bootstrap: add tracing and tracing-tree based tracing setup

Author: jieyouxu

src/bootstrap/src/bin/main.rs

@@ -15,8 +15,20 @@ use bootstrap::{
     human_readable_changes, t,
 };
 use build_helper::ci::CiEnv;
-
+// NOTE: These are conditionally enabled if the user specifies `BOOTSTRAP_TRACING=...`, which builds
+// bootstrap with `--features=tracing`.
+#[cfg(feature = "tracing")]
+use tracing::*;
+#[cfg(feature = "tracing")]
+use tracing_subscriber::EnvFilter;
+#[cfg(feature = "tracing")]
+use tracing_subscriber::prelude::*;
+
+#[cfg_attr(feature = "tracing", instrument(level = "trace", name = "main"))]
 fn main() {
+    #[cfg(feature = "tracing")]
+    setup_tracing();
+
     let args = env::args().skip(1).collect::<Vec<_>>();
 
     if Flags::try_parse_verbose_help(&args) {
@@ -187,3 +199,19 @@ fn check_version(config: &Config) -> Option<String> {
 
     Some(msg)
 }
+
+#[cfg(feature = "tracing")]
+fn setup_tracing() {
+    let filter = EnvFilter::from_env("BOOTSTRAP_TRACING");
+    let layer = tracing_tree::HierarchicalLayer::default()
+        .with_writer(std::io::stderr)
+        .with_ansi(true)
+        .with_targets(true)
+        .with_bracketed_fields(true)
+        .with_indent_amount(2)
+        .with_indent_lines(true);
+    let subscriber = tracing_subscriber::registry().with(filter).with(layer);
+
+    tracing::subscriber::set_global_default(subscriber).unwrap();
+    trace!("tracing subscriber setup");
+}