Unused variables, better as error or warning?

[dsimcha]

== Quote from bearophile (bearophileHUGS at lycos.com)'s article
> A small Reddit thread regarding if unused variables and imports are better as
errors or warnings:
> http://www.reddit.com/r/programming/comments/d3emo
> In my opinion in this case errors are too much, warning are enough.
> Few situations for those warnings:
> - warning for unused variables (as GC, C# and other compilers do);
> - warning when a variable get used in some ways, and then its last assignment
gets unused (done by a kind of C compiler);
> - unused imports (useful to keep code clean and remove unnecessary module
dependences);
> - unused functions (but this is harder to do in a clean way in a language that
has templates, so this may be omitted).
> Among those four warnings the most useful are the first two ones. In C once the
unused variable warning of GCC has found at compile time a bug in my code (I did
forget to increment that variable in the loop). So I have loved this warning ever
since.
> Bye,
> bearophile

If we make unused imports an error, how is anyone supposed to do import
someSmallLibrary.all?  If you make unused imports an error, the collective cost of
extra import declaration boilerplate will probably be larger than the GDP of some
African countries.