feat(xatu): report empty let expressions

AGENTS.md

@@ -96,11 +96,15 @@
 - `rules_rust` `rust_test` targets cannot set both `crate` and `srcs`.
   Use one input style per test target.
 - Rust tests in this monorepo live in separate `*_test.rs` files, not inline `#[cfg(test)]` modules.
+- `rnix::ast::HasEntry::entries()` includes assignments and `inherit` statements.
+  Use `attrpath_values()` when code needs only assignment entries.
 - Ariadne Rust tests are declared in Bazel `BUILD` files; do not add `[[test]]` entries to
   `src/ariadne/Cargo.toml` unless explicitly requested.
 - In this monorepo, "dotfiles" refers to the dotfiles/NixOS configuration under `src/dotfiles/` and `configs/nixos/`.
 - The flake does not currently define `formatter.x86_64-linux`.
   Do not use `nix fmt` as a validation command until a formatter is added.
+- The `fix` app passes its current directory to Statix and Deadnix.
+  Run `nix run .#fix` from the repository root so path exclusions match.
 - Ariadne extraction must preserve the game-root season prefix for archived source.
   Different game installs can contain compiled scripts with the same internal path.
 - Clew owns Ren'Py extraction, preparation, and compiled-script AST export. Ariadne owns analysis over Clew-produced data.

src/xatu/BUILD

@@ -10,6 +10,7 @@ rust_library(
         "rules.rs",
         "rules/deprecated_to_path.rs",
         "rules/empty_inherit.rs",
+        "rules/empty_let_in.rs",
         "rules/unquoted_uri.rs",
         "xatu.rs",
     ],
@@ -33,6 +34,7 @@ rust_test(
         "testdata/clean.nix",
         "testdata/invalid.nix",
         "testdata/lint_findings.nix",
+        "testdata/lint_interactions.nix",
         "testdata/lint_variants.nix",
     ],
     deps = [

src/xatu/ast.rs

@@ -29,6 +29,7 @@ impl std::fmt::Display for SourceRange {
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum NodeData {
     Apply(Apply),
+    LetIn(LetIn),
     Inherit(Inherit),
 }
 
@@ -54,6 +55,11 @@ pub struct Select {
     pub attrs: Vec<Ident>,
 }
 
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct LetIn {
+    pub binding_count: usize,
+}
+
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct Inherit {
     pub attr_count: usize,

src/xatu/parser.rs

@@ -1,3 +1,4 @@
+use rnix::ast::HasEntry;
 use rowan::ast::AstNode;
 
 use crate::{FileId, ast};
@@ -37,6 +38,7 @@ fn convert_syntax(syntax: rnix::SyntaxNode) -> (Vec<ast::Node>, Vec<ast::Token>)
 fn convert_node(node: rnix::SyntaxNode) -> Option<ast::Node> {
     match node.kind() {
         rnix::SyntaxKind::NODE_APPLY => convert_apply(node),
+        rnix::SyntaxKind::NODE_LET_IN => convert_let_in(node),
         rnix::SyntaxKind::NODE_INHERIT => convert_inherit(node),
         _ => None,
     }
@@ -102,6 +104,27 @@ impl TryFrom<rnix::ast::Select> for ast::Select {
     }
 }
 
+fn convert_let_in(node: rnix::SyntaxNode) -> Option<ast::Node> {
+    let let_in = rnix::ast::LetIn::cast(node)?;
+
+    Some(ast::Node {
+        range: let_in.syntax().text_range().into(),
+        data: ast::NodeData::LetIn(let_in.into()),
+    })
+}
+
+impl From<rnix::ast::LetIn> for ast::LetIn {
+    fn from(let_in: rnix::ast::LetIn) -> Self {
+        Self {
+            binding_count: let_in.attrpath_values().count()
+                + let_in
+                    .inherits()
+                    .map(|inherit| inherit.attrs().count())
+                    .sum::<usize>(),
+        }
+    }
+}
+
 fn convert_inherit(node: rnix::SyntaxNode) -> Option<ast::Node> {
     let inherit = rnix::ast::Inherit::cast(node)?;
 

src/xatu/rules.rs

@@ -2,10 +2,12 @@ use crate::FileId;
 use crate::ast;
 use crate::rules::deprecated_to_path::DeprecatedToPath;
 use crate::rules::empty_inherit::EmptyInherit;
+use crate::rules::empty_let_in::EmptyLetIn;
 use crate::rules::unquoted_uri::UnquotedUri;
 
 pub mod deprecated_to_path;
 pub mod empty_inherit;
+pub mod empty_let_in;
 pub mod unquoted_uri;
 
 #[derive(Debug)]
@@ -54,6 +56,7 @@ pub fn run_rules(file: &ast::ParsedFile) -> Vec<Diagnostic> {
         [
             run_node_rule::<DeprecatedToPath>(node),
             run_node_rule::<EmptyInherit>(node),
+            run_node_rule::<EmptyLetIn>(node),
         ]
         .into_iter()
         .flatten()

src/xatu/rules/empty_let_in.rs

@@ -0,0 +1,24 @@
+use crate::{ast, rules::NodeRule};
+
+pub struct EmptyLetIn;
+
+impl NodeRule for EmptyLetIn {
+    type Node = ast::LetIn;
+
+    const ID: &'static str = "empty_let_in";
+
+    fn select(data: &ast::NodeData) -> Option<&Self::Node> {
+        match data {
+            ast::NodeData::LetIn(let_in) => Some(let_in),
+            _ => None,
+        }
+    }
+
+    fn check(node: &Self::Node) -> Option<&'static str> {
+        if node.binding_count == 0 {
+            Some("let expression has no bindings")
+        } else {
+            None
+        }
+    }
+}

src/xatu/testdata/clean.nix

@@ -1,6 +1,11 @@
 { pkgs }:
 
 {
+  nonEmpty =
+    let
+      value = pkgs;
+    in
+    value;
   path = ./. + "/foo";
   url = "https://example.com";
   inherit pkgs;

src/xatu/testdata/lint_findings.nix

@@ -2,6 +2,7 @@
 
 {
   inherit;
+  empty = let in src;
   path = builtins.toPath src;
   url = https://example.com;
 }

src/xatu/testdata/lint_interactions.nix

@@ -0,0 +1,8 @@
+{ src }:
+
+let
+  inherit;
+in
+{
+  value = src;
+}

src/xatu/testdata/lint_variants.nix

@@ -1,5 +1,10 @@
 { src }:
 
 {
+  commentedEmpty =
+    let
+      # This comment must not suppress the finding.
+    in
+    src;
   path = toPath src;
 }

src/xatu/xatu_test.rs

@@ -113,6 +113,51 @@ fn xatu_reports_empty_inherit() -> googletest::Result<()> {
     Ok(())
 }
 
+#[gtest]
+fn xatu_reports_empty_let_in() -> googletest::Result<()> {
+    let source = fixture_source("testdata/lint_findings.nix")?;
+    let range_start = source.source.find("let in src").or_fail()?;
+    let range_end = range_start + "let in src".len();
+    let report = xatu::future_sight(vec![source]).or_fail()?;
+
+    let finding = report
+        .diagnostics
+        .iter()
+        .map(|diagnostic| &diagnostic.finding)
+        .find(|finding| finding.rule_id == "empty_let_in")
+        .or_fail()?;
+    let expected_range = format!("{range_start}..{range_end}");
+    expect_that!(finding.range.to_string(), eq(&expected_range));
+    Ok(())
+}
+
+#[gtest]
+fn xatu_reports_commented_empty_let_in() -> googletest::Result<()> {
+    let report = xatu::future_sight(vec![fixture_source("testdata/lint_variants.nix")?]).or_fail()?;
+
+    let findings = report
+        .diagnostics
+        .iter()
+        .map(|diagnostic| diagnostic.finding.rule_id)
+        .collect::<Vec<_>>();
+    expect_that!(findings, contains(eq(&"empty_let_in")));
+    Ok(())
+}
+
+#[gtest]
+fn xatu_reports_empty_let_with_empty_inherit() -> googletest::Result<()> {
+    let report =
+        xatu::future_sight(vec![fixture_source("testdata/lint_interactions.nix")?]).or_fail()?;
+
+    let findings = report
+        .diagnostics
+        .iter()
+        .map(|diagnostic| diagnostic.finding.rule_id)
+        .collect::<Vec<_>>();
+    expect_that!(findings, eq(&vec!["empty_let_in", "empty_inherit"]));
+    Ok(())
+}
+
 #[gtest]
 fn xatu_reports_unquoted_uri() -> googletest::Result<()> {
     let report = xatu::future_sight(vec![fixture_source("testdata/lint_findings.nix")?]).or_fail()?;