From 9c11fd264451a3d2b8617ee5e47e6db3fcb148d8 Mon Sep 17 00:00:00 2001
From: Tom French <15848336+TomAFrench@users.noreply.github.com>
Date: Fri, 28 Jun 2024 15:12:05 +0100
Subject: [PATCH] fix: ignore calls to `Intrinsic::AsWitness` during brillig
 codegen (#5350)

# Description

## Problem\*

Resolves <!-- Link to GitHub Issue -->

## Summary\*

This PR addresses an issue where if a call is made to `std::as_witness`
from a brillig runtime then the compiler will panic with the unreachable
error directly below where the new match arm is added.

## 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.
---
 .../src/brillig/brillig_gen/brillig_block.rs                | 5 +++++
 test_programs/execution_success/as_witness/Nargo.toml       | 6 ++++++
 test_programs/execution_success/as_witness/Prover.toml      | 1 +
 test_programs/execution_success/as_witness/src/main.nr      | 5 +++++
 4 files changed, 17 insertions(+)
 create mode 100644 test_programs/execution_success/as_witness/Nargo.toml
 create mode 100644 test_programs/execution_success/as_witness/Prover.toml
 create mode 100644 test_programs/execution_success/as_witness/src/main.nr

diff --git a/compiler/noirc_evaluator/src/brillig/brillig_gen/brillig_block.rs b/compiler/noirc_evaluator/src/brillig/brillig_gen/brillig_block.rs
index f10ff834f6c..c2bc1e32cd6 100644
--- a/compiler/noirc_evaluator/src/brillig/brillig_gen/brillig_block.rs
+++ b/compiler/noirc_evaluator/src/brillig/brillig_gen/brillig_block.rs
@@ -583,6 +583,11 @@ impl<'block> BrilligBlock<'block> {
                         1,
                     );
                 }
+
+                // `Intrinsic::AsWitness` is used to provide hints to acir-gen on optimal expression splitting.
+                // It is then useless in the brillig runtime and so we can ignore it
+                Value::Intrinsic(Intrinsic::AsWitness) => (),
+
                 _ => {
                     unreachable!("unsupported function call type {:?}", dfg[*func])
                 }
diff --git a/test_programs/execution_success/as_witness/Nargo.toml b/test_programs/execution_success/as_witness/Nargo.toml
new file mode 100644
index 00000000000..18f3f99b5b5
--- /dev/null
+++ b/test_programs/execution_success/as_witness/Nargo.toml
@@ -0,0 +1,6 @@
+[package]
+name = "as_witness"
+type = "bin"
+authors = [""]
+
+[dependencies]
diff --git a/test_programs/execution_success/as_witness/Prover.toml b/test_programs/execution_success/as_witness/Prover.toml
new file mode 100644
index 00000000000..cd8a5b5e03c
--- /dev/null
+++ b/test_programs/execution_success/as_witness/Prover.toml
@@ -0,0 +1 @@
+a = 42
\ No newline at end of file
diff --git a/test_programs/execution_success/as_witness/src/main.nr b/test_programs/execution_success/as_witness/src/main.nr
new file mode 100644
index 00000000000..a24f4af7669
--- /dev/null
+++ b/test_programs/execution_success/as_witness/src/main.nr
@@ -0,0 +1,5 @@
+// Simple example of checking where two arrays are different
+fn main(a: Field) -> pub Field {
+    std::as_witness(a);
+    a
+}