Pull out nightly checking to edges

Parsing the code block's LangString (```foo) previously checked itself
to see if we were on nightly; that isn't the right place to do so. Move
that check slightly outwards to better abstract LangString.

(This is also an optimization as we avoid the costly environment
variable load of RUSTC_BOOTSTRAP).

Author: Mark-Simulacrum

src/librustdoc/html/markdown.rs

@@ -129,12 +129,14 @@ thread_local!(pub static PLAYGROUND: RefCell<Option<(Option<String>, String)>> =
 /// Adds syntax highlighting and playground Run buttons to rust code blocks.
 struct CodeBlocks<'a, I: Iterator<Item = Event<'a>>> {
     inner: I,
+    check_error_codes: bool,
 }
 
 impl<'a, I: Iterator<Item = Event<'a>>> CodeBlocks<'a, I> {
     fn new(iter: I) -> Self {
         CodeBlocks {
             inner: iter,
+            check_error_codes: UnstableFeatures::from_environment().is_nightly_build(),
         }
     }
 }
@@ -147,7 +149,7 @@ impl<'a, I: Iterator<Item = Event<'a>>> Iterator for CodeBlocks<'a, I> {
         let compile_fail;
         let ignore;
         if let Some(Event::Start(Tag::CodeBlock(lang))) = event {
-            let parse_result = LangString::parse(&lang);
+            let parse_result = LangString::parse(&lang, self.check_error_codes);
             if !parse_result.rust {
                 return Some(Event::Start(Tag::CodeBlock(lang)));
             }
@@ -471,6 +473,7 @@ pub fn find_testable_code(doc: &str, tests: &mut ::test::Collector, position: Sp
                           sess: Option<&session::Session>) {
     tests.set_position(position);
 
+    let is_nightly = UnstableFeatures::from_environment().is_nightly_build();
     let mut parser = Parser::new(doc);
     let mut prev_offset = 0;
     let mut nb_lines = 0;
@@ -481,7 +484,7 @@ pub fn find_testable_code(doc: &str, tests: &mut ::test::Collector, position: Sp
                 let block_info = if s.is_empty() {
                     LangString::all_false()
                 } else {
-                    LangString::parse(&*s)
+                    LangString::parse(&*s, is_nightly)
                 };
                 if !block_info.rust {
                     continue
@@ -569,14 +572,10 @@ impl LangString {
         }
     }
 
-    fn parse(string: &str) -> LangString {
+    fn parse(string: &str, allow_error_code_check: bool) -> LangString {
         let mut seen_rust_tags = false;
         let mut seen_other_tags = false;
         let mut data = LangString::all_false();
-        let mut allow_error_code_check = false;
-        if UnstableFeatures::from_environment().is_nightly_build() {
-            allow_error_code_check = true;
-        }
 
         data.original = string.to_owned();
         let tokens = string.split(|c: char|
@@ -842,7 +841,7 @@ mod tests {
         fn t(s: &str,
             should_panic: bool, no_run: bool, ignore: bool, rust: bool, test_harness: bool,
             compile_fail: bool, allow_fail: bool, error_codes: Vec<String>) {
-            assert_eq!(LangString::parse(s), LangString {
+            assert_eq!(LangString::parse(s, true), LangString {
                 should_panic,
                 no_run,
                 ignore,