rustc-dev-guide subtree update

src/doc/rustc-dev-guide/src/tracing.md

@@ -109,7 +109,7 @@ Miri, use `MIRI_LOG` instead. You get the idea :)
 
 See the [`tracing`] crate's docs, and specifically the docs for [`debug!`] to
 see the full syntax you can use. (Note: unlike the compiler, the [`tracing`]
-crate and its examples use the `RUST_LOG` environment variable. rustc, rustdoc,
+crate and its examples use the `RUSTC_LOG` environment variable. rustc, rustdoc,
 and other tools set custom environment variables.)
 
 **Note that unless you use a very strict filter, the logger will emit a lot of