Add an assert_bounds utility function.

src/compare/util.rs

@@ -37,3 +37,30 @@ pub fn assert_name<'s>(emacs: &'s Token<'s>, name: &str) -> Result<(), Box<dyn s
     }
     Ok(())
 }
+
+pub fn assert_bounds<'s, S: Source<'s>>(
+    source: &'s str,
+    emacs: &'s Token<'s>,
+    rust: &'s S,
+) -> Result<(), Box<dyn std::error::Error>> {
+    let children = emacs.as_list()?;
+    let attributes_child = children
+        .iter()
+        .nth(1)
+        .ok_or("Should have an attributes child.")?;
+    let attributes_map = attributes_child.as_map()?;
+    let begin = attributes_map
+        .get(":begin")
+        .ok_or("Missing :begin attribute.")?
+        .as_atom()?;
+    let end = attributes_map
+        .get(":end")
+        .ok_or("Missing :end attribute.")?
+        .as_atom()?;
+    let (rust_begin, rust_end) = get_offsets(source, rust);
+    if (rust_begin + 1).to_string() != begin || (rust_end + 1).to_string() != end {
+        Err(format!("Rust bounds ({rust_begin}, {rust_end}) do not match emacs bounds ({emacs_begin}, {emacs_end})", rust_begin = rust_begin + 1, rust_end = rust_end + 1, emacs_begin=begin, emacs_end=end))?;
+    }
+
+    Ok(())
+}