i8 and u8::to_string() specialisation (far less asm).

library/alloc/src/string.rs

@@ -2224,6 +2224,60 @@ impl ToString for char {
     }
 }
 
+#[stable(feature = "u8_to_string_specialization", since = "1.999.0")]
+impl ToString for u8 {
+    #[inline]
+    fn to_string(&self) -> String {
+        let mut result = String::with_capacity(3);
+        let mut n = *self;
+        if n >= 100 {
+            result.push((b'0' + n / 100) as char);
+            n %= 100;
+        }
+        if !result.is_empty() || n >= 10 {
+            result.push((b'0' + n / 10) as char);
+            n %= 10;
+        };
+        result.push((b'0' + n) as char);
+        result
+    }
+}
+
+#[stable(feature = "i8_to_string_specialization", since = "1.999.0")]
+impl ToString for i8 {
+    #[inline]
+    fn to_string(&self) -> String {
+        let mut vec = vec![0; 4];
+        let n = *self;
+        let mut free = 0;
+        let mut n: u8 = if n.is_negative() {
+            vec[free] = b'-';
+            free += 1;
+            i8::unsigned_abs(n)
+        } else {
+            n as u8
+        };
+        if n >= 10 {
+            if n >= 100 {
+                n -= 100;
+                vec[free] = b'1';
+                free += 1;
+            }
+            debug_assert!(n < 100);
+            vec[free] = b'0' + n / 10;
+            free += 1;
+            n %= 10;
+        }
+        debug_assert!(n < 10);
+        vec[free] = b'0' + n;
+        free += 1;
+        vec.truncate(free);
+
+        // SAFETY: Vec only contains ascii so valid utf8
+        unsafe { String::from_utf8_unchecked(vec) }
+    }
+}
+
 #[stable(feature = "str_to_string_specialization", since = "1.9.0")]
 impl ToString for str {
     #[inline]