feat(natu): report expected let order

src/natu/natu.rs

@@ -1,3 +1,5 @@
+use std::collections::HashSet;
+
 mod ast;
 mod file_store;
 mod parser;
@@ -23,7 +25,45 @@ impl Report {
 
 impl std::fmt::Display for Report {
     fn fmt(&self, formatter: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let mut reported_let_order_files = HashSet::new();
+
         for diagnostic in &self.diagnostics {
+            if diagnostic.finding.rule_id == "let_order" {
+                if !reported_let_order_files.insert(diagnostic.file_id) {
+                    continue;
+                }
+
+                let file = self.files.get(diagnostic.file_id);
+                writeln!(
+                    formatter,
+                    "{} [let_order] Let bindings must follow dependency order",
+                    file.path.display()
+                )?;
+
+                let mut findings = self
+                    .diagnostics
+                    .iter()
+                    .filter(|candidate| {
+                        candidate.file_id == diagnostic.file_id
+                            && candidate.finding.rule_id == "let_order"
+                    })
+                    .collect::<Vec<_>>();
+                findings.sort_by_key(|finding| finding.finding.range.start);
+
+                for finding in findings {
+                    let (start, _) = line_range(&file.source, finding.finding.range);
+                    let expected_order =
+                        finding.finding.expected_order.as_ref().expect("let order");
+                    writeln!(
+                        formatter,
+                        "  let at line {start}: {}",
+                        expected_order.join(", ")
+                    )?;
+                }
+
+                continue;
+            }
+
             let file = self.files.get(diagnostic.file_id);
             let (start, end) = line_range(&file.source, diagnostic.finding.range);
             let location = if start == end {