minor: improve readability

It's important that module interface doesn't depend on features. Better
hide features in bodies.

Author: matklad

crates/stdx/src/lib.rs

@@ -19,13 +19,11 @@ pub fn timeit(label: &'static str) -> impl Drop {
 }
 
 /// Prints backtrace to stderr, useful for debugging.
-#[cfg(feature = "backtrace")]
-pub fn print_backtrace() {
-    let bt = backtrace::Backtrace::new();
-    eprintln!("{:?}", bt);
-}
-#[cfg(not(feature = "backtrace"))]
 pub fn print_backtrace() {
+    #[cfg(feature = "backtrace")]
+    eprintln!("{:?}", backtrace::Backtrace::new());
+
+    #[cfg(not(feature = "backtrace"))]
     eprintln!(
         r#"Enable the backtrace feature.
 Uncomment `default = [ "backtrace" ]` in `crates/stdx/Cargo.toml`.