show env_logger working in wiggle tracing example

Cargo.lock

@@ -2113,6 +2113,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a7c6b59d116d218cb2d990eb06b77b64043e0268ef7323aae63d8b30ae462923"
 dependencies = [
  "cfg-if",
+ "log",
  "tracing-attributes",
  "tracing-core",
 ]
@@ -2656,6 +2657,7 @@ dependencies = [
 name = "wiggle-test"
 version = "0.17.0"
 dependencies = [
+ "env_logger 0.7.1",
  "proptest",
  "thiserror",
  "tracing",

crates/wiggle/test-helpers/Cargo.toml

@@ -12,12 +12,13 @@ include = ["src/**/*", "LICENSE"]
 
 [dependencies]
 proptest = "0.9"
-wiggle = { path = ".." }
+wiggle = { path = "..", features = ["tracing_log"] }
 
 [dev-dependencies]
 thiserror = "1.0"
 tracing = "0.1.14"
 tracing-subscriber = "0.2.4"
+env_logger="0.7"
 
 
 [badges]

crates/wiggle/test-helpers/examples/tracing.rs

@@ -68,14 +68,20 @@ impl<'a> one_error_conversion::OneErrorConversion for WasiCtx<'a> {
 }
 
 fn main() {
+    if std::env::var("RUST_LOG").is_err() {
+        // with no RUST_LOG env variable: use the tracing subscriber.
         let subscriber = tracing_subscriber::fmt()
             // all spans/events with a level higher than TRACE (e.g, debug, info, warn, etc.)
             // will be written to stdout.
             .with_max_level(tracing::Level::TRACE)
             // builds the subscriber.
             .finish();
+        tracing::subscriber::set_global_default(subscriber).expect("set global tracing subscriber");
+    } else {
+        // with RUST_LOG set: use the env_logger backend to tracing.
+        env_logger::init();
+    }
 
-    tracing::subscriber::with_default(subscriber, || {
     let ctx = WasiCtx::new();
     let host_memory = HostMemory::new();
 
@@ -114,5 +120,4 @@ fn main() {
         "Invalid argument: out-of-bounds: 2",
         "Expected log entry for strike=2",
     );
-    });
 }