Unexpected operation of vec::as_mut_slice() with binary read

[alexpyattaev]

I think there is an issue with implementation of vec::as_mut_slice()
Here in the docs it is suggested that as_mut_slice() can be used to read data into a vec from an IO reader. So, below example should happily read a bunch of bytes from stdin, and print them out to stdout as byte vectors.

    let mut buf:Vec<u8> = Vec::with_capacity(512);
    stdin_lock.read(buf.as_mut_slice());

Now, actually running this will read nothing into buf, and will not produce errors or warnings.
The reason this is happening is subtle - buf has capacity, but once it is converted into a slice, that slice is actually empty. For this reason, nothing gets read.

[sfackler]

You are creating a vec of zero length but a capacity of 512 in the first example. as_mut_slice returns an empty slice when called on an empty vec.

[alexpyattaev]

Yes, ended up figuring this out myself as well. Took me way longer than it should though =(

[alexpyattaev]

To avoid people stumbling into this trap, the docs could probably give an example like this to emphasize the possible trap

   let mut buf:Vec<u8> = Vec::with_capacity(size);
    buf.resize(size, 0);
   std::io::stdin().read_exact(buf.as_mutable_slice());

On the same note, should read() even accept empty slices?