Function decompressed_size 

pub fn decompressed_size(data: &[u8]) -> Option<u64>

Reads the decompressed size out of the start of a frame, if it says.

Returns None if data does not begin with a zstd frame header, or if the frame does not record its content size - which is normal for anything produced by a streaming compressor that was not told the size in advance.

This is the size to reserve when decompressing into your own buffer:

let frame = zstd::encode_all(&b"some data"[..], 3)?;

let mut output = match zstd::decompressed_size(&frame) {
    Some(size) => Vec::with_capacity(size as usize),
    None => Vec::new(),
};
zstd::stream::copy_decode(&frame[..], &mut output)?;