feat(natu): detect redundant let expressions

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 # NOLINT(redundant_let)\n  cfg = config.example; # NOLINT(let_order)\n  value = cfg;\nin value\n".into(),
     }])
     .or_fail()?;
 
@@ -101,6 +101,66 @@ entry
     Ok(())
 }
 
+#[gtest]
+fn natu_reports_redundant_let() -> googletest::Result<()> {
+    let report = natu::future_sight(vec![SourceFile {
+        path: PathBuf::from("testdata/redundant_let.nix"),
+        source: r#"
+let
+  helper = 1;
+  value = helper;
+in
+value
+"#
+        .into(),
+    }])
+    .or_fail()?;
+
+    let count = report
+        .diagnostics
+        .iter()
+        .filter(|diagnostic| diagnostic.finding.rule_id == "redundant_let")
+        .count();
+    expect_that!(count, eq(1));
+    Ok(())
+}
+
+#[gtest]
+fn natu_accepts_nonredundant_let() -> googletest::Result<()> {
+    let report = natu::future_sight(vec![
+        SourceFile {
+            path: PathBuf::from("testdata/recursive_let.nix"),
+            source: r#"
+let
+  recursive = recursive;
+in
+recursive
+"#
+            .into(),
+        },
+        SourceFile {
+            path: PathBuf::from("testdata/dependent_let.nix"),
+            source: r#"
+let
+  value = 1;
+  dependent = value;
+in
+value
+"#
+            .into(),
+        },
+    ])
+    .or_fail()?;
+
+    let count = report
+        .diagnostics
+        .iter()
+        .filter(|diagnostic| diagnostic.finding.rule_id == "redundant_let")
+        .count();
+    expect_that!(count, eq(0));
+    Ok(())
+}
+
 #[gtest]
 fn natu_reports_redundant_parameter_forwarding() -> googletest::Result<()> {
     let report = natu::future_sight(vec![