From a61ccb01e6b35605e80e3cfbee65a638cea4d4a4 Mon Sep 17 00:00:00 2001
From: jfecher <jake@aztecprotocol.com>
Date: Tue, 26 Mar 2024 14:40:00 -0500
Subject: [PATCH] fix: Slice coercions (#4640)

# Description

## Problem\*

Resolves #4639

## Summary\*

The array to slice coercion was broken - and it seemingly has always
been so. The coercion never actually called `replace_expr` to replace
the original expression with the `as_slice(original_expr)` coercion.

## Additional Context



## Documentation\*

Check one:
- [x] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist\*

- [x] I have tested the changes locally.
- [x] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.
---
 compiler/noirc_frontend/src/hir_def/types.rs  | 15 ++++++++++-----
 .../slice_coercion/Nargo.toml                 |  7 +++++++
 .../slice_coercion/Prover.toml                |  2 ++
 .../slice_coercion/src/main.nr                | 19 +++++++++++++++++++
 4 files changed, 38 insertions(+), 5 deletions(-)
 create mode 100644 test_programs/execution_success/slice_coercion/Nargo.toml
 create mode 100644 test_programs/execution_success/slice_coercion/Prover.toml
 create mode 100644 test_programs/execution_success/slice_coercion/src/main.nr

diff --git a/compiler/noirc_frontend/src/hir_def/types.rs b/compiler/noirc_frontend/src/hir_def/types.rs
index 3c5627f739b..65592407d7d 100644
--- a/compiler/noirc_frontend/src/hir_def/types.rs
+++ b/compiler/noirc_frontend/src/hir_def/types.rs
@@ -1662,14 +1662,19 @@ fn convert_array_expression_to_slice(
     let as_slice = HirExpression::Ident(HirIdent::non_trait_method(as_slice_id, location));
     let func = interner.push_expr(as_slice);
 
-    let arguments = vec![expression];
+    // Copy the expression and give it a new ExprId. The old one
+    // will be mutated in place into a Call expression.
+    let argument = interner.expression(&expression);
+    let argument = interner.push_expr(argument);
+    interner.push_expr_type(argument, array_type.clone());
+    interner.push_expr_location(argument, location.span, location.file);
+
+    let arguments = vec![argument];
     let call = HirExpression::Call(HirCallExpression { func, arguments, location });
-    let call = interner.push_expr(call);
+    interner.replace_expr(&expression, call);
 
-    interner.push_expr_location(call, location.span, location.file);
     interner.push_expr_location(func, location.span, location.file);
-
-    interner.push_expr_type(call, target_type.clone());
+    interner.push_expr_type(expression, target_type.clone());
 
     let func_type = Type::Function(vec![array_type], Box::new(target_type), Box::new(Type::Unit));
     interner.push_expr_type(func, func_type);
diff --git a/test_programs/execution_success/slice_coercion/Nargo.toml b/test_programs/execution_success/slice_coercion/Nargo.toml
new file mode 100644
index 00000000000..659677cc560
--- /dev/null
+++ b/test_programs/execution_success/slice_coercion/Nargo.toml
@@ -0,0 +1,7 @@
+[package]
+name = "slice_coercion"
+type = "bin"
+authors = [""]
+compiler_version = ">=0.25.0"
+
+[dependencies]
diff --git a/test_programs/execution_success/slice_coercion/Prover.toml b/test_programs/execution_success/slice_coercion/Prover.toml
new file mode 100644
index 00000000000..09c44a855b0
--- /dev/null
+++ b/test_programs/execution_success/slice_coercion/Prover.toml
@@ -0,0 +1,2 @@
+first = 3
+expected = 3
diff --git a/test_programs/execution_success/slice_coercion/src/main.nr b/test_programs/execution_success/slice_coercion/src/main.nr
new file mode 100644
index 00000000000..a3785e79afa
--- /dev/null
+++ b/test_programs/execution_success/slice_coercion/src/main.nr
@@ -0,0 +1,19 @@
+struct Hasher {
+    fields: [Field],
+}
+
+impl Hasher {
+    pub fn new() -> Self {
+        Self { fields: [] }
+    }
+
+    pub fn add(&mut self, field: Field) {
+        self.fields = self.fields.push_back(field);
+    }
+}
+
+fn main(expected: pub Field, first: Field) {
+    let mut hasher = Hasher::new();
+    hasher.add(first);
+    assert(hasher.fields[0] == expected);
+}