feat(natu): detect single-use let bindings

src/natu/natu_test.rs

@@ -69,7 +69,7 @@ fn natu_reports_out_of_order_let_bindings() -> googletest::Result<()> {
 fn natu_preserves_suppressed_bindings() -> googletest::Result<()> {
     let report = natu::future_sight(vec![SourceFile {
         path: PathBuf::from("testdata/nolint.nix"),
-        source: "{ config, ... }: let\n  cfg = config.example; # NOLINT(let_order)\n  value = cfg;\nin value\n".into(),
+        source: "{ config, ... }: let\n  cfg = config.example; # NOLINT(let_order, single_use_binding)\n  value = cfg; # NOLINT(single_use_binding)\nin value\n".into(),
     }])
     .or_fail()?;
 
@@ -159,6 +159,49 @@ transform value
     Ok(())
 }
 
+#[gtest]
+fn natu_accepts_value_captured_by_reused_lambda() -> googletest::Result<()> {
+    let report = natu::future_sight(vec![SourceFile {
+        path: PathBuf::from("testdata/captured_value.nix"),
+        source: r#"
+let
+  mapper = _: captured;
+  captured = 1;
+in
+[ (mapper 1) (mapper 2) ]
+"#
+        .into(),
+    }])
+    .or_fail()?;
+
+    expect_that!(report.is_clean(), is_true());
+    Ok(())
+}
+
+#[gtest]
+fn natu_reports_single_use_binding() -> googletest::Result<()> {
+    let report = natu::future_sight(vec![SourceFile {
+        path: PathBuf::from("testdata/single_use_binding.nix"),
+        source: r#"
+let
+  single = 1;
+  recursive = recursive;
+in
+{ inherit single recursive; }
+"#
+        .into(),
+    }])
+    .or_fail()?;
+
+    let count = report
+        .diagnostics
+        .iter()
+        .filter(|diagnostic| diagnostic.finding.rule_id == "single_use_binding")
+        .count();
+    expect_that!(count, eq(1));
+    Ok(())
+}
+
 #[gtest]
 fn natu_reports_list_to_attrs_after_map() -> googletest::Result<()> {
     let report = natu::future_sight(vec![SourceFile {