feat(log): emit build-started JSON message with run_id

When both `-Zbuild-analysis` and `--message-format=json` are enabled,
emit a `build-started` JSON message to stdout containing the `run_id`.

This allows external tools to associate the JSON output stream
with the corresponding log file in `~/.cargo/log/{run_id}.jsonl`.

While a better design of JSON message and structured logging is needed,
this provides a temporary workaround for those needing the old
`--timings=json` flag

Closes #16576

Author: weihanglo

src/cargo/ops/cargo_compile/mod.rs

@@ -58,6 +58,8 @@ use crate::util::BuildLogger;
 use crate::util::context::{GlobalContext, WarningHandling};
 use crate::util::interning::InternedString;
 use crate::util::log_message::LogMessage;
+use crate::util::machine_message;
+use crate::util::machine_message::Message as _;
 use crate::util::{CargoResult, StableHasher};
 
 mod compile_filter;
@@ -181,6 +183,12 @@ pub fn compile_ws<'a>(
             target_dir: ws.target_dir().as_path_unlocked().to_path_buf(),
             workspace_root: ws.root().to_path_buf(),
         });
+
+        if options.build_config.emit_json() {
+            let run_id = logger.run_id().to_string();
+            let msg = machine_message::BuildStarted { run_id: &run_id }.to_json_string();
+            writeln!(ws.gctx().shell().out(), "{msg}")?;
+        }
     }
 
     let bcx = create_bcx(ws, options, &interner, logger.as_ref())?;

src/cargo/util/machine_message.rs

@@ -103,3 +103,14 @@ impl Message for BuildFinished {
         "build-finished"
     }
 }
+
+#[derive(Serialize)]
+pub struct BuildStarted<'a> {
+    pub run_id: &'a str,
+}
+
+impl Message for BuildStarted<'_> {
+    fn reason(&self) -> &str {
+        "build-started"
+    }
+}

tests/testsuite/build_analysis.rs

@@ -732,6 +732,10 @@ fn json_message_build_started_with_run_id() {
         .with_stdout_data(
             str![[r#"
 [
+  {
+    "reason": "build-started",
+    "run_id": "[..]T[..]Z-[..]"
+  },
   {
     "reason": "compiler-artifact",
     "...": "{...}"