Rollup merge of #142098 - GuillaumeGomez:int_format_into, r=Amanieu

Implement `int_format_into` feature

I took over #138338 with `@madhav-madhusoodanan's` approval.

Since #136264, a lot of changes happened so I made use of them to reduce the number of changes.

ACP approval: rust-lang/libs-team#546 (comment)

## Associated Issue
- #138215

r? `@hanna-kruppe`

Author: matthiaskrgr

library/alloc/src/string.rs

@@ -2875,7 +2875,8 @@ macro_rules! impl_to_string {
                     out = String::with_capacity(SIZE);
                 }
 
-                out.push_str(self.unsigned_abs()._fmt(&mut buf));
+                // SAFETY: `buf` is always big enough to contain all the digits.
+                unsafe { out.push_str(self.unsigned_abs()._fmt(&mut buf)); }
                 out
             }
         }
@@ -2887,7 +2888,8 @@ macro_rules! impl_to_string {
                 const SIZE: usize = $unsigned::MAX.ilog10() as usize + 1;
                 let mut buf = [core::mem::MaybeUninit::<u8>::uninit(); SIZE];
 
-                self._fmt(&mut buf).to_string()
+                // SAFETY: `buf` is always big enough to contain all the digits.
+                unsafe { self._fmt(&mut buf).to_string() }
             }
         }
         )*

library/alloctests/tests/lib.rs

@@ -9,6 +9,7 @@
 #![feature(downcast_unchecked)]
 #![feature(exact_size_is_empty)]
 #![feature(hashmap_internals)]
+#![feature(int_format_into)]
 #![feature(linked_list_cursors)]
 #![feature(map_try_insert)]
 #![feature(pattern)]

library/alloctests/tests/num.rs

@@ -1,27 +1,33 @@
-use std::fmt::{Debug, Display};
+use core::fmt::NumBuffer;
 use std::str::FromStr;
 
-fn assert_nb<Int: ToString + FromStr + Debug + Display + Eq>(value: Int) {
-    let s = value.to_string();
-    let s2 = format!("s: {}.", value);
+macro_rules! assert_nb {
+    ($int:ident, $value:expr) => {
+        let value: $int = $value;
+        let s = value.to_string();
+        let s2 = format!("s: {}.", value);
 
-    assert_eq!(format!("s: {s}."), s2);
-    let Ok(ret) = Int::from_str(&s) else {
-        panic!("failed to convert into to string");
+        assert_eq!(format!("s: {s}."), s2);
+        let Ok(ret) = $int::from_str(&s) else {
+            panic!("failed to convert into to string");
+        };
+        assert_eq!(ret, value);
+
+        let mut buffer = NumBuffer::<$int>::new();
+        assert_eq!(value.format_into(&mut buffer), s.as_str());
     };
-    assert_eq!(ret, value);
 }
 
 macro_rules! uint_to_s {
     ($($fn_name:ident, $int:ident,)+) => {
         $(
             #[test]
             fn $fn_name() {
-                assert_nb::<$int>($int::MIN);
-                assert_nb::<$int>($int::MAX);
-                assert_nb::<$int>(1);
-                assert_nb::<$int>($int::MIN / 2);
-                assert_nb::<$int>($int::MAX / 2);
+                assert_nb!($int, $int::MIN);
+                assert_nb!($int, $int::MAX);
+                assert_nb!($int, 1);
+                assert_nb!($int, $int::MIN / 2);
+                assert_nb!($int, $int::MAX / 2);
             }
         )+
     }
@@ -31,13 +37,13 @@ macro_rules! int_to_s {
         $(
             #[test]
             fn $fn_name() {
-                assert_nb::<$int>($int::MIN);
-                assert_nb::<$int>($int::MAX);
-                assert_nb::<$int>(1);
-                assert_nb::<$int>(0);
-                assert_nb::<$int>(-1);
-                assert_nb::<$int>($int::MIN / 2);
-                assert_nb::<$int>($int::MAX / 2);
+                assert_nb!($int, $int::MIN);
+                assert_nb!($int, $int::MAX);
+                assert_nb!($int, 1);
+                assert_nb!($int, 0);
+                assert_nb!($int, -1);
+                assert_nb!($int, $int::MIN / 2);
+                assert_nb!($int, $int::MAX / 2);
             }
         )+
     }

library/core/src/fmt/mod.rs

@@ -15,6 +15,7 @@ mod float;
 #[cfg(no_fp_fmt_parse)]
 mod nofloat;
 mod num;
+mod num_buffer;
 mod rt;
 
 #[stable(feature = "fmt_flags_align", since = "1.28.0")]
@@ -33,6 +34,9 @@ pub enum Alignment {
     Center,
 }
 
+#[unstable(feature = "int_format_into", issue = "138215")]
+pub use num_buffer::{NumBuffer, NumBufferTrait};
+
 #[stable(feature = "debug_builders", since = "1.2.0")]
 pub use self::builders::{DebugList, DebugMap, DebugSet, DebugStruct, DebugTuple};
 #[unstable(feature = "debug_closure_helpers", issue = "117729")]