improve codegen of fmt_num to delete unreachable panic

library/core/benches/fmt.rs

@@ -148,3 +148,17 @@ fn write_u64_min(bh: &mut Bencher) {
         test::black_box(format!("{}", 0u64));
     });
 }
+
+#[bench]
+fn write_u8_max(bh: &mut Bencher) {
+    bh.iter(|| {
+        test::black_box(format!("{}", u8::MAX));
+    });
+}
+
+#[bench]
+fn write_u8_min(bh: &mut Bencher) {
+    bh.iter(|| {
+        test::black_box(format!("{}", 0u8));
+    });
+}

library/core/src/fmt/num.rs

@@ -79,23 +79,23 @@ unsafe trait GenericRadix: Sized {
         if is_nonnegative {
             // Accumulate each digit of the number from the least significant
             // to the most significant figure.
-            for byte in buf.iter_mut().rev() {
+            loop {
                 let n = x % base; // Get the current place value.
                 x = x / base; // Deaccumulate the number.
-                byte.write(Self::digit(n.to_u8())); // Store the digit in the buffer.
                 curr -= 1;
+                buf[curr].write(Self::digit(n.to_u8())); // Store the digit in the buffer.
                 if x == zero {
                     // No more digits left to accumulate.
                     break;
                 };
             }
         } else {
             // Do the same as above, but accounting for two's complement.
-            for byte in buf.iter_mut().rev() {
+            loop {
                 let n = zero - (x % base); // Get the current place value.
                 x = x / base; // Deaccumulate the number.
-                byte.write(Self::digit(n.to_u8())); // Store the digit in the buffer.
                 curr -= 1;
+                buf[curr].write(Self::digit(n.to_u8())); // Store the digit in the buffer.
                 if x == zero {
                     // No more digits left to accumulate.
                     break;