chore: Save witness in correct format that bb.js expects (#1876)

* save witness in correct format that bb.js expects and updates acvm-backend-barretenberg dep to show all old CI passing

* add feature flag for witness writing

* rename line

* use one flat witness feature flag

* fix flat_witness feature flag

* switch between witness map serialization feature flag on a feature flag

Cargo.toml

@@ -58,3 +58,4 @@ wasm-bindgen-test = "0.3.33"
 
 [patch.crates-io]
 async-lsp = { git = "https://github.com/oxalica/async-lsp", rev = "09dbcc11046f7a188a80137f8d36484d86c78c78" }
+acvm-backend-barretenberg = { git = "https://github.com/noir-lang/acvm-backend-barretenberg.git", rev = "0842911beed6c54b7efcd721372fb73431c95bbf" }

crates/nargo_cli/Cargo.toml

@@ -56,3 +56,4 @@ default = ["plonk_bn254"]
 # The plonk backend can only use bn254, so we do not specify the field
 plonk_bn254 = ["acvm-backend-barretenberg/native"]
 plonk_bn254_wasm = ["acvm-backend-barretenberg/wasm"]
+flat_witness = ["acvm-backend-barretenberg/native"]

crates/nargo_cli/src/backends.rs

@@ -1,6 +1,6 @@
 pub(crate) use acvm_backend_barretenberg::Barretenberg as ConcreteBackend;
 
-#[cfg(not(any(feature = "plonk_bn254", feature = "plonk_bn254_wasm")))]
+#[cfg(not(any(feature = "plonk_bn254", feature = "plonk_bn254_wasm", feature = "flat_witness")))]
 compile_error!("please specify a backend to compile with");
 
 #[cfg(all(feature = "plonk_bn254", feature = "plonk_bn254_wasm"))]

crates/nargo_cli/src/cli/fs/witness.rs

@@ -6,16 +6,31 @@ use super::{create_named_dir, write_to_file};
 use crate::{constants::WITNESS_EXT, errors::FilesystemError};
 
 pub(crate) fn save_witness_to_dir<P: AsRef<Path>>(
-    witness: WitnessMap,
+    witnesses: WitnessMap,
     witness_name: &str,
     witness_dir: P,
 ) -> Result<PathBuf, FilesystemError> {
     create_named_dir(witness_dir.as_ref(), "witness");
     let witness_path = witness_dir.as_ref().join(witness_name).with_extension(WITNESS_EXT);
 
-    let buf: Vec<u8> = witness.try_into()?;
+    let buf: Vec<u8> = serialize_witness_map(witnesses)?;
 
     write_to_file(buf.as_slice(), &witness_path);
 
     Ok(witness_path)
 }
+
+#[cfg(not(feature = "flat_witness"))]
+fn serialize_witness_map(witnesses: WitnessMap) -> Result<Vec<u8>, FilesystemError> {
+    let buf: Vec<u8> = witnesses.try_into()?;
+    Ok(buf)
+}
+
+#[cfg(feature = "flat_witness")]
+fn serialize_witness_map(witnesses: WitnessMap) -> Result<Vec<u8>, FilesystemError> {
+    let mut buf: Vec<u8> = Vec::new();
+    for (_, value) in witnesses {
+        buf.extend_from_slice(&value.to_be_bytes());
+    }
+    Ok(buf)
+}