feat: add --check option to nargo fmt for dry-run formatting verification

Cargo.toml

@@ -118,6 +118,7 @@ hex = "0.4.2"
 const_format = "0.2.30"
 num-bigint = "0.4"
 num-traits = "0.2"
+similar-asserts = "1.5.0"
 
 [profile.dev]
 # This is required to be able to run `cargo test` in acvm_js due to the `locals exceeds maximum` error.

tooling/nargo_cli/Cargo.toml

@@ -43,6 +43,7 @@ tower.workspace = true
 async-lsp = { workspace = true, features = ["client-monitor", "stdio", "tracing", "tokio"] }
 const_format.workspace = true
 hex.workspace = true
+similar-asserts.workspace = true
 termcolor = "1.1.2"
 color-eyre = "0.6.2"
 tokio = { version = "1.0", features = ["io-std"] }

tooling/nargo_cli/src/cli/fmt_cmd.rs

@@ -13,9 +13,15 @@
 
 /// Format the Noir files in a workspace
 #[derive(Debug, Clone, Args)]
-pub(crate) struct FormatCommand {}
+pub(crate) struct FormatCommand {
+    /// Run noirfmt in check mode
+    #[arg(long)]
+    check: bool,
+}
+
+pub(crate) fn run(args: FormatCommand, config: NargoConfig) -> Result<(), CliError> {
+    let check_mode = args.check;
 
-pub(crate) fn run(_args: FormatCommand, config: NargoConfig) -> Result<(), CliError> {
     let toml_path = get_package_manifest(&config.program_dir)?;
     let workspace = resolve_workspace_from_toml(
         &toml_path,
@@ -26,6 +32,8 @@
     let config = nargo_fmt::Config::read(&config.program_dir)
         .map_err(|err| CliError::Generic(err.to_string()))?;
 
+    let mut check_exit_code_one = false;
+
     for package in &workspace {
         let mut file_manager =
             FileManager::new(&package.root_dir, Box::new(|path| std::fs::read_to_string(path)));
@@ -53,16 +61,40 @@
                 return Ok(());
             }
 
-            let source = nargo_fmt::format(
-                file_manager.fetch_file(file_id).source(),
-                parsed_module,
-                &config,
-            );
+            let original = file_manager.fetch_file(file_id).source();
+            let formatted = nargo_fmt::format(original, parsed_module, &config);
+
+            if check_mode {
+                let diff = similar_asserts::SimpleDiff::from_str(
+                    original,
+                    &formatted,
+                    "original",
+                    "formatted",
+                )
+                .to_string();
+
+                if !diff.lines().next().is_some_and(|line| line.contains("Invisible differences")) {
+                    if !check_exit_code_one {
+                        check_exit_code_one = true;
+                    }
 
-            std::fs::write(entry.path(), source)
+                    println!("{diff}");
+                }
+
+                Ok(())
+            } else {
+                std::fs::write(entry.path(), formatted)
+            }
         })
         .map_err(|error| CliError::Generic(error.to_string()))?;
     }
+
+    if check_exit_code_one {
+        std::process::exit(1);
+    } else if check_mode {
+        println!("No formatting changes were detected");
+    }
+
     Ok(())
 }
 

tooling/nargo_fmt/Cargo.toml

@@ -13,4 +13,4 @@ toml.workspace = true
 thiserror.workspace = true
 
 [dev-dependencies]
-similar-asserts = "1.5.0"
+similar-asserts.workspace = true