From c6ea58410cb6145cf2791f0ae0d1918cf0d5bc62 Mon Sep 17 00:00:00 2001
From: Akshay <nerdy@peppe.rs>
Date: Fri, 8 Oct 2021 21:47:07 +0530
Subject: new lint: eta_reduction

---
 lib/src/lints.rs               |  1 +
 lib/src/lints/eta_reduction.rs | 51 ++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 52 insertions(+)
 create mode 100644 lib/src/lints/eta_reduction.rs

diff --git a/lib/src/lints.rs b/lib/src/lints.rs
index 7da4933..868cd26 100644
--- a/lib/src/lints.rs
+++ b/lib/src/lints.rs
@@ -7,4 +7,5 @@ lint_map! {
     manual_inherit_from,
     legacy_let_syntax,
     collapsible_let_in,
+    eta_reduction,
 }
diff --git a/lib/src/lints/eta_reduction.rs b/lib/src/lints/eta_reduction.rs
new file mode 100644
index 0000000..8c34205
--- /dev/null
+++ b/lib/src/lints/eta_reduction.rs
@@ -0,0 +1,51 @@
+use crate::{Lint, Metadata, Report, Rule, Suggestion};
+
+use if_chain::if_chain;
+use macros::lint;
+use rnix::{
+    types::{Lambda, Ident, Apply, TypedNode, TokenWrapper},
+    NodeOrToken, SyntaxElement, SyntaxKind,
+};
+
+#[lint(
+    name = "eta reduction",
+    note = "This function expression is eta reducible",
+    code = 7,
+    match_with = SyntaxKind::NODE_LAMBDA
+)]
+struct EtaReduction;
+
+impl Rule for EtaReduction {
+    fn validate(&self, node: &SyntaxElement) -> Option<Report> {
+        if_chain! {
+            if let NodeOrToken::Node(node) = node;
+            if let Some(lambda_expr) = Lambda::cast(node.clone());
+
+            if let Some(arg_node) = lambda_expr.arg();
+            if let Some(arg) = Ident::cast(arg_node);
+
+            if let Some(body_node) = lambda_expr.body();
+            if let Some(body) = Apply::cast(body_node);
+
+            if let Some(value_node) = body.value();
+            if let Some(value) = Ident::cast(value_node);
+
+            if arg.as_str() == value.as_str() ;
+
+            then {
+                let at = node.text_range();
+                let replacement = body.lambda()?;
+                let message = 
+                    format!(
+                        "Found eta-reduction: `{}`",
+                        replacement.text().to_string()
+                    );
+                Some(Self::report().suggest(at, message, Suggestion::new(at, replacement)))
+            } else {
+                None
+            }
+        }
+    }
+}
+
+
-- 
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