chore(ci): add mocked backend binary to improve `compile_success_empt…

…y` tests (noir-lang#2554)

crates/acvm_backend_barretenberg/src/lib.rs

@@ -17,7 +17,7 @@ pub fn backends_directory() -> PathBuf {
 
 #[cfg(test)]
 fn get_bb() -> Backend {
-    let bb = Backend::default();
+    let bb = Backend::new("acvm-backend-barretenberg".to_string());
     crate::assert_binary_exists(&bb);
     bb
 }
@@ -36,12 +36,6 @@ pub struct Backend {
     name: String,
 }
 
-impl Default for Backend {
-    fn default() -> Self {
-        Self { name: "acvm-backend-barretenberg".to_string() }
-    }
-}
-
 impl Backend {
     pub fn new(name: String) -> Backend {
         Backend { name }
@@ -52,9 +46,13 @@ impl Backend {
     }
 
     fn binary_path(&self) -> PathBuf {
-        const BINARY_NAME: &str = "backend_binary";
+        if let Some(binary_path) = std::env::var_os("NARGO_BACKEND_PATH") {
+            PathBuf::from(binary_path)
+        } else {
+            const BINARY_NAME: &str = "backend_binary";
 
-        self.backend_directory().join(BINARY_NAME)
+            self.backend_directory().join(BINARY_NAME)
+        }
     }
 }
 

crates/nargo_cli/Cargo.toml

@@ -54,6 +54,7 @@ tokio-util = { version = "0.7.8", features = ["compat"] }
 
 [dev-dependencies]
 tempdir = "0.3.7"
+dirs.workspace = true
 assert_cmd = "2.0.8"
 assert_fs = "1.0.10"
 predicates = "2.1.5"
@@ -66,6 +67,7 @@ pprof = { version = "0.12", features = [
     "criterion",
 ] }
 iai = "0.1.1"
+test-binary = "3.0.1"
 
 [[bench]]
 name = "criterion"

crates/nargo_cli/build.rs

@@ -104,15 +104,23 @@ fn generate_compile_success_empty_tests(test_file: &mut File, test_data_dir: &Pa
             r#"
 #[test]
 fn compile_success_empty_{test_name}() {{
-    let test_program_dir = PathBuf::from("{test_dir}");
+    
+    // We use a mocked backend for this test as we do not rely on the returned circuit size
+    // but we must call a backend as part of querying the number of opcodes in the circuit.
 
+    let test_program_dir = PathBuf::from("{test_dir}");
     let mut cmd = Command::cargo_bin("nargo").unwrap();
+    cmd.env("NARGO_BACKEND_PATH", path_to_mock_backend());
     cmd.arg("--program-dir").arg(test_program_dir);
     cmd.arg("info");
     cmd.arg("--json");
 
     let output = cmd.output().expect("Failed to execute command");
-    
+
+    if !output.status.success() {{
+        panic!("`nargo info` failed with: {{}}", String::from_utf8(output.stderr).unwrap());
+    }}
+
     // `compile_success_empty` tests should be able to compile down to an empty circuit.
     let json: serde_json::Value = serde_json::from_slice(&output.stdout).expect("JSON was not well-formatted");
     let num_opcodes = &json["programs"][0]["acir_opcodes"];

crates/nargo_cli/test-binaries/mock_backend/Cargo.toml

@@ -0,0 +1,10 @@
+[workspace]
+
+[package]
+name = "mock_backend"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

crates/nargo_cli/test-binaries/mock_backend/src/main.rs

@@ -0,0 +1,10 @@
+#![forbid(unsafe_code)]
+#![warn(unreachable_pub)]
+#![warn(clippy::semicolon_if_nothing_returned)]
+#![cfg_attr(not(test), warn(unused_crate_dependencies, unused_extern_crates))]
+
+use std::io::Write;
+
+fn main() {
+    std::io::stdout().write_all(&0u64.to_be_bytes()).unwrap();
+}

crates/nargo_cli/tests/execute.rs

@@ -13,6 +13,8 @@ mod tests {
 
     use super::*;
 
+    test_binary::build_test_binary_once!(mock_backend, "test-binaries");
+
     // include tests generated by `build.rs`
     include!(concat!(env!("OUT_DIR"), "/execute.rs"));
 }