feat: extract serde_core out of serde

serde_core/src/format.rs

@@ -0,0 +1,30 @@
+use crate::lib::fmt::{self, Write};
+use crate::lib::str;
+
+pub(super) struct Buf<'a> {
+    bytes: &'a mut [u8],
+    offset: usize,
+}
+
+impl<'a> Buf<'a> {
+    pub fn new(bytes: &'a mut [u8]) -> Self {
+        Buf { bytes, offset: 0 }
+    }
+
+    pub fn as_str(&self) -> &str {
+        let slice = &self.bytes[..self.offset];
+        unsafe { str::from_utf8_unchecked(slice) }
+    }
+}
+
+impl<'a> Write for Buf<'a> {
+    fn write_str(&mut self, s: &str) -> fmt::Result {
+        if self.offset + s.len() > self.bytes.len() {
+            Err(fmt::Error)
+        } else {
+            self.bytes[self.offset..self.offset + s.len()].copy_from_slice(s.as_bytes());
+            self.offset += s.len();
+            Ok(())
+        }
+    }
+}