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

Implement `int_format_into` feature

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

Since https://github.com/rust-lang/rust/pull/136264, a lot of changes happened so I made use of them to reduce the number of changes.

ACP approval: https://github.com/rust-lang/libs-team/issues/546#issuecomment-2707244569

## Associated Issue
- https://github.com/rust-lang/rust/issues/138215

r? `@hanna-kruppe`

1 files changed, 4 insertions, 2 deletions

diff --git a/library/alloc/src/string.rs b/library/alloc/src/string.rs
index 5f69f699867..f7a6a646b63 100644
--- a/library/alloc/src/string.rs
+++ b/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() }
             }
         }
         )*