Disable "pretty" mode tests on stable

Author: denzp

src/runtest.rs

@@ -263,6 +263,12 @@ impl<'test> TestCx<'test> {
         }
     }
 
+    #[cfg(feature = "stable")]
+    fn run_pretty_test(&self) {
+        self.fatal("pretty-printing tests can only be used with nightly Rust".into());
+    }
+
+    #[cfg(not(feature = "stable"))]
     fn run_pretty_test(&self) {
         if self.props.pp_exact.is_some() {
             logv(self.config, "testing for exact pretty-printing".to_owned());

test-project/tests/tests.rs

@@ -19,5 +19,7 @@ fn run_mode(mode: &'static str) {
 fn compile_test() {
     run_mode("compile-fail");
     run_mode("run-pass");
+
+    #[cfg(not(feature = "stable"))]
     run_mode("pretty");
 }