Update WriteBuf

Author: osa1

rts/motoko-rts/src/print.rs

@@ -58,6 +58,8 @@ macro_rules! format {
     })
 }
 
+/// A stack-allocated buffer that implements `core::fmt::Write`. `Write` methods will write to the
+/// buffer until it's filled and then ignore the rest, without failing.
 pub(crate) struct WriteBuf<'a> {
     buf: &'a mut [u8],
     offset: usize,
@@ -76,23 +78,18 @@ impl<'a> WriteBuf<'a> {
     }
 }
 
-// https://stackoverflow.com/questions/39488327/how-to-format-output-to-a-byte-array-with-no-std-and-no-allocator
 impl<'a> fmt::Write for WriteBuf<'a> {
     fn write_str(&mut self, s: &str) -> fmt::Result {
+        // Amount of space left in the write buffer
+        let buf_space_left = self.buf.len() - self.offset;
+        let buf = &mut self.buf[self.offset..];
+        // Copy the bytes to the buffer. Note that the buffer and the copied slice have to have the
+        // same length otherwise `copy_from_slice` panics.
         let bytes = s.as_bytes();
-        // Skip over already-copied data
-        let remainder = &mut self.buf[self.offset..];
-        // Check if there is space remaining (return error instead of panicking)
-        if remainder.len() < bytes.len() {
-            return Err(core::fmt::Error);
-        }
-        // Make the two slices the same length
-        let remainder = &mut remainder[..bytes.len()];
-        // Copy
-        remainder.copy_from_slice(bytes);
-        // Update offset to avoid overwriting
-        self.offset += bytes.len();
-
+        let copy_len = core::cmp::min(buf_space_left, bytes.len());
+        (&mut buf[..copy_len]).copy_from_slice(&bytes[..copy_len]);
+        // Update offset
+        self.offset += copy_len;
         Ok(())
     }
 }