feat(natu): detect single-use let bindings

src/natu/rules/single_use_binding.rs

@@ -0,0 +1,45 @@
+use crate::{
+    ast,
+    scope_graph::{Binding, LetScope, ScopeGraph},
+};
+
+pub struct Finding {
+    pub range: ast::SourceRange,
+    pub name: String,
+}
+
+pub fn check(graph: &ScopeGraph) -> Vec<Finding> {
+    graph
+        .let_scopes()
+        .iter()
+        .flat_map(|let_scope| {
+            let_scope.bindings.iter().filter_map(|binding| {
+                is_single_use(graph, let_scope, binding).then(|| Finding {
+                    range: binding.range,
+                    name: binding.name.clone(),
+                })
+            })
+        })
+        .collect()
+}
+
+fn is_single_use(graph: &ScopeGraph, let_scope: &LetScope, binding: &Binding) -> bool {
+    let self_references = binding
+        .references
+        .iter()
+        .filter(|reference| graph.resolve(**reference).contains(&binding.declaration))
+        .count();
+    let external_references = let_scope
+        .bindings
+        .iter()
+        .filter(|candidate| candidate.declaration != binding.declaration)
+        .flat_map(|candidate| candidate.references.iter())
+        .chain(let_scope.entry_references.iter())
+        .filter(|reference| graph.resolve(**reference).contains(&binding.declaration))
+        .collect::<Vec<_>>();
+
+    self_references == 0
+        && external_references.len() == 1
+        && (binding.is_lambda
+            || !graph.crosses_lambda_scope(*external_references[0], binding.declaration))
+}