Keep the context that we are inside macro in nested macro

[topecongiro]

Closes #2830.
Closes #2857.

[nrc]

Thanks for the PR!

[nrc]

Could you use an RAII pattern here so there is no change of missing the replace at the end of the function?

[topecongiro]

Hmm, I am sorry, but I am not sure how to apply an RAII pattern here. Does RefCell provides such thing? Either way, I have tried to mimic the pattern here. Please let me know if this is not what you expected, or there is a better way to do this!

[nrc]

RAII in Rust would be something like:

struct InsideMacroRestore<'a> {
    context: &RewriteContext,
    old_value: bool,
}

impl<'a> InsideMacroRestore<'a> {
    // Call this as `let _imr = InsideMacroRestore(context, true);` which sets
    // the value and stores the old one.
    fn replace(context: &'a RewriteContext, new_value: bool) -> InsideMacroRestore<'a> {
        let old_value = context.inside_macro.replace(new_value);
        InsideMacroRestore { context, old_value }
    }
}

impl<'a> Drop for InsideMacroRestore<'a> {
    // Called automatically when `_imr` goes out of scope, which restores the old
    // value of inside_macro
    fn drop(&mut self) {
        self.context.inside_macro.replace(self.old_value)
    }
}

MutexLock is an example of such thing, although there the object is actually useful, here we just want the Drop impl

[topecongiro]

Added commits to use RAII pattern and fix #2857.