1 files changed, 99 insertions, 0 deletions

diff --git a/lib/src/lints/useless_parens.rs b/lib/src/lints/useless_parens.rs
new file mode 100644
index 0000000..d93f06c
--- /dev/null
+++ b/lib/src/lints/useless_parens.rs
@@ -0,0 +1,99 @@
+use crate::{Lint, Metadata, Report, Rule, Suggestion, Diagnostic};
+
+use if_chain::if_chain;
+use macros::lint;
+use rnix::{
+    types::{ParsedType, KeyValue, Paren, TypedNode, Wrapper},
+    NodeOrToken, SyntaxElement, SyntaxKind,
+};
+
+#[lint(
+    name = "useless parens",
+    note = "These parentheses can be omitted",
+    code = 8,
+    match_with = [
+        SyntaxKind::NODE_KEY_VALUE,
+        SyntaxKind::NODE_PAREN,
+        SyntaxKind::NODE_LET_IN,
+    ]
+)]
+struct UselessParens;
+
+impl Rule for UselessParens {
+    fn validate(&self, node: &SyntaxElement) -> Option<Report> {
+        if_chain! {
+            if let NodeOrToken::Node(node) = node;
+            if let Some(parsed_type_node) = ParsedType::cast(node.clone());
+
+            if let Some(diagnostic) = do_thing(parsed_type_node);
+            then {
+                let mut report = Self::report();
+                report.diagnostics.push(diagnostic);
+                Some(report)
+            } else {
+                None
+            }
+        }
+    }
+}
+
+fn do_thing(parsed_type_node: ParsedType) -> Option<Diagnostic> {
+    match parsed_type_node {
+        ParsedType::KeyValue(kv) => if_chain! {
+            if let Some(value_node) = kv.value();
+            let value_range = value_node.text_range();
+            if let Some(value_in_parens) = Paren::cast(value_node);
+            if let Some(inner) = value_in_parens.inner();
+            then {
+                let at = value_range;
+                let message = "Useless parentheses around value in `let` binding";
+                let replacement = inner;
+                Some(Diagnostic::suggest(at, message, Suggestion::new(at, replacement)))
+            } else {
+                None
+            }
+        },
+        ParsedType::LetIn(let_in) => if_chain! {
+            if let Some(body_node) = let_in.body();
+            let body_range = body_node.text_range();
+            if let Some(body_as_parens) = Paren::cast(body_node);
+            if let Some(inner) = body_as_parens.inner();
+            then {
+                let at = body_range;
+                let message = "Useless parentheses around body of `let` expression";
+                let replacement = inner;
+                Some(Diagnostic::suggest(at, message, Suggestion::new(at, replacement)))
+            } else {
+                None
+            }
+        },
+        ParsedType::Paren(paren_expr) => if_chain! {
+            let paren_expr_range = paren_expr.node().text_range();
+            if let Some(father_node) = paren_expr.node().parent();
+
+            // ensure that we don't lint inside let-in statements
+            // we already lint such cases in previous match stmt
+            if KeyValue::cast(father_node).is_none();
+
+            if let Some(inner_node) = paren_expr.inner();
+            if let Some(parsed_inner) = ParsedType::cast(inner_node);
+            if matches!(
+                parsed_inner,
+                ParsedType::List(_) 
+                | ParsedType::Paren(_)
+                | ParsedType::Str(_)
+                | ParsedType::AttrSet(_)
+                | ParsedType::Select(_)
+            );
+            then {
+                let at = paren_expr_range;
+                let message = "Useless parentheses around primitive expression";
+                let replacement = parsed_inner.node().clone();
+                Some(Diagnostic::suggest(at, message, Suggestion::new(at, replacement)))
+            } else {
+                None
+            }
+        },
+        _ => None
+    }
+}