Revert Buf::as_str safety change from PR 2319

Author: dtolnay

serde/src/de/format.rs

@@ -7,13 +7,13 @@ pub(super) struct Buf<'a> {
 }
 
 impl<'a> Buf<'a> {
-    pub(super) fn new(bytes: &'a mut [u8]) -> Self {
+    pub fn new(bytes: &'a mut [u8]) -> Self {
         Buf { bytes, offset: 0 }
     }
 
-    pub(super) unsafe fn as_str(&self) -> &str {
+    pub fn as_str(&self) -> &str {
         let slice = &self.bytes[..self.offset];
-        str::from_utf8_unchecked(slice)
+        unsafe { str::from_utf8_unchecked(slice) }
     }
 }
 

serde/src/de/mod.rs

@@ -1376,11 +1376,7 @@ pub trait Visitor<'de>: Sized {
             let mut buf = [0u8; 58];
             let mut writer = format::Buf::new(&mut buf);
             fmt::Write::write_fmt(&mut writer, format_args!("integer `{}` as i128", v)).unwrap();
-
-            // Safety: This is safe because we only wrote UTF-8 into the buffer.
-            let s = unsafe { writer.as_str() };
-
-            Err(Error::invalid_type(Unexpected::Other(s), &self))
+            Err(Error::invalid_type(Unexpected::Other(writer.as_str()), &self))
         }
     }
 
@@ -1442,11 +1438,7 @@ pub trait Visitor<'de>: Sized {
             let mut buf = [0u8; 57];
             let mut writer = format::Buf::new(&mut buf);
             fmt::Write::write_fmt(&mut writer, format_args!("integer `{}` as u128", v)).unwrap();
-
-            // Safety: This is safe because we only wrote UTF-8 into the buffer.
-            let s = unsafe { writer.as_str() };
-
-            Err(Error::invalid_type(Unexpected::Other(s), &self))
+            Err(Error::invalid_type(Unexpected::Other(writer.as_str()), &self))
         }
     }