deduplicate inline is_nightly_build implementations

Author: TimNN

src/librustc/session/config.rs

@@ -1572,10 +1572,7 @@ pub mod nightly_options {
 
     pub fn check_nightly_options(matches: &getopts::Matches, flags: &[RustcOptGroup]) {
         let has_z_unstable_option = matches.opt_strs("Z").iter().any(|x| *x == "unstable-options");
-        let really_allows_unstable_options = match UnstableFeatures::from_environment() {
-            UnstableFeatures::Disallow => false,
-            _ => true,
-        };
+        let really_allows_unstable_options = UnstableFeatures::from_environment().is_nightly_build();
 
         for opt in flags.iter() {
             if opt.stability == OptionStability::Stable {

src/librustc_driver/lib.rs

@@ -649,10 +649,7 @@ impl RustcDefaultCalls {
                     }
                 }
                 PrintRequest::Cfg => {
-                    let allow_unstable_cfg = match UnstableFeatures::from_environment() {
-                        UnstableFeatures::Disallow => false,
-                        _ => true,
-                    };
+                    let allow_unstable_cfg = UnstableFeatures::from_environment().is_nightly_build();
 
                     for cfg in cfg {
                         if !allow_unstable_cfg && GatedCfg::gate(&*cfg).is_some() {

src/librustdoc/html/markdown.rs

@@ -477,13 +477,10 @@ impl LangString {
         let mut data = LangString::all_false();
         let mut allow_compile_fail = false;
         let mut allow_error_code_check = false;
-        match UnstableFeatures::from_environment() {
-            UnstableFeatures::Allow | UnstableFeatures::Cheat => {
-                allow_compile_fail = true;
-                allow_error_code_check = true;
-            }
-            _ => {},
-        };
+        if UnstableFeatures::from_environment().is_nightly_build() {
+            allow_compile_fail = true;
+            allow_error_code_check = true;
+        }
 
         let tokens = string.split(|c: char|
             !(c == '_' || c == '-' || c.is_alphanumeric())