Formatter::sign is &'static str

The contents were always UTF-8 anyway, and &str has an equivalent representation
to &[u8], so this should not affect performance while removing unsafety at
edges.

It may be worth exploring a further adjustment that stores a single byte
(instead of 16) as the contents are always "", "-", or "+".

Author: Mark-Simulacrum

Diff for: src/libcore/fmt/mod.rs

@@ -1356,11 +1356,11 @@ impl<'a> Formatter<'a> {
             let mut align = old_align;
             if self.sign_aware_zero_pad() {
                 // a sign always goes first
-                let sign = unsafe { str::from_utf8_unchecked(formatted.sign) };
+                let sign = formatted.sign;
                 self.buf.write_str(sign)?;
 
                 // remove the sign from the formatted parts
-                formatted.sign = b"";
+                formatted.sign = "";
                 width = width.saturating_sub(sign.len());
                 align = rt::v1::Alignment::Right;
                 self.fill = '0';
@@ -1392,7 +1392,7 @@ impl<'a> Formatter<'a> {
         }
 
         if !formatted.sign.is_empty() {
-            write_bytes(self.buf, formatted.sign)?;
+            self.buf.write_str(formatted.sign)?;
         }
         for part in formatted.parts {
             match *part {

Diff for: src/libcore/fmt/num.rs

@@ -369,11 +369,11 @@ macro_rules! impl_Exp {
                 flt2dec::Part::Copy(exp_slice)
             ];
             let sign = if !is_nonnegative {
-                &b"-"[..]
+                "-"
             } else if f.sign_plus() {
-                &b"+"[..]
+                "+"
             } else {
-                &b""[..]
+                ""
             };
             let formatted = flt2dec::Formatted{sign, parts};
             f.pad_formatted_parts(&formatted)

Diff for: src/libcore/num/flt2dec/mod.rs

@@ -237,7 +237,7 @@ impl<'a> Part<'a> {
 #[derive(Clone)]
 pub struct Formatted<'a> {
     /// A byte slice representing a sign, either `""`, `"-"` or `"+"`.
-    pub sign: &'static [u8],
+    pub sign: &'static str,
     /// Formatted parts to be rendered after a sign and optional zero padding.
     pub parts: &'a [Part<'a>],
 }
@@ -259,7 +259,7 @@ impl<'a> Formatted<'a> {
         if out.len() < self.sign.len() {
             return None;
         }
-        out[..self.sign.len()].copy_from_slice(self.sign);
+        out[..self.sign.len()].copy_from_slice(self.sign.as_bytes());
 
         let mut written = self.sign.len();
         for part in self.parts {
@@ -402,38 +402,38 @@ pub enum Sign {
 }
 
 /// Returns the static byte string corresponding to the sign to be formatted.
-/// It can be either `b""`, `b"+"` or `b"-"`.
-fn determine_sign(sign: Sign, decoded: &FullDecoded, negative: bool) -> &'static [u8] {
+/// It can be either `""`, `"+"` or `"-"`.
+fn determine_sign(sign: Sign, decoded: &FullDecoded, negative: bool) -> &'static str {
     match (*decoded, sign) {
-        (FullDecoded::Nan, _) => b"",
-        (FullDecoded::Zero, Sign::Minus) => b"",
+        (FullDecoded::Nan, _) => "",
+        (FullDecoded::Zero, Sign::Minus) => "",
         (FullDecoded::Zero, Sign::MinusRaw) => {
             if negative {
-                b"-"
+                "-"
             } else {
-                b""
+                ""
             }
         }
-        (FullDecoded::Zero, Sign::MinusPlus) => b"+",
+        (FullDecoded::Zero, Sign::MinusPlus) => "+",
         (FullDecoded::Zero, Sign::MinusPlusRaw) => {
             if negative {
-                b"-"
+                "-"
             } else {
-                b"+"
+                "+"
             }
         }
         (_, Sign::Minus) | (_, Sign::MinusRaw) => {
             if negative {
-                b"-"
+                "-"
             } else {
-                b""
+                ""
             }
         }
         (_, Sign::MinusPlus) | (_, Sign::MinusPlusRaw) => {
             if negative {
-                b"-"
+                "-"
             } else {
-                b"+"
+                "+"
             }
         }
     }