Move #[track_caller] behind feature gate

Cargo.toml

@@ -3,7 +3,7 @@ name = "grass"
 version = "0.1.0"
 description = "An SCSS compiler in Rust"
 readme = "README.md"
-license = "MIT OR Apache-2.0"
+license = "MIT"
 categories = ["command-line-utilities", "web-programming"]
 keywords = ["scss", "sass", "css"]
 repository = "https://github.com/connorskees/grass"
@@ -26,6 +26,7 @@ clap = { version = "2.33.0", optional = true }
 [features]
 default = ["commandline"]
 commandline = ["clap"]
+nightly = []
 
 [dev-dependencies]
 # test imports

src/lib.rs

@@ -39,7 +39,7 @@
     // than they should
     clippy::module_name_repetitions
 )]
-#![feature(track_caller)]
+#![cfg_attr(feature = "nightly", feature(track_caller))]
 // todo! handle erroring on styles at the toplevel
 use std::fmt::{self, Display};
 use std::fs;

src/utils.rs

@@ -18,7 +18,7 @@ pub(crate) fn devour_whitespace<I: Iterator<Item = W>, W: IsWhitespace>(s: &mut
     found_whitespace
 }
 
-#[track_caller]
+#[cfg_attr(feature = "nightly", track_caller)]
 pub(crate) fn deref_variable(name: &str, scope: &Scope) -> Vec<Token> {
     let mut toks = scope
         .vars