Function encode_varint 

pub fn encode_varint(value: u64, buf: &mut [u8]) -> usize

Encode a u64 value as an unsigned LEB128 varint into the provided buffer.

Returns

The number of bytes written (1–10).

Panics

Panics if buf is shorter than the required encoding length. A 10-byte buffer is always sufficient for any u64.

Wire format examples

Value	Encoded bytes	Length
0	[0x00]	1
1	[0x01]	1
127	[0x7F]	1
128	[0x80, 0x01]	2
300	[0xAC, 0x02]	2
16383	[0xFF, 0x7F]	2
16384	[0x80, 0x80, 0x01]	3