Unused variables, better as error or warning?

[bearophile]

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