Revert Buf::as_str safety change from PR 2319

serde/src/de/format.rs

@@ -1,7 +1,7 @@
 use lib::fmt::{self, Write};
 use lib::str;
 
-pub struct Buf<'a> {
+pub(super) struct Buf<'a> {
     bytes: &'a mut [u8],
     offset: usize,
 }

serde/src/ser/impls.rs

@@ -736,8 +736,9 @@ impl Serialize for net::Ipv4Addr {
                 // Skip over delimiters that we initialized buf with
                 written += format_u8(*oct, &mut buf[written + 1..]) + 1;
             }
-            // We've only written ASCII bytes to the buffer, so it is valid UTF-8
-            serializer.serialize_str(unsafe { str::from_utf8_unchecked(&buf[..written]) })
+            // Safety: We've only written ASCII bytes to the buffer, so it is valid UTF-8
+            let buf = unsafe { str::from_utf8_unchecked(&buf[..written]) };
+            serializer.serialize_str(buf)
         } else {
             self.octets().serialize(serializer)
         }