subscriber: Warn if RUST_LOG is set and env-filter is not enabled

Close tokio-rs#1697

tracing-subscriber/src/fmt/mod.rs

@@ -619,6 +619,22 @@ where
     /// because a global collector was already installed by another
     /// call to `try_init`.
     pub fn try_init(self) -> Result<(), Box<dyn Error + Send + Sync + 'static>> {
+        #[cfg(all(not(feature = "env-filter"), feature = "std"))]
+        {
+            // This should refer to EnvFilter::DEFAULT_ENV, but it is unavailable when the
+            // "env-filter" feature is disabled, so we just hardcode it here
+            if std::env::var_os("RUST_LOG").is_some() {
+                eprint!("\
+tracing-subscriber: Ignoring `RUST_LOG` as the `env-filter` feature is not enabled.
+
+To fix this, replaces the corresponding line with the following in your \
+Cargo.toml:
+    tracing-subscriber = {{ version = \"{}\", features = [\"env-filter\"] }}
+",
+                    env!("CARGO_PKG_VERSION"));
+            }
+        }
+
         use crate::util::SubscriberInitExt;
         self.finish().try_init()?;