feat: remove query to backend to get expression width (#4975)

# Description

## Problem\*

Progress on #4961 

## Summary\*

This PR removes the query to the backend in order to get the expression
width, for now we just have a blanket default of ~4~ 3 (Looks like we're
still waiting to update to a version of `bb` which uses 4). I've decided
to split the ability to specify a package-level override of this default
as I'd need to do some mucky stuff related to `compile_options`.

## 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.
We don't have docs related to the expression width afaik.
# 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_driver/src/lib.rs

@@ -54,8 +54,8 @@ pub const NOIR_ARTIFACT_VERSION_STRING: &str =
 #[derive(Args, Clone, Debug, Default)]
 pub struct CompileOptions {
     /// Override the expression width requested by the backend.
-    #[arg(long, value_parser = parse_expression_width)]
-    pub expression_width: Option<ExpressionWidth>,
+    #[arg(long, value_parser = parse_expression_width, default_value = "3")]
+    pub expression_width: ExpressionWidth,
 
     /// Force a full recompilation.
     #[arg(long = "force")]

tooling/backend_interface/Cargo.toml

@@ -13,7 +13,6 @@ license.workspace = true
 acvm.workspace = true
 dirs.workspace = true
 thiserror.workspace = true
-serde.workspace = true
 serde_json.workspace = true
 bb_abstraction_leaks.workspace = true
 tracing.workspace = true

tooling/backend_interface/src/cli/mod.rs

@@ -2,7 +2,6 @@
 
 mod contract;
 mod gates;
-mod info;
 mod proof_as_fields;
 mod prove;
 mod verify;
@@ -12,7 +11,6 @@ mod write_vk;
 
 pub(crate) use contract::ContractCommand;
 pub(crate) use gates::GatesCommand;
-pub(crate) use info::InfoCommand;
 pub(crate) use proof_as_fields::ProofAsFieldsCommand;
 pub(crate) use prove::ProveCommand;
 pub(crate) use verify::VerifyCommand;

tooling/backend_interface/src/proof_system.rs

@@ -3,16 +3,16 @@ use std::io::Write;
 use std::path::Path;
 
 use acvm::acir::{
-    circuit::{ExpressionWidth, Program},
+    circuit::Program,
     native_types::{WitnessMap, WitnessStack},
 };
 use acvm::FieldElement;
 use tempfile::tempdir;
 use tracing::warn;
 
 use crate::cli::{
-    GatesCommand, InfoCommand, ProofAsFieldsCommand, ProveCommand, VerifyCommand,
-    VkAsFieldsCommand, WriteVkCommand,
+    GatesCommand, ProofAsFieldsCommand, ProveCommand, VerifyCommand, VkAsFieldsCommand,
+    WriteVkCommand,
 };
 use crate::{Backend, BackendError};
 
@@ -33,25 +33,6 @@ impl Backend {
             .run(binary_path)
     }
 
-    pub fn get_backend_info(&self) -> Result<ExpressionWidth, BackendError> {
-        let binary_path = self.assert_binary_exists()?;
-        self.assert_correct_version()?;
-        InfoCommand { crs_path: self.crs_directory() }.run(binary_path)
-    }
-
-    /// If we cannot get a valid backend, returns `ExpressionWidth::Bound { width: 4 }``
-    /// The function also prints a message saying we could not find a backend
-    pub fn get_backend_info_or_default(&self) -> ExpressionWidth {
-        if let Ok(expression_width) = self.get_backend_info() {
-            expression_width
-        } else {
-            warn!(
-                "No valid backend found, ExpressionWidth defaulting to Bounded with a width of 4"
-            );
-            ExpressionWidth::Bounded { width: 4 }
-        }
-    }
-
     #[tracing::instrument(level = "trace", skip_all)]
     pub fn prove(
         &self,

tooling/backend_interface/test-binaries/mock_backend/src/main.rs

@@ -7,7 +7,6 @@ use clap::{Parser, Subcommand};
 
 mod contract_cmd;
 mod gates_cmd;
-mod info_cmd;
 mod prove_cmd;
 mod verify_cmd;
 mod write_vk_cmd;
@@ -21,7 +20,6 @@ struct BackendCli {
 
 #[derive(Subcommand, Clone, Debug)]
 enum BackendCommand {
-    Info(info_cmd::InfoCommand),
     Contract(contract_cmd::ContractCommand),
     Gates(gates_cmd::GatesCommand),
     Prove(prove_cmd::ProveCommand),
@@ -34,7 +32,6 @@ fn main() {
     let BackendCli { command } = BackendCli::parse();
 
     match command {
-        BackendCommand::Info(args) => info_cmd::run(args),
         BackendCommand::Contract(args) => contract_cmd::run(args),
         BackendCommand::Gates(args) => gates_cmd::run(args),
         BackendCommand::Prove(args) => prove_cmd::run(args),

tooling/nargo_cli/src/cli/check_cmd.rs

@@ -1,4 +1,3 @@
-use crate::backends::Backend;
 use crate::errors::CliError;
 
 use clap::Args;
@@ -42,11 +41,7 @@ pub(crate) struct CheckCommand {
     compile_options: CompileOptions,
 }
 
-pub(crate) fn run(
-    _backend: &Backend,
-    args: CheckCommand,
-    config: NargoConfig,
-) -> Result<(), CliError> {
+pub(crate) fn run(args: CheckCommand, config: NargoConfig) -> Result<(), CliError> {
     let toml_path = get_package_manifest(&config.program_dir)?;
     let default_selection =
         if args.workspace { PackageSelection::All } else { PackageSelection::DefaultOrAll };

tooling/nargo_cli/src/cli/codegen_verifier_cmd.rs

@@ -44,7 +44,6 @@ pub(crate) fn run(
     insert_all_files_for_workspace_into_file_manager(&workspace, &mut workspace_file_manager);
     let parsed_files = parse_all(&workspace_file_manager);
 
-    let expression_width = backend.get_backend_info()?;
     let binary_packages = workspace.into_iter().filter(|package| package.is_binary());
     for package in binary_packages {
         let compilation_result = compile_program(
@@ -62,7 +61,7 @@ pub(crate) fn run(
             args.compile_options.silence_warnings,
         )?;
 
-        let program = nargo::ops::transform_program(program, expression_width);
+        let program = nargo::ops::transform_program(program, args.compile_options.expression_width);
 
         // TODO(https://github.com/noir-lang/noir/issues/4428):
         // We do not expect to have a smart contract verifier for a foldable program with multiple circuits.

tooling/nargo_cli/src/cli/compile_cmd.rs

@@ -20,7 +20,6 @@ use noirc_frontend::hir::ParsedFiles;
 use notify::{EventKind, RecursiveMode, Watcher};
 use notify_debouncer_full::new_debouncer;
 
-use crate::backends::Backend;
 use crate::errors::CliError;
 
 use super::fs::program::only_acir;
@@ -47,11 +46,7 @@ pub(crate) struct CompileCommand {
     watch: bool,
 }
 
-pub(crate) fn run(
-    backend: &Backend,
-    mut args: CompileCommand,
-    config: NargoConfig,
-) -> Result<(), CliError> {
+pub(crate) fn run(args: CompileCommand, config: NargoConfig) -> Result<(), CliError> {
     let toml_path = get_package_manifest(&config.program_dir)?;
     let default_selection =
         if args.workspace { PackageSelection::All } else { PackageSelection::DefaultOrAll };
@@ -63,10 +58,6 @@ pub(crate) fn run(
         Some(NOIR_ARTIFACT_VERSION_STRING.to_owned()),
     )?;
 
-    if args.compile_options.expression_width.is_none() {
-        args.compile_options.expression_width = Some(backend.get_backend_info_or_default());
-    };
-
     if args.watch {
         watch_workspace(&workspace, &args.compile_options)
             .map_err(|err| CliError::Generic(err.to_string()))?;
@@ -128,8 +119,6 @@ fn compile_workspace_full(
     insert_all_files_for_workspace_into_file_manager(workspace, &mut workspace_file_manager);
     let parsed_files = parse_all(&workspace_file_manager);
 
-    let expression_width =
-        compile_options.expression_width.expect("expression width should have been set");
     let compiled_workspace =
         compile_workspace(&workspace_file_manager, &parsed_files, workspace, compile_options);
 
@@ -149,12 +138,12 @@ fn compile_workspace_full(
     // Save build artifacts to disk.
     let only_acir = compile_options.only_acir;
     for (package, program) in binary_packages.into_iter().zip(compiled_programs) {
-        let program = nargo::ops::transform_program(program, expression_width);
+        let program = nargo::ops::transform_program(program, compile_options.expression_width);
         save_program(program.clone(), &package, &workspace.target_directory_path(), only_acir);
     }
     let circuit_dir = workspace.target_directory_path();
     for (package, contract) in contract_packages.into_iter().zip(compiled_contracts) {
-        let contract = nargo::ops::transform_contract(contract, expression_width);
+        let contract = nargo::ops::transform_contract(contract, compile_options.expression_width);
         save_contract(contract, &package, &circuit_dir);
     }
 

tooling/nargo_cli/src/cli/dap_cmd.rs

@@ -1,6 +1,5 @@
 use acvm::acir::circuit::ExpressionWidth;
 use acvm::acir::native_types::WitnessMap;
-use backend_interface::Backend;
 use clap::Args;
 use nargo::constants::PROVER_INPUT_FILE;
 use nargo::workspace::Workspace;
@@ -29,8 +28,8 @@ use noir_debugger::errors::{DapError, LoadError};
 #[derive(Debug, Clone, Args)]
 pub(crate) struct DapCommand {
     /// Override the expression width requested by the backend.
-    #[arg(long, value_parser = parse_expression_width)]
-    expression_width: Option<ExpressionWidth>,
+    #[arg(long, value_parser = parse_expression_width, default_value = "3")]
+    expression_width: ExpressionWidth,
 
     #[clap(long)]
     preflight_check: bool,
@@ -249,14 +248,7 @@ fn run_preflight_check(
     Ok(())
 }
 
-pub(crate) fn run(
-    backend: &Backend,
-    args: DapCommand,
-    _config: NargoConfig,
-) -> Result<(), CliError> {
-    let expression_width =
-        args.expression_width.unwrap_or_else(|| backend.get_backend_info_or_default());
-
+pub(crate) fn run(args: DapCommand, _config: NargoConfig) -> Result<(), CliError> {
     // When the --preflight-check flag is present, we run Noir's DAP server in "pre-flight mode", which test runs
     // the DAP initialization code without actually starting the DAP server.
     //
@@ -270,12 +262,12 @@ pub(crate) fn run(
     // the DAP loop is established, which otherwise are considered "out of band" by the maintainers of the DAP spec.
     // More details here: https://github.com/microsoft/vscode/issues/108138
     if args.preflight_check {
-        return run_preflight_check(expression_width, args).map_err(CliError::DapError);
+        return run_preflight_check(args.expression_width, args).map_err(CliError::DapError);
     }
 
     let output = BufWriter::new(std::io::stdout());
     let input = BufReader::new(std::io::stdin());
     let server = Server::new(input, output);
 
-    loop_uninitialized_dap(server, expression_width).map_err(CliError::DapError)
+    loop_uninitialized_dap(server, args.expression_width).map_err(CliError::DapError)
 }

tooling/nargo_cli/src/cli/debug_cmd.rs

@@ -24,7 +24,6 @@ use noirc_frontend::hir::ParsedFiles;
 
 use super::fs::{inputs::read_inputs_from_file, witness::save_witness_to_dir};
 use super::NargoConfig;
-use crate::backends::Backend;
 use crate::errors::CliError;
 
 /// Executes a circuit in debug mode
@@ -53,11 +52,7 @@ pub(crate) struct DebugCommand {
     skip_instrumentation: Option<bool>,
 }
 
-pub(crate) fn run(
-    backend: &Backend,
-    args: DebugCommand,
-    config: NargoConfig,
-) -> Result<(), CliError> {
+pub(crate) fn run(args: DebugCommand, config: NargoConfig) -> Result<(), CliError> {
     let acir_mode = args.acir_mode;
     let skip_instrumentation = args.skip_instrumentation.unwrap_or(acir_mode);
 
@@ -69,10 +64,6 @@ pub(crate) fn run(
         Some(NOIR_ARTIFACT_VERSION_STRING.to_string()),
     )?;
     let target_dir = &workspace.target_directory_path();
-    let expression_width = args
-        .compile_options
-        .expression_width
-        .unwrap_or_else(|| backend.get_backend_info_or_default());
 
     let Some(package) = workspace.into_iter().find(|p| p.is_binary()) else {
         println!(
@@ -89,7 +80,8 @@ pub(crate) fn run(
         args.compile_options.clone(),
     )?;
 
-    let compiled_program = nargo::ops::transform_program(compiled_program, expression_width);
+    let compiled_program =
+        nargo::ops::transform_program(compiled_program, args.compile_options.expression_width);
 
     run_async(package, compiled_program, &args.prover_name, &args.witness_name, target_dir)
 }