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, 48 insertions, 0 deletions

diff --git a/library/alloc/src/string.rs b/library/alloc/src/string.rs
index 4e42a5da7ea..37614a7ca45 100644
--- a/library/alloc/src/string.rs
+++ b/library/alloc/src/string.rs
@@ -2826,7 +2826,54 @@ impl SpecToString for bool {
     }
 }
 
+macro_rules! impl_to_string {
+    ($($signed:ident, $unsigned:ident,)*) => {
+        $(
+        #[cfg(not(no_global_oom_handling))]
+        #[cfg(not(feature = "optimize_for_size"))]
+        impl SpecToString for $signed {
+            #[inline]
+            fn spec_to_string(&self) -> String {
+                const SIZE: usize = $signed::MAX.ilog(10) as usize + 1;
+                let mut buf = [core::mem::MaybeUninit::<u8>::uninit(); SIZE];
+                // Only difference between signed and unsigned are these 8 lines.
+                let mut out;
+                if *self < 0 {
+                    out = String::with_capacity(SIZE + 1);
+                    out.push('-');
+                } else {
+                    out = String::with_capacity(SIZE);
+                }
+
+                out.push_str(self.unsigned_abs()._fmt(&mut buf));
+                out
+            }
+        }
+        #[cfg(not(no_global_oom_handling))]
+        #[cfg(not(feature = "optimize_for_size"))]
+        impl SpecToString for $unsigned {
+            #[inline]
+            fn spec_to_string(&self) -> String {
+                const SIZE: usize = $unsigned::MAX.ilog(10) as usize + 1;
+                let mut buf = [core::mem::MaybeUninit::<u8>::uninit(); SIZE];
+
+                self._fmt(&mut buf).to_string()
+            }
+        }
+        )*
+    }
+}
+
+impl_to_string! {
+    i8, u8,
+    i16, u16,
+    i32, u32,
+    i64, u64,
+    isize, usize,
+}
+
 #[cfg(not(no_global_oom_handling))]
+#[cfg(feature = "optimize_for_size")]
 impl SpecToString for u8 {
     #[inline]
     fn spec_to_string(&self) -> String {
@@ -2846,6 +2893,7 @@ impl SpecToString for u8 {
 }
 
 #[cfg(not(no_global_oom_handling))]
+#[cfg(feature = "optimize_for_size")]
 impl SpecToString for i8 {
     #[inline]
     fn spec_to_string(&self) -> String {