Our CI started failing at the formatting check step after updating to 1.7.0-nightly (9a66e4471 2023-11-19); however, when run locally cargo +nightly fmt --check passes. I verified that they both are using the same version of rustfmt, the only differences being the platform/target.

CI: nightly-x86_64-unknown-linux-gnu installed - rustc 1.76.0-nightly (9a66e4471 2023-11-19)
Local: nightly-aarch64-apple-darwin - Up to date : 1.76.0-nightly (9a66e4471 2023-11-19)

Note that rustfmt 1.7.0-nightly (28317017 2023-11-17) produces the same output regardless of platform, so I think that the issue only exists in the most recent nightly (rustfmt 1.7.0-nightly (9a66e447 2023-11-19)) on the nightly-aarch64-apple-darwin target.

Example

CI Output (nightly-x86_64-unknown-linux-gnu)

use serde_json::json;

fn main() {
    let map = json!({ "key": "value" });

    let _value: String = map
        .get("key")
        .map(|v| v.as_str())
        .flatten()
        .unwrap_or_default()
        .to_string();
}

Local Output (nightly-aarch64-apple-darwin)

use serde_json::json;

fn main() {
    let map = json!({ "key": "value" });

    let _value: String =
        map.get("key")
            .map(|v| v.as_str())
            .flatten()
            .unwrap_or_default()
            .to_string();
}

Output Diff

diff --git a/src/main.rs b/src/main.rs
index 789b018..771d6ca 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -3,10 +3,10 @@ use serde_json::json;
 fn main() {
     let map = json!({ "key": "value" });

-    let _value: String =
-        map.get("key")
-            .map(|v| v.as_str())
-            .flatten()
-            .unwrap_or_default()
-            .to_string();
+    let _value: String = map
+        .get("key")
+        .map(|v| v.as_str())
+        .flatten()
+        .unwrap_or_default()
+        .to_string();
 }