Function expect_end 

pub fn expect_end(
    data: &[u8],
    end: usize,
    at: usize,
    what: &str,
) -> Result<(), Error>

Require a value that stopped at end to have used all of data.

Octets left over are DecodingKind::Trailing, arguments past the value’s last member, unless they open with a closing tag, which closes nothing, or with a tag header that doesn’t decode: those are DecodingKind::InvalidTag.

at is the offset the error reports: end itself when data is the whole input, or the frame’s own offset in the larger input when data is a frame body sliced out of it (offsets into the slice would mislead).