Fix regression that swallowed json diagnostic explanations (#16075)

Fixes #16074

Author: epage

src/cargo/core/compiler/mod.rs

@@ -2168,12 +2168,14 @@ fn on_stderr_line_inner(
                 rendered: String,
                 #[serde(flatten, borrow)]
                 other: std::collections::BTreeMap<Cow<'a, str>, serde_json::Value>,
-                code: Option<DiagnosticCode>,
+                code: Option<DiagnosticCode<'a>>,
             }
 
             #[derive(serde::Deserialize, serde::Serialize)]
-            struct DiagnosticCode {
+            struct DiagnosticCode<'a> {
                 code: String,
+                #[serde(flatten, borrow)]
+                other: std::collections::BTreeMap<Cow<'a, str>, serde_json::Value>,
             }
 
             if let Ok(mut error) =

tests/testsuite/test.rs

@@ -4455,6 +4455,47 @@ fn json_artifact_includes_executable_for_library_tests() {
         .run();
 }
 
+#[cargo_test]
+fn json_diagnostic_includes_explanation() {
+    let p = project()
+        .file(
+            "src/main.rs",
+            "fn main() { const OH_NO: &'static mut usize = &mut 1; }",
+        )
+        .build();
+
+    p.cargo("check --message-format=json")
+        .with_stdout_data(
+            str![[r#"
+[
+  {
+    "manifest_path": "[ROOT]/foo/Cargo.toml",
+    "message": {
+      "$message_type": "diagnostic",
+      "children": "{...}",
+      "code": {
+        "code": "E0764",
+        "explanation": "{...}"
+      },
+      "level": "error",
+      "message": "{...}",
+      "rendered": "{...}",
+      "spans": "{...}"
+    },
+    "package_id": "{...}",
+    "reason": "compiler-message",
+    "target": "{...}"
+  },
+  "{...}"
+]
+"#]]
+            .is_json()
+            .against_jsonlines(),
+        )
+        .with_status(101)
+        .run();
+}
+
 #[cargo_test]
 fn json_artifact_includes_executable_for_integration_tests() {
     let p = project()