test_derive! output diff on failure

[joshlf]

If test_derive! outputted a diff on failure instead of the full text of both the expected and received output, it would make it much easier to debug. There are crates which do this out-of-the-box, so it should be relatively straightforward.

[joshlf]

Whipped up something with dissimilar. The output is a little odd because all of the tokens are still space-separated, but it basically works.

#[doc(hidden)]
pub fn assert_eq_streams(expect: TokenStream2, res: TokenStream2) {
    fn token_stream_eq(a: TokenStream2, b: TokenStream2) -> bool {
        a.into_iter()
            .zip(b.into_iter())
            .all(|(a, b)| token_tree_eq(&a, &b))
    }

    fn token_tree_eq(a: &proc_macro2::TokenTree, b: &proc_macro2::TokenTree) -> bool {
        use proc_macro2::TokenTree::*;
        match (a, b) {
            (Group(a), Group(b)) => {
                a.delimiter() == b.delimiter() && token_stream_eq(a.stream(), b.stream())
            }
            (Ident(a), Ident(b)) => format!("{}", a) == format!("{}", b),
            (Punct(a), Punct(b)) => a.as_char() == b.as_char() && a.spacing() == b.spacing(),
            (Literal(a), Literal(b)) => format!("{}", a) == format!("{}", b),
            _ => false,
        }
    }

    if !token_stream_eq(expect.clone(), res.clone()) {
        let diff = dissimilar::diff(&expect.to_string(), &res.to_string())
            .into_iter()
            .map(|chunk| {
                let (prefix, chunk) = match chunk {
                    Chunk::Equal(chunk) => (" ", chunk),
                    Chunk::Delete(chunk) => ("-", chunk),
                    Chunk::Insert(chunk) => ("+", chunk),
                };
                [prefix, chunk, "\n"]
            })
            .flatten()
            .collect::<String>();
        panic!(
            "\
test_derive failed:
diff (expected vs got):

{}
\n",
            diff
        );
    }
}

[joshlf]

Put up an implementation: #66

[ComputerDruid]

I just encountered this on a failure I was debugging and ended up independently hacking in a diffing fix (I used assert2, but very similar idea) because I couldn't spot the difference visually

So please do consider adding some kind of diffing to test_derive, as it's very hard to figure out what's changed, otherwise.

[joshlf]

We ended up just reimplementing this from scratch, and it turned out to be a relatively small amount of code if you want to just copy what we did: https://github.com/google/zerocopy/blob/51bc4df582e16f3afbe31d65291c3fb78ee092d3/zerocopy-derive/src/output_tests.rs#L31-L96