5 files changed, 84 insertions, 1 deletions

diff --git a/bin/tests/data/bool_simplification.nix b/bin/tests/data/bool_simplification.nix
new file mode 100644
index 0000000..64c2c7a
--- /dev/null
+++ b/bin/tests/data/bool_simplification.nix
@@ -0,0 +1,7 @@
+let
+  _ = !(a == b);
+  # do not match here
+  _ = !(a != b);
+  _ = a != b;
+in
+  null
diff --git a/bin/tests/main.rs b/bin/tests/main.rs
index bec64d2..3403f65 100644
--- a/bin/tests/main.rs
+++ b/bin/tests/main.rs
@@ -63,5 +63,6 @@ test_lint! {
     empty_inherit,
     faster_groupby => session_info!("2.5"),
     faster_zipattrswith => session_info!("2.6"),
-    deprecated_to_path => session_info!("2.4")
+    deprecated_to_path => session_info!("2.4"),
+    bool_simplification
 }
diff --git a/bin/tests/snapshots/main__bool_simplification.snap b/bin/tests/snapshots/main__bool_simplification.snap
new file mode 100644
index 0000000..9e3c991
--- /dev/null
+++ b/bin/tests/snapshots/main__bool_simplification.snap
@@ -0,0 +1,13 @@
+---
+source: bin/tests/main.rs
+expression: "&out"
+
+---
+[W18] Warning: This boolean expression can be simplified
+   ╭─[data/bool_simplification.nix:2:7]
+   │
+ 2 │   _ = !(a == b);
+   ·       ────┬────  
+   ·           ╰────── Try != instead of !(... == ...)
+───╯
+
diff --git a/lib/src/lints.rs b/lib/src/lints.rs
index 439fd8f..582cabe 100644
--- a/lib/src/lints.rs
+++ b/lib/src/lints.rs
@@ -18,4 +18,5 @@ lints! {
     faster_groupby,
     faster_zipattrswith,
     deprecated_to_path,
+    bool_simplification,
 }
diff --git a/lib/src/lints/bool_simplification.rs b/lib/src/lints/bool_simplification.rs
new file mode 100644
index 0000000..9ccb1f6
--- /dev/null
+++ b/lib/src/lints/bool_simplification.rs
@@ -0,0 +1,61 @@
+use crate::{make, session::SessionInfo, Metadata, Report, Rule, Suggestion};
+
+use if_chain::if_chain;
+use macros::lint;
+use rnix::{
+    types::{BinOp, BinOpKind, Paren, TypedNode, UnaryOp, UnaryOpKind, Wrapper},
+    NodeOrToken, SyntaxElement, SyntaxKind,
+};
+
+/// ## What it does
+/// Checks for boolean expressions that can be simplified.
+///
+/// ## Why is this bad?
+/// Complex booleans affect readibility.
+///
+/// ## Example
+/// ```nix
+/// if !(x == y) then 0 else 1
+/// ```
+///
+/// Use `!=` instead:
+///
+/// ```nix
+/// if x != y then 0 else 1
+/// ```
+#[lint(
+    name = "bool_simplification",
+    note = "This boolean expression can be simplified",
+    code = 18,
+    match_with = SyntaxKind::NODE_UNARY_OP
+)]
+struct BoolSimplification;
+
+impl Rule for BoolSimplification {
+    fn validate(&self, node: &SyntaxElement, _sess: &SessionInfo) -> Option<Report> {
+        if_chain! {
+            if let NodeOrToken::Node(node) = node;
+            if let Some(unary_expr) = UnaryOp::cast(node.clone());
+            if unary_expr.operator() == UnaryOpKind::Invert;
+            if let Some(value_expr) = unary_expr.value();
+            if let Some(paren_expr) = Paren::cast(value_expr.clone());
+            if let Some(inner_expr) = paren_expr.inner();
+            if let Some(bin_expr) = BinOp::cast(inner_expr);
+            if let Some(BinOpKind::Equal) = bin_expr.operator();
+            then {
+                let at = node.text_range();
+                let message = "Try `!=` instead of `!(... == ...)`";
+
+                let lhs = bin_expr.lhs()?;
+                let rhs = bin_expr.rhs()?;
+                let replacement = make::binary(&lhs, "!=", &rhs).node().clone();
+                Some(
+                    self.report()
+                        .suggest(at, message, Suggestion::new(at, replacement)),
+                )
+            } else {
+                None
+            }
+        }
+    }
+}