3 files changed, 63 insertions, 30 deletions
diff --git a/library/std/src/lib.rs b/library/std/src/lib.rs
index 475a1d9fd99..a8d6645794a 100644
--- a/library/std/src/lib.rs
+++ b/library/std/src/lib.rs
@@ -294,6 +294,8 @@
 #![feature(std_internals)]
 #![feature(str_internals)]
 #![feature(strict_provenance)]
+#![feature(maybe_uninit_uninit_array)]
+#![feature(const_maybe_uninit_uninit_array)]
 //
 // Library features (alloc):
 #![feature(alloc_layout_extra)]
diff --git a/library/std/src/net/ip.rs b/library/std/src/net/ip.rs
index 41ca9ba8425..189754a161e 100644
--- a/library/std/src/net/ip.rs
+++ b/library/std/src/net/ip.rs
@@ -3,12 +3,14 @@
 mod tests;
 
 use crate::cmp::Ordering;
-use crate::fmt::{self, Write as FmtWrite};
-use crate::io::Write as IoWrite;
+use crate::fmt::{self, Write};
 use crate::mem::transmute;
 use crate::sys::net::netc as c;
 use crate::sys_common::{FromInner, IntoInner};
 
+mod display_buffer;
+use display_buffer::IpDisplayBuffer;
+
 /// An IP address, either IPv4 or IPv6.
 ///
 /// This enum can contain either an [`Ipv4Addr`] or an [`Ipv6Addr`], see their
@@ -991,21 +993,19 @@ impl From<Ipv6Addr> for IpAddr {
 impl fmt::Display for Ipv4Addr {
     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
         let octets = self.octets();
-        // Fast Path: if there's no alignment stuff, write directly to the buffer
+
+        // If there are no alignment requirements, write the IP address directly to `f`.
+        // Otherwise, write it to a local buffer and then use `f.pad`.
         if fmt.precision().is_none() && fmt.width().is_none() {
             write!(fmt, "{}.{}.{}.{}", octets[0], octets[1], octets[2], octets[3])
         } else {
-            const IPV4_BUF_LEN: usize = 15; // Long enough for the longest possible IPv4 address
-            let mut buf = [0u8; IPV4_BUF_LEN];
-            let mut buf_slice = &mut buf[..];
+            const LONGEST_IPV4_ADDR: &str = "255.255.255.255";
 
-            // Note: The call to write should never fail, hence the unwrap
-            write!(buf_slice, "{}.{}.{}.{}", octets[0], octets[1], octets[2], octets[3]).unwrap();
-            let len = IPV4_BUF_LEN - buf_slice.len();
+            let mut buf = IpDisplayBuffer::<{ LONGEST_IPV4_ADDR.len() }>::new();
+            // Buffer is long enough for the longest possible IPv4 address, so this should never fail.
+            write!(buf, "{}.{}.{}.{}", octets[0], octets[1], octets[2], octets[3]).unwrap();
 
-            // This unsafe is OK because we know what is being written to the buffer
-            let buf = unsafe { crate::str::from_utf8_unchecked(&buf[..len]) };
-            fmt.pad(buf)
+            fmt.pad(buf.as_str())
         }
     }
 }
@@ -1708,8 +1708,8 @@ impl Ipv6Addr {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl fmt::Display for Ipv6Addr {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        // If there are no alignment requirements, write out the IP address to
-        // f. Otherwise, write it to a local buffer, then use f.pad.
+        // If there are no alignment requirements, write the IP address directly to `f`.
+        // Otherwise, write it to a local buffer and then use `f.pad`.
         if f.precision().is_none() && f.width().is_none() {
             let segments = self.segments();
 
@@ -1780,22 +1780,13 @@ impl fmt::Display for Ipv6Addr {
                 }
             }
         } else {
-            // Slow path: write the address to a local buffer, then use f.pad.
-            // Defined recursively by using the fast path to write to the
-            // buffer.
-
-            // This is the largest possible size of an IPv6 address
-            const IPV6_BUF_LEN: usize = (4 * 8) + 7;
-            let mut buf = [0u8; IPV6_BUF_LEN];
-            let mut buf_slice = &mut buf[..];
-
-            // Note: This call to write should never fail, so unwrap is okay.
-            write!(buf_slice, "{}", self).unwrap();
-            let len = IPV6_BUF_LEN - buf_slice.len();
-
-            // This is safe because we know exactly what can be in this buffer
-            let buf = unsafe { crate::str::from_utf8_unchecked(&buf[..len]) };
-            f.pad(buf)
+            const LONGEST_IPV6_ADDR: &str = "ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff";
+
+            let mut buf = IpDisplayBuffer::<{ LONGEST_IPV6_ADDR.len() }>::new();
+            // Buffer is long enough for the longest possible IPv6 address, so this should never fail.
+            write!(buf, "{}", self).unwrap();
+
+            f.pad(buf.as_str())
         }
     }
 }
diff --git a/library/std/src/net/ip/display_buffer.rs b/library/std/src/net/ip/display_buffer.rs
new file mode 100644
index 00000000000..bd852d5da8e
--- /dev/null
+++ b/library/std/src/net/ip/display_buffer.rs
@@ -0,0 +1,40 @@
+use crate::fmt;
+use crate::mem::MaybeUninit;
+use crate::str;
+
+/// Used for slow path in `Display` implementations when alignment is required.
+pub struct IpDisplayBuffer<const SIZE: usize> {
+    buf: [MaybeUninit<u8>; SIZE],
+    len: usize,
+}
+
+impl<const SIZE: usize> IpDisplayBuffer<SIZE> {
+    #[inline]
+    pub const fn new() -> Self {
+        Self { buf: MaybeUninit::uninit_array(), len: 0 }
+    }
+
+    #[inline]
+    pub fn as_str(&self) -> &str {
+        // SAFETY: `buf` is only written to by the `fmt::Write::write_str` implementation
+        // which writes a valid UTF-8 string to `buf` and correctly sets `len`.
+        unsafe {
+            let s = MaybeUninit::slice_assume_init_ref(&self.buf[..self.len]);
+            str::from_utf8_unchecked(s)
+        }
+    }
+}
+
+impl<const SIZE: usize> fmt::Write for IpDisplayBuffer<SIZE> {
+    fn write_str(&mut self, s: &str) -> fmt::Result {
+        let bytes = s.as_bytes();
+
+        if let Some(buf) = self.buf.get_mut(self.len..(self.len + bytes.len())) {
+            MaybeUninit::write_slice(buf, bytes);
+            self.len += bytes.len();
+            Ok(())
+        } else {
+            Err(fmt::Error)
+        }
+    }
+}