feat(scaffold): emit canonical rustfmt.toml (max_width 120)

poly's Rust formatter now defers to rustfmt's own config discovery, so an
explicit rustfmt.toml pins the width both poly and cargo fmt use. Every
alef-managed repo standardizes on 120 to match poly's global line_length.

Author: Goldziher

CHANGELOG.md

@@ -7,6 +7,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- **scaffold**: the poly scaffold now also emits a canonical repo-root `rustfmt.toml`
+  (`max_width = 120`, alef-managed). poly's Rust formatter defers to rustfmt's own
+  config discovery (matching `cargo fmt`), so this pins the width both tools use;
+  without it rustfmt falls back to its 100 default. Every alef-managed repo
+  standardizes on 120 to match poly's global `line_length` default.
+
 ## [0.30.11] - 2026-07-02
 
 ### Added

src/scaffold/languages/poly.rs

@@ -254,9 +254,21 @@ pub(crate) fn scaffold_poly_config(config: &ResolvedCrateConfig, languages: &[La
         ));
     }
 
-    vec![GeneratedFile {
-        path: PathBuf::from("poly.toml"),
-        content: out,
-        generated_header: true,
-    }]
+    // Canonical rustfmt config. poly's Rust formatter defers to rustfmt's own
+    // config discovery (matching `cargo fmt`), so an explicit `rustfmt.toml`
+    // pins the width both tools use. Without it rustfmt falls back to its 100
+    // default; every alef repo standardizes on 120 to match poly's global
+    // `line_length` default and stay consistent across the polyglot ecosystem.
+    vec![
+        GeneratedFile {
+            path: PathBuf::from("poly.toml"),
+            content: out,
+            generated_header: true,
+        },
+        GeneratedFile {
+            path: PathBuf::from("rustfmt.toml"),
+            content: "max_width = 120\n".to_string(),
+            generated_header: true,
+        },
+    ]
 }

src/scaffold/tests/poly.rs

@@ -77,6 +77,24 @@ fn emits_a_generated_poly_toml_replacing_precommit() {
     );
 }
 
+#[test]
+fn emits_a_canonical_rustfmt_toml_at_width_120() {
+    let config = test_config();
+    let api = test_api();
+    let files = scaffold(&api, &config, &[Language::Python, Language::Node]).unwrap();
+
+    let rustfmt = files
+        .iter()
+        .find(|f| f.path.to_string_lossy() == "rustfmt.toml")
+        .expect("scaffold should emit a repo-root rustfmt.toml");
+    assert!(rustfmt.generated_header, "rustfmt.toml must be alef-managed");
+    assert!(
+        rustfmt.content.contains("max_width = 120"),
+        "rustfmt.toml must pin width 120 (poly defers to rustfmt discovery); got {:?}",
+        rustfmt.content
+    );
+}
+
 #[test]
 fn poly_toml_drives_hooks_builtins_and_excludes() {
     let config = test_config();