Preserve signed zero on roundtrip

This commit removes the previous mechanism of differentiating
between "Debug" and "Display" formattings for the sign of -0 so as
to comply with the IEEE 754 standard's requirements on external
character sequences preserving various attributes of a floating
point representation.

In addition, numerous tests are fixed.

Author: workingjubilee

library/alloc/tests/fmt.rs

@@ -154,8 +154,7 @@ fn test_format_macro_interface() {
     t!(format!("{:+10.3e}", -1.2345e6f64), "  -1.234e6");
 
     // Float edge cases
-    t!(format!("{}", -0.0), "0");
-    t!(format!("{:?}", -0.0), "-0.0");
+    t!(format!("{}", -0.0), "-0");
     t!(format!("{:?}", 0.0), "0.0");
 
     // sign aware zero padding

library/core/src/fmt/float.rs

@@ -54,21 +54,14 @@ where
 }
 
 // Common code of floating point Debug and Display.
-fn float_to_decimal_common<T>(
-    fmt: &mut Formatter<'_>,
-    num: &T,
-    negative_zero: bool,
-    min_precision: usize,
-) -> Result
+fn float_to_decimal_common<T>(fmt: &mut Formatter<'_>, num: &T, min_precision: usize) -> Result
 where
     T: flt2dec::DecodableFloat,
 {
     let force_sign = fmt.sign_plus();
-    let sign = match (force_sign, negative_zero) {
-        (false, false) => flt2dec::Sign::Minus,
-        (false, true) => flt2dec::Sign::MinusRaw,
-        (true, false) => flt2dec::Sign::MinusPlus,
-        (true, true) => flt2dec::Sign::MinusPlusRaw,
+    let sign = match force_sign {
+        false => flt2dec::Sign::Minus,
+        true => flt2dec::Sign::MinusPlus,
     };
 
     if let Some(precision) = fmt.precision {
@@ -156,14 +149,14 @@ macro_rules! floating {
         #[stable(feature = "rust1", since = "1.0.0")]
         impl Debug for $ty {
             fn fmt(&self, fmt: &mut Formatter<'_>) -> Result {
-                float_to_decimal_common(fmt, self, true, 1)
+                float_to_decimal_common(fmt, self, 1)
             }
         }
 
         #[stable(feature = "rust1", since = "1.0.0")]
         impl Display for $ty {
             fn fmt(&self, fmt: &mut Formatter<'_>) -> Result {
-                float_to_decimal_common(fmt, self, false, 0)
+                float_to_decimal_common(fmt, self, 0)
             }
         }
 

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

@@ -399,45 +399,25 @@ fn digits_to_exp_str<'a>(
 /// Sign formatting options.
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 pub enum Sign {
-    /// Prints `-` only for the negative non-zero values.
-    Minus, // -inf -1  0  0  1  inf nan
-    /// Prints `-` only for any negative values (including the negative zero).
-    MinusRaw, // -inf -1 -0  0  1  inf nan
-    /// Prints `-` for the negative non-zero values, or `+` otherwise.
-    MinusPlus, // -inf -1 +0 +0 +1 +inf nan
-    /// Prints `-` for any negative values (including the negative zero), or `+` otherwise.
-    MinusPlusRaw, // -inf -1 -0 +0 +1 +inf nan
+    /// Prints `-` for any negative value.
+    Minus, // -inf -1 -0  0  1  inf nan
+    /// Prints `-` for any negative value, or `+` otherwise.
+    MinusPlus, // -inf -1 -0 +0 +1 +inf nan
 }
 
 /// Returns the static byte string corresponding to the sign to be formatted.
 /// It can be either `""`, `"+"` or `"-"`.
 fn determine_sign(sign: Sign, decoded: &FullDecoded, negative: bool) -> &'static str {
     match (*decoded, sign) {
         (FullDecoded::Nan, _) => "",
-        (FullDecoded::Zero, Sign::Minus) => "",
-        (FullDecoded::Zero, Sign::MinusRaw) => {
+        (_, Sign::Minus) => {
             if negative {
                 "-"
             } else {
                 ""
             }
         }
-        (FullDecoded::Zero, Sign::MinusPlus) => "+",
-        (FullDecoded::Zero, Sign::MinusPlusRaw) => {
-            if negative {
-                "-"
-            } else {
-                "+"
-            }
-        }
-        (_, Sign::Minus | Sign::MinusRaw) => {
-            if negative {
-                "-"
-            } else {
-                ""
-            }
-        }
-        (_, Sign::MinusPlus | Sign::MinusPlusRaw) => {
+        (_, Sign::MinusPlus) => {
             if negative {
                 "-"
             } else {