1 files changed, 72 insertions, 0 deletions

diff --git a/lib/src/lints/faster_groupby.rs b/lib/src/lints/faster_groupby.rs
new file mode 100644
index 0000000..c496125
--- /dev/null
+++ b/lib/src/lints/faster_groupby.rs
@@ -0,0 +1,72 @@
+use crate::{
+    make,
+    session::{SessionInfo, Version},
+    Metadata, Report, Rule, Suggestion,
+};
+
+use if_chain::if_chain;
+use macros::lint;
+use rnix::{
+    types::{Select, TypedNode},
+    NodeOrToken, SyntaxElement, SyntaxKind,
+};
+
+/// ## What it does
+/// Checks for `lib.groupBy`.
+///
+/// ## Why is this bad?
+/// Nix 2.5 introduces `builtins.groupBy` which is faster and does
+/// not require a lib import.
+///
+/// ## Example
+///
+/// ```nix
+/// lib.groupBy (x: if x > 2 then "big" else "small") [ 1 2 3 4 5 6 ];
+/// # { big = [ 3 4 5 6 ]; small = [ 1 2 ]; }
+/// ```
+///
+/// Replace `lib.groupBy` with `builtins.groupBy`:
+///
+/// ```
+/// builtins.groupBy (x: if x > 2 then "big" else "small") [ 1 2 3 4 5 6 ];
+/// ```
+#[lint(
+    name = "faster_groupby",
+    note = "Found lib.groupBy",
+    code = 15,
+    match_with = SyntaxKind::NODE_SELECT
+)]
+struct FasterGroupBy;
+
+impl Rule for FasterGroupBy {
+    fn validate(&self, node: &SyntaxElement, sess: &SessionInfo) -> Option<Report> {
+        let lint_version = "nix (Nix) 2.5".parse::<Version>().unwrap();
+        if_chain! {
+            if sess.version() >=  &lint_version;
+            if let NodeOrToken::Node(node) = node;
+            if let Some(select_expr) = Select::cast(node.clone());
+            if let Some(select_from) = select_expr.set();
+            if let Some(group_by_attr) = select_expr.index();
+
+            // a heuristic to lint on nixpkgs.lib.groupBy
+            // and lib.groupBy and its variants
+            if select_from.text().to_string() != "builtins";
+            if group_by_attr.text().to_string() == "groupBy";
+
+            then {
+                let at = node.text_range();
+                let replacement = {
+                    let builtins = make::ident("builtins");
+                    make::select(builtins.node(), &group_by_attr).node().clone()
+                };
+                let message = format!("Prefer `builtins.groupBy` over `{}.groupBy`", select_from);
+                Some(
+                    self.report()
+                        .suggest(at, message, Suggestion::new(at, replacement)),
+                )
+            } else {
+                None
+            }
+        }
+    }
+}