Q:

What is the Arduino compiler's version of `__unused` for function parameters?

When I first started with the Arduino environment I noticed that it had File | Preferences | Compiler Warnings set to None, so I set it to All - and there were many, many warnings in the supplied libraries! So I quickly set it to None again - until later.
Well, "later" is now, and I started going through the warnings to fix the code to quiet the compiler.
Note that I don't want to disable any warnings - I want to placate the compiler at each spot so that it says to itself "No problem here!"

If the compiler complains of an unused variable, I comment out the variable;
If it complains of a signed/unsigned comparison, I change the type;
If it complains of an unused parameter in a function, I...

...hmmm. If this was C++, I'd simply comment out the parameter name, leaving the type naked:
void Fn(int /*param*/) {
} // Fn(param)

Arduino reports:
error: parameter name omitted

But that isn't legal in C. With some compilers you can use the __unused attribute:
void Fn(int __unused param) {
} // Fn(param)

Arduino reports:
error: expected ';', ',' or ')' before 'param'
    It's interpreting __unused as the actual parameter name

or if they're fussy:
void Fn(int __attribute__((unused)) param) {
} // Fn(param)

Arduino reports:
warning: unused parameter 'param' [-Wunused-parameter]
    That's not the correct __attribute__, obviously

So none of the above work in the Arduino environment. Suggestions?

I've been programming in C and C++ for over twenty years, and I know that the #1 thing you should always do is ask the compiler to tell you about everything it notices. You need a warning-free compile at all times! Otherwise, you spend hours or days trying to find a bug - only to discover that the compiler has been warning you about it all along...

A:

Well, I typed all that question up for posting, then tried one last thing. It worked!
void Fn(__attribute__((unused)) int param) {
} // Fn(param)

So, rather than let a good question go to waste: here's the answer!
Now all we need to do is get the maintainers of all the libraries to put these in... and fix the other warnings...

