From 8270cac39d23f65ca5e2462746a0760662c4af7a Mon Sep 17 00:00:00 2001
From: jfecher <jake@aztecprotocol.com>
Date: Tue, 10 Oct 2023 12:33:54 -0500
Subject: [PATCH] feat: Implement automatic dereferencing for index expressions
 (#3082)

---
 .../noirc_frontend/src/hir/type_check/expr.rs    | 16 +++++++++++++---
 .../compile_success_empty/auto_deref/Nargo.toml  |  7 +++++++
 .../compile_success_empty/auto_deref/src/main.nr |  5 +++++
 3 files changed, 25 insertions(+), 3 deletions(-)
 create mode 100644 tooling/nargo_cli/tests/compile_success_empty/auto_deref/Nargo.toml
 create mode 100644 tooling/nargo_cli/tests/compile_success_empty/auto_deref/src/main.nr

diff --git a/compiler/noirc_frontend/src/hir/type_check/expr.rs b/compiler/noirc_frontend/src/hir/type_check/expr.rs
index 76a0cca4758..3b61f2b2b6b 100644
--- a/compiler/noirc_frontend/src/hir/type_check/expr.rs
+++ b/compiler/noirc_frontend/src/hir/type_check/expr.rs
@@ -128,7 +128,7 @@ impl<'interner> TypeChecker<'interner> {
                         Type::Error
                     })
             }
-            HirExpression::Index(index_expr) => self.check_index_expression(index_expr),
+            HirExpression::Index(index_expr) => self.check_index_expression(expr_id, index_expr),
             HirExpression::Call(call_expr) => {
                 self.check_if_deprecated(&call_expr.func);
 
@@ -396,7 +396,11 @@ impl<'interner> TypeChecker<'interner> {
         }
     }
 
-    fn check_index_expression(&mut self, index_expr: expr::HirIndexExpression) -> Type {
+    fn check_index_expression(
+        &mut self,
+        id: &ExprId,
+        mut index_expr: expr::HirIndexExpression,
+    ) -> Type {
         let index_type = self.check_expression(&index_expr.index);
         let span = self.interner.expr_span(&index_expr.index);
 
@@ -408,14 +412,20 @@ impl<'interner> TypeChecker<'interner> {
             }
         });
 
+        // When writing `a[i]`, if `a : &mut ...` then automatically dereference `a` as many
+        // times as needed to get the underlying array.
         let lhs_type = self.check_expression(&index_expr.collection);
+        let (new_lhs, lhs_type) = self.insert_auto_dereferences(index_expr.collection, lhs_type);
+        index_expr.collection = new_lhs;
+        self.interner.replace_expr(id, HirExpression::Index(index_expr));
+
         match lhs_type.follow_bindings() {
             // XXX: We can check the array bounds here also, but it may be better to constant fold first
             // and have ConstId instead of ExprId for constants
             Type::Array(_, base_type) => *base_type,
             Type::Error => Type::Error,
             typ => {
-                let span = self.interner.expr_span(&index_expr.collection);
+                let span = self.interner.expr_span(&new_lhs);
                 self.errors.push(TypeCheckError::TypeMismatch {
                     expected_typ: "Array".to_owned(),
                     expr_typ: typ.to_string(),
diff --git a/tooling/nargo_cli/tests/compile_success_empty/auto_deref/Nargo.toml b/tooling/nargo_cli/tests/compile_success_empty/auto_deref/Nargo.toml
new file mode 100644
index 00000000000..cc1c9a8d8f7
--- /dev/null
+++ b/tooling/nargo_cli/tests/compile_success_empty/auto_deref/Nargo.toml
@@ -0,0 +1,7 @@
+[package]
+name = "auto_deref"
+type = "bin"
+authors = [""]
+compiler_version = "0.16.0"
+
+[dependencies]
\ No newline at end of file
diff --git a/tooling/nargo_cli/tests/compile_success_empty/auto_deref/src/main.nr b/tooling/nargo_cli/tests/compile_success_empty/auto_deref/src/main.nr
new file mode 100644
index 00000000000..1225885d765
--- /dev/null
+++ b/tooling/nargo_cli/tests/compile_success_empty/auto_deref/src/main.nr
@@ -0,0 +1,5 @@
+
+fn main() {
+    let a = &mut [1, 2, 3];
+    assert(a[0] == 1);
+}