What I wrote up is the spec-defined desugaring of a single-branch `if let`, aside from desugaring bugs there should be no counter-example.

The snippet you link to should be replaceable by something along the lines of

    match &passages.asset {
        TranscriptionAsset::Track {
            artist_name: Some(artist_name),
            ..
        } => {
            metadata
                .entry(ARTIST_METADATA.to_string())
                .or_default()
                .push(artist_name.clone());
        }
        _ => {}
    }

no `unwrap` needed. I don't know where you got that idea.