Function compress_sized 

pub fn compress_sized<R: Read>(
    source: R,
    level: i32,
    size: u64,
) -> Result<Vec<u8>>

Compress everything from source, whose length you already know.

The length is recorded in the frame header, which is what lets whoever decompresses it size their buffer up front - worth about a factor of two to them. encode_all() cannot do this, because it has no way to know how much is coming.

size must be exactly how many bytes source yields; zstd fails the frame if it turns out to be wrong.

For data already in a slice, use crate::bulk::compress(), which knows the length without being told. For a file, compress_file().

let data = b"the length of this is known up front";

let frame = zstd::compress_sized(&data[..], 3, data.len() as u64)?;

assert_eq!(zstd::decompressed_size(&frame), Some(data.len() as u64));