What's the deal with "Warning: explicit element-wise assignment..."

[Andrei Alexandrescu via Digitalmars-d]

We've just enabled warnings as errors in our build system at work and 
suddenly:

Warning: explicit element-wise assignment <exprnew> is better than <expr>
where <exprnew> and <expr> are expressions picked from the code. For 
example, this wasn't accepted:

writeAvail_[0 .. buf.length] = buf;

but this was:

writeAvail_[0 .. buf.length] = buf[];

The type of the involved variables were as trivial as ubyte[] and in 
ubyte[] respectively.

What's the idea?


Andrei