From f8966fe2a8c982ff9334ab24f05fa6fd198007a5 Mon Sep 17 00:00:00 2001
From: alexion <contact@alexion.dev>
Date: Fri, 18 Sep 2026 23:57:08 -0400
Subject: [PATCH] feat(xatu): report deprecated toPath calls

---
 src/xatu/BUILD                                |  3 +
 src/xatu/ast.rs                               | 23 +++++++
 src/xatu/parser.rs                            | 62 +++++++++++++++++++
 src/xatu/rules.rs                             | 19 +++---
 src/xatu/rules/deprecated_to_path.rs          | 39 ++++++++++++
 src/xatu/rules/empty_inherit.rs               |  1 +
 .../testdata/deprecated_to_path_ident.nix     |  1 +
 .../testdata/deprecated_to_path_select.nix    |  1 +
 src/xatu/xatu_test.rs                         | 32 +++++++++-
 9 files changed, 173 insertions(+), 8 deletions(-)
 create mode 100644 src/xatu/rules/deprecated_to_path.rs
 create mode 100644 src/xatu/testdata/deprecated_to_path_ident.nix
 create mode 100644 src/xatu/testdata/deprecated_to_path_select.nix

diff --git a/src/xatu/BUILD b/src/xatu/BUILD
index 3b3550b..85abfea 100644
--- a/src/xatu/BUILD
+++ b/src/xatu/BUILD
@@ -8,6 +8,7 @@ rust_library(
         "file_store.rs",
         "parser.rs",
         "rules.rs",
+        "rules/deprecated_to_path.rs",
         "rules/empty_inherit.rs",
         "xatu.rs",
     ],
@@ -29,6 +30,8 @@ rust_test(
     srcs = ["xatu_test.rs"],
     data = [
         "testdata/clean.nix",
+        "testdata/deprecated_to_path_ident.nix",
+        "testdata/deprecated_to_path_select.nix",
         "testdata/empty_inherit.nix",
         "testdata/invalid.nix",
     ],
diff --git a/src/xatu/ast.rs b/src/xatu/ast.rs
index 3e22150..d64e63c 100644
--- a/src/xatu/ast.rs
+++ b/src/xatu/ast.rs
@@ -1,5 +1,27 @@
 use crate::FileId;
 
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct Apply {
+    pub function: Expr,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum Expr {
+    Ident(Ident),
+    Select(Box<Select>),
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct Ident {
+    pub name: String,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct Select {
+    pub receiver: Expr,
+    pub attrs: Vec<Ident>,
+}
+
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct Inherit {
     pub attr_count: usize,
@@ -7,6 +29,7 @@ pub struct Inherit {
 
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum NodeData {
+    Apply(Apply),
     Inherit(Inherit),
 }
 
diff --git a/src/xatu/parser.rs b/src/xatu/parser.rs
index a6a389e..0c92745 100644
--- a/src/xatu/parser.rs
+++ b/src/xatu/parser.rs
@@ -52,11 +52,22 @@ fn parse_error_range(error: &rnix::ParseError, source_len: usize) -> ast::Source
 
 fn convert_node(node: rnix::SyntaxNode) -> Option<ast::Node> {
     match node.kind() {
+        rnix::SyntaxKind::NODE_APPLY => convert_apply(node),
         rnix::SyntaxKind::NODE_INHERIT => convert_inherit(node),
         _ => None,
     }
 }
 
+fn convert_apply(node: rnix::SyntaxNode) -> Option<ast::Node> {
+    let apply = rnix::ast::Apply::cast(node)?;
+    let range = apply.syntax().text_range().into();
+
+    Some(ast::Node {
+        range,
+        data: ast::NodeData::Apply(apply.try_into().ok()?),
+    })
+}
+
 fn convert_inherit(node: rnix::SyntaxNode) -> Option<ast::Node> {
     let inherit = rnix::ast::Inherit::cast(node)?;
 
@@ -75,6 +86,57 @@ impl From<rnix::TextRange> for ast::SourceRange {
     }
 }
 
+impl TryFrom<rnix::ast::Apply> for ast::Apply {
+    type Error = ();
+
+    fn try_from(apply: rnix::ast::Apply) -> Result<Self, Self::Error> {
+        Ok(Self {
+            function: apply.lambda().ok_or(())?.try_into()?,
+        })
+    }
+}
+
+impl TryFrom<rnix::ast::Expr> for ast::Expr {
+    type Error = ();
+
+    fn try_from(expr: rnix::ast::Expr) -> Result<Self, Self::Error> {
+        match expr {
+            rnix::ast::Expr::Ident(ident) => Ok(Self::Ident(ident.try_into()?)),
+            rnix::ast::Expr::Select(select) => Ok(Self::Select(Box::new(select.try_into()?))),
+            _ => Err(()),
+        }
+    }
+}
+
+impl TryFrom<rnix::ast::Ident> for ast::Ident {
+    type Error = ();
+
+    fn try_from(ident: rnix::ast::Ident) -> Result<Self, Self::Error> {
+        Ok(Self {
+            name: ident.ident_token().ok_or(())?.text().to_owned(),
+        })
+    }
+}
+
+impl TryFrom<rnix::ast::Select> for ast::Select {
+    type Error = ();
+
+    fn try_from(select: rnix::ast::Select) -> Result<Self, Self::Error> {
+        Ok(Self {
+            receiver: select.expr().ok_or(())?.try_into()?,
+            attrs: select
+                .attrpath()
+                .ok_or(())?
+                .attrs()
+                .map(|attr| match attr {
+                    rnix::ast::Attr::Ident(ident) => ident.try_into(),
+                    _ => Err(()),
+                })
+                .collect::<Result<Vec<_>, _>>()?,
+        })
+    }
+}
+
 impl From<rnix::ast::Inherit> for ast::Inherit {
     fn from(inherit: rnix::ast::Inherit) -> Self {
         Self {
diff --git a/src/xatu/rules.rs b/src/xatu/rules.rs
index 28edf39..567e660 100644
--- a/src/xatu/rules.rs
+++ b/src/xatu/rules.rs
@@ -1,7 +1,9 @@
 use crate::FileId;
 use crate::ast;
+use crate::rules::deprecated_to_path::DeprecatedToPath;
 use crate::rules::empty_inherit::EmptyInherit;
 
+pub mod deprecated_to_path;
 pub mod empty_inherit;
 
 #[derive(Debug)]
@@ -54,13 +56,16 @@ pub fn run_rules(file: &ast::ParsedFile) -> Vec<Diagnostic> {
     file.nodes
         .iter()
         .flat_map(|node| {
-            [run_node_rule::<EmptyInherit>(node)]
-                .into_iter()
-                .flatten()
-                .map(|finding| Diagnostic {
-                    file_id: file.file_id,
-                    finding,
-                })
+            [
+                run_node_rule::<EmptyInherit>(node),
+                run_node_rule::<DeprecatedToPath>(node),
+            ]
+            .into_iter()
+            .flatten()
+            .map(|finding| Diagnostic {
+                file_id: file.file_id,
+                finding,
+            })
         })
         .collect()
 }
diff --git a/src/xatu/rules/deprecated_to_path.rs b/src/xatu/rules/deprecated_to_path.rs
new file mode 100644
index 0000000..68b7790
--- /dev/null
+++ b/src/xatu/rules/deprecated_to_path.rs
@@ -0,0 +1,39 @@
+use crate::{ast, rules::NodeRule};
+
+pub struct DeprecatedToPath;
+
+impl NodeRule for DeprecatedToPath {
+    type Node = ast::Apply;
+
+    const ID: &'static str = "deprecated_to_path";
+
+    fn select(data: &ast::NodeData) -> Option<&Self::Node> {
+        match data {
+            ast::NodeData::Apply(apply) => Some(apply),
+            _ => None,
+        }
+    }
+
+    fn check(node: &Self::Node) -> Option<&'static str> {
+        if calls_to_path(&node.function) {
+            Some("builtins.toPath is deprecated")
+        } else {
+            None
+        }
+    }
+}
+
+fn calls_to_path(function: &ast::Expr) -> bool {
+    match function {
+        ast::Expr::Ident(ident) => ident.name == "toPath",
+        ast::Expr::Select(select) => is_builtins_to_path(select),
+    }
+}
+
+fn is_builtins_to_path(select: &ast::Select) -> bool {
+    matches!(
+        &select.receiver,
+        ast::Expr::Ident(ident) if ident.name == "builtins"
+    ) && select.attrs.len() == 1
+        && select.attrs[0].name == "toPath"
+}
diff --git a/src/xatu/rules/empty_inherit.rs b/src/xatu/rules/empty_inherit.rs
index c044a65..fee8bae 100644
--- a/src/xatu/rules/empty_inherit.rs
+++ b/src/xatu/rules/empty_inherit.rs
@@ -10,6 +10,7 @@ impl NodeRule for EmptyInherit {
     fn select(data: &ast::NodeData) -> Option<&Self::Node> {
         match data {
             ast::NodeData::Inherit(inherit) => Some(inherit),
+            _ => None,
         }
     }
 
diff --git a/src/xatu/testdata/deprecated_to_path_ident.nix b/src/xatu/testdata/deprecated_to_path_ident.nix
new file mode 100644
index 0000000..5d2d9bb
--- /dev/null
+++ b/src/xatu/testdata/deprecated_to_path_ident.nix
@@ -0,0 +1 @@
+toPath x
diff --git a/src/xatu/testdata/deprecated_to_path_select.nix b/src/xatu/testdata/deprecated_to_path_select.nix
new file mode 100644
index 0000000..fa6ea4f
--- /dev/null
+++ b/src/xatu/testdata/deprecated_to_path_select.nix
@@ -0,0 +1 @@
+builtins.toPath x
diff --git a/src/xatu/xatu_test.rs b/src/xatu/xatu_test.rs
index e1ef141..6d10aaf 100644
--- a/src/xatu/xatu_test.rs
+++ b/src/xatu/xatu_test.rs
@@ -37,10 +37,40 @@ fn future_sight_rejects_parse_errors() {
     assert_eq!(error.code(), Code::FailedPrecondition);
 }
 
+#[test]
+fn future_sight_reports_deprecated_to_path_ident_findings() {
+    let report = xatu::future_sight(vec![source_file("testdata/deprecated_to_path_ident.nix")])
+        .expect("fixture should parse");
+
+    assert_eq!(report.diagnostics.len(), 1);
+
+    let diagnostic = &report.diagnostics[0];
+    assert_eq!(
+        report.files.get(diagnostic.file_id).path,
+        PathBuf::from("testdata/deprecated_to_path_ident.nix"),
+    );
+    assert_eq!(diagnostic.finding.rule_id, "deprecated_to_path");
+}
+
+#[test]
+fn future_sight_reports_deprecated_to_path_select_findings() {
+    let report = xatu::future_sight(vec![source_file("testdata/deprecated_to_path_select.nix")])
+        .expect("fixture should parse");
+
+    assert_eq!(report.diagnostics.len(), 1);
+
+    let diagnostic = &report.diagnostics[0];
+    assert_eq!(
+        report.files.get(diagnostic.file_id).path,
+        PathBuf::from("testdata/deprecated_to_path_select.nix"),
+    );
+    assert_eq!(diagnostic.finding.rule_id, "deprecated_to_path");
+}
+
 #[test]
 fn future_sight_reports_empty_inherit_findings() {
     let report = xatu::future_sight(vec![source_file("testdata/empty_inherit.nix")])
-    .expect("fixture should parse");
+        .expect("fixture should parse");
 
     assert_eq!(report.diagnostics.len(), 1);