Auto merge of #136264 - GuillaumeGomez:optimize-integers-to-string, r=Amanieu

Optimize `ToString` implementation for integers

Part of https://github.com/rust-lang/rust/issues/135543.

Follow-up of https://github.com/rust-lang/rust/pull/133247 and https://github.com/rust-lang/rust/pull/128204.

The benchmark results are:

| name| 1.87.0-nightly (3ea711f17 2025-03-09) | With this PR | diff |
|-|-|-|-|
| bench_i16 | 32.06 ns/iter (+/- 0.12) | 17.62 ns/iter (+/- 0.03) | -45% |
| bench_i32 | 31.61 ns/iter (+/- 0.04) | 15.10 ns/iter (+/- 0.06) | -52% |
| bench_i64 | 31.71 ns/iter (+/- 0.07) | 15.02 ns/iter (+/- 0.20) | -52% |
| bench_i8 | 13.21 ns/iter (+/- 0.14) | 14.93 ns/iter (+/- 0.16) | +13% |
| bench_u16 | 31.20 ns/iter (+/- 0.06) | 16.14 ns/iter (+/- 0.11) | -48% |
| bench_u32 | 33.27 ns/iter (+/- 0.05) | 16.18 ns/iter (+/- 0.10) | -51% |
| bench_u64 | 31.44 ns/iter (+/- 0.06) | 16.62 ns/iter (+/- 0.21) | -47% |
| bench_u8 | 10.57 ns/iter (+/- 0.30) | 13.00 ns/iter (+/- 0.43) | +22% |

More information about it in [the original comment](https://github.com/rust-lang/rust/pull/136264#discussion_r1987542954).

r? `@workingjubilee`

1 files changed, 19 insertions, 9 deletions

diff --git a/library/core/src/fmt/num.rs b/library/core/src/fmt/num.rs
index 4467b37bd45..ba30518d70b 100644
--- a/library/core/src/fmt/num.rs
+++ b/library/core/src/fmt/num.rs
@@ -208,7 +208,11 @@ macro_rules! impl_Display {
             fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                 #[cfg(not(feature = "optimize_for_size"))]
                 {
-                    self._fmt(true, f)
+                    const MAX_DEC_N: usize = $unsigned::MAX.ilog(10) as usize + 1;
+                    // Buffer decimals for $unsigned with right alignment.
+                    let mut buf = [MaybeUninit::<u8>::uninit(); MAX_DEC_N];
+
+                    f.pad_integral(true, "", self._fmt(&mut buf))
                 }
                 #[cfg(feature = "optimize_for_size")]
                 {
@@ -222,7 +226,11 @@ macro_rules! impl_Display {
             fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                 #[cfg(not(feature = "optimize_for_size"))]
                 {
-                    return self.unsigned_abs()._fmt(*self >= 0, f);
+                    const MAX_DEC_N: usize = $unsigned::MAX.ilog(10) as usize + 1;
+                    // Buffer decimals for $unsigned with right alignment.
+                    let mut buf = [MaybeUninit::<u8>::uninit(); MAX_DEC_N];
+
+                    f.pad_integral(*self >= 0, "", self.unsigned_abs()._fmt(&mut buf))
                 }
                 #[cfg(feature = "optimize_for_size")]
                 {
@@ -233,10 +241,13 @@ macro_rules! impl_Display {
 
         #[cfg(not(feature = "optimize_for_size"))]
         impl $unsigned {
-            fn _fmt(self, is_nonnegative: bool, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-                const MAX_DEC_N: usize = $unsigned::MAX.ilog(10) as usize + 1;
-                // Buffer decimals for $unsigned with right alignment.
-                let mut buf = [MaybeUninit::<u8>::uninit(); MAX_DEC_N];
+            #[doc(hidden)]
+            #[unstable(
+                feature = "fmt_internals",
+                reason = "specialized method meant to only be used by `SpecToString` implementation",
+                issue = "none"
+            )]
+            pub fn _fmt<'a>(self, buf: &'a mut [MaybeUninit::<u8>]) -> &'a str {
                 // Count the number of bytes in buf that are not initialized.
                 let mut offset = buf.len();
                 // Consume the least-significant decimals from a working copy.
@@ -301,13 +312,12 @@ macro_rules! impl_Display {
                 // SAFETY: All buf content since offset is set.
                 let written = unsafe { buf.get_unchecked(offset..) };
                 // SAFETY: Writes use ASCII from the lookup table exclusively.
-                let as_str = unsafe {
+                unsafe {
                     str::from_utf8_unchecked(slice::from_raw_parts(
                           MaybeUninit::slice_as_ptr(written),
                           written.len(),
                     ))
-                };
-                f.pad_integral(is_nonnegative, "", as_str)
+                }
             }
         })*