Minor style feedback:

⛳️

let new_len = self.stack.len().checked_sub(n).ok_or( /* error code goes here */ );
// do other stuff here

I think this is a bit more explicit and concise!

This can be combined into a singe statement, but either style is fine.

I agree with you, it would has been more clear in your way but the code will more complicated at the end because either:

• wasmparser::primitives:BinaryReaderError doesn't implement convert::From
• warning: unreachable expression
• unused varaible will be detected by the compiler somehow so we will need to rename new_len to _new_len.

Hmm, sorry I'm not following, I'll write out fully what I meant:

I think the entire function can just be:

pub fn peekn_extra(&self, n: usize) -> Result<&[(BasicValueEnum, ExtraInfo)], BinaryReaderError> {
   let new_len = self.stack.len().checked_sub(n)
            .ok_or(BinaryReaderError {
                message: "invalid value stack",
                offset: -1isize as usize,
            });
   Ok(&self.stack[new_len..])
}

Let me know if that doesn't make sense

I've written the same and got some error but my bad i forgot to propagate the Err ;)