BufReader .consume() confuses the Reader seek point when the buffer is empty

[jorisgio]

The behavior of .consume when the buffer is empty is weird :

let mut f = BufReader::new(Cursor::new(vec![65, 255, 42]));
f.consume(1);
let x = &mut [0; 1];
println!("{}", f.read(x).unwrap()); // prints 0

while :

let mut f = BufReader::new(Cursor::new(vec![65, 255, 42]));
let x = &mut [0; 1];
f.read(x).unwrap();
f.consume(1);
println!("{}", f.read(x).unwrap()); // prints 1, x[0] is 42

[alexcrichton]

Currently the contract of consume is that you can only pass a number >= the number of bytes you receive from fill_buf. In this case the method is never being called, causing the weird behavior.

I believe this is a documentation issue as the documentation for the consume method is pretty curt.

[jorisgio]

Maybe adding an assert_debug to check that could be useful too ?

[cskr]

@alexcrichton What's the intended use of consume if it must always be passed a value >= the return value of fill_buf?

[alexcrichton]

Oh sorry, I meant <=! The function is intended to tell the buffer how many bytes you've consumed from the return value of fill_buf