new lint: deprecated_to_path

bin/tests/data/deprecated_to_path.nix

@@ -0,0 +1,6 @@
+[
+  (builtins.toPath x)
+  (toPath x)
+  (toPath "/abc/def")
+  (builtins.toPath "/some/path")
+]

bin/tests/main.rs

@@ -62,5 +62,6 @@ test_lint! {
     deprecated_is_null,
     empty_inherit,
     faster_groupby => session_info!("2.5"),
-    faster_zipattrswith => session_info!("2.6")
+    faster_zipattrswith => session_info!("2.6"),
+    deprecated_to_path => session_info!("2.4")
 }

bin/tests/snapshots/main__deprecated_to_path.snap

@@ -0,0 +1,34 @@
+---
+source: bin/tests/main.rs
+expression: "&out"
+
+---
+[W17] Warning: Found usage of deprecated builtin toPath
+   ╭─[data/deprecated_to_path.nix:2:4]
+   │
+ 2 │   (builtins.toPath x)
+   ·    ────────┬────────  
+   ·            ╰────────── builtins.toPath is deprecated, see :doc builtins.toPath within the REPL for more
+───╯
+[W17] Warning: Found usage of deprecated builtin toPath
+   ╭─[data/deprecated_to_path.nix:3:4]
+   │
+ 3 │   (toPath x)
+   ·    ────┬───  
+   ·        ╰───── toPath is deprecated, see :doc builtins.toPath within the REPL for more
+───╯
+[W17] Warning: Found usage of deprecated builtin toPath
+   ╭─[data/deprecated_to_path.nix:4:4]
+   │
+ 4 │   (toPath "/abc/def")
+   ·    ────────┬────────  
+   ·            ╰────────── toPath is deprecated, see :doc builtins.toPath within the REPL for more
+───╯
+[W17] Warning: Found usage of deprecated builtin toPath
+   ╭─[data/deprecated_to_path.nix:5:4]
+   │
+ 5 │   (builtins.toPath "/some/path")
+   ·    ──────────────┬─────────────  
+   ·                  ╰─────────────── builtins.toPath is deprecated, see :doc builtins.toPath within the REPL for more
+───╯
+

lib/src/lints.rs

@@ -17,4 +17,5 @@ lints! {
     empty_inherit,
     faster_groupby,
     faster_zipattrswith,
+    deprecated_to_path,
 }

lib/src/lints/deprecated_to_path.rs

@@ -0,0 +1,59 @@
+use crate::{session::SessionInfo, Metadata, Report, Rule};
+
+use if_chain::if_chain;
+use macros::lint;
+use rnix::{
+    types::{Apply, TypedNode},
+    NodeOrToken, SyntaxElement, SyntaxKind,
+};
+
+/// ## What it does
+/// Checks for usage of the `toPath` function.
+///
+/// ## Why is this bad?
+/// `toPath` is deprecated.
+///
+/// ## Example
+///
+/// ```nix
+/// builtins.toPath "/path"
+/// ```
+///
+/// Try these instead:
+///
+/// ```nix
+/// # to convert the string to an absolute path:
+/// /. + "/path"
+/// # => /abc
+///
+/// # to convert the string to a path relative to the current directory:
+/// ./. + "/bin"
+/// # => /home/np/statix/bin
+/// ```
+#[lint(
+    name = "deprecated_to_path",
+    note = "Found usage of deprecated builtin toPath",
+    code = 17,
+    match_with = SyntaxKind::NODE_APPLY
+)]
+struct DeprecatedIsNull;
+
+static ALLOWED_PATHS: &[&str; 2] = &["builtins.toPath", "toPath"];
+
+impl Rule for DeprecatedIsNull {
+    fn validate(&self, node: &SyntaxElement, _sess: &SessionInfo) -> Option<Report> {
+        if_chain! {
+            if let NodeOrToken::Node(node) = node;
+            if let Some(apply) = Apply::cast(node.clone());
+            let lambda_path = apply.lambda()?.to_string();
+            if ALLOWED_PATHS.iter().any(|&p| p == lambda_path.as_str());
+            then {
+                let at = node.text_range();
+                let message = format!("`{}` is deprecated, see `:doc builtins.toPath` within the REPL for more", lambda_path);
+                Some(self.report().diagnostic(at, message))
+            } else {
+                None
+            }
+        }
+    }
+}