feat(natu): detect returned 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 # NOLINT(redundant_let)\n  cfg = config.example; # NOLINT(let_order)\n  value = cfg;\nin value\n".into(),
     }])
     .or_fail()?;
 
@@ -107,7 +107,8 @@ fn natu_reports_redundant_let() -> googletest::Result<()> {
         path: PathBuf::from("testdata/redundant_let.nix"),
         source: r#"
 let
-  value = 1;
+  helper = 1;
+  value = helper;
 in
 value
 "#
@@ -126,8 +127,9 @@ value
 
 #[gtest]
 fn natu_accepts_nonredundant_let() -> googletest::Result<()> {
-    let report = natu::future_sight(vec![SourceFile {
-        path: PathBuf::from("testdata/nonredundant_let.nix"),
+    let report = natu::future_sight(vec![
+        SourceFile {
+            path: PathBuf::from("testdata/recursive_let.nix"),
             source: r#"
 let
   recursive = recursive;
@@ -135,7 +137,19 @@ 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

src/natu/rules.rs

@@ -97,7 +97,7 @@ pub fn run_rules(file: &ast::ParsedFile, graph: &ScopeGraph, source: &str) -> Ve
             .map(|finding| Finding {
                 rule_id: "redundant_let",
                 range: finding.range,
-                message: "Inline this single-binding let expression",
+                message: "Inline this returned let binding",
                 binding: None,
                 expected_order: None,
             });

src/natu/rules/redundant_let.rs

@@ -45,11 +45,18 @@ fn check_expr(expr: &ast::Expr, findings: &mut Vec<Finding>) {
 }
 
 fn is_redundant(bindings: &[ast::Binding], body: &ast::Expr) -> bool {
-    let [binding] = bindings else {
+    let Some((binding_index, binding)) = bindings
+        .iter()
+        .enumerate()
+        .find(|(_, binding)| is_ident(body, &binding.key.name))
+    else {
         return false;
     };
 
-    is_ident(body, &binding.key.name) && !references_name(&binding.value, &binding.key.name)
+    !references_name(&binding.value, &binding.key.name)
+        && !bindings.iter().enumerate().any(|(candidate_index, candidate)| {
+            candidate_index != binding_index && references_name(&candidate.value, &binding.key.name)
+        })
 }
 
 fn is_ident(expr: &ast::Expr, name: &str) -> bool {