1}", 9.849f32));
     assert_eq!("9.9", format!("{:.1}", 9.851f32));
-    assert_eq!("1", format!("{:.0}", 0.5f32));
+    assert_eq!("0", format!("{:.0}", 0.5f32));
     assert_eq!("1.2345679e6", format!("{:e}", 1234567.89f32));
     assert_eq!("1.2345679e3", format!("{:e}", 1234.56789f32));
     assert_eq!("1.2345679E6", format!("{:E}", 1234567.89f32));

library/core/tests/num/flt2dec/mod.rs

@@ -138,7 +138,7 @@ where
 
     // check exact rounding for zero- and negative-width cases
     let start;
-    if expected[0] >= b'5' {
+    if expected[0] > b'5' {
         try_fixed!(f(&decoded) => &mut buf, expectedk, b"1", expectedk + 1;
                    "zero-width rounding-up mismatch for v={v}: \
                     actual {actual:?}, expected {expected:?}",
@@ -1007,7 +1007,7 @@ where
     assert_eq!(to_string(f, 999.5, Minus, 3), "999.500");
     assert_eq!(to_string(f, 999.5, Minus, 30), "999.500000000000000000000000000000");
 
-    assert_eq!(to_string(f, 0.5, Minus, 0), "1");
+    assert_eq!(to_string(f, 0.5, Minus, 0), "0");
     assert_eq!(to_string(f, 0.5, Minus, 1), "0.5");
     assert_eq!(to_string(f, 0.5, Minus, 2), "0.50");
     assert_eq!(to_string(f, 0.5, Minus, 3), "0.500");