Add a sentence before rustc errors

Author: GuillaumeGomez

Diff for: src/librustdoc/html/highlight.rs

@@ -44,7 +44,7 @@ pub fn render_with_highlighting(src: &str, class: Option<&str>,
     }
     write_header(class, &mut out).unwrap();
 
-    let lexer = match lexer::StringReader::new_without_err(&sess, fm, None) {
+    let lexer = match lexer::StringReader::new_without_err(&sess, fm, None, "Output from rustc:") {
         Ok(l) => l,
         Err(_) => {
             let first_line = src.lines().next().unwrap_or_else(|| "");

Diff for: src/libsyntax/parse/lexer/mod.rs

@@ -240,9 +240,11 @@ impl<'a> StringReader<'a> {
 
     pub fn new_without_err(sess: &'a ParseSess,
                            source_file: Lrc<syntax_pos::SourceFile>,
-                           override_span: Option<Span>) -> Result<Self, ()> {
+                           override_span: Option<Span>,
+                           prepend_error_text: &str) -> Result<Self, ()> {
         let mut sr = StringReader::new_raw(sess, source_file, override_span);
         if sr.advance_token().is_err() {
+            eprintln!("{}", prepend_error_text);
             sr.emit_fatal_errors();
             return Err(());
         }

Diff for: src/test/rustdoc-ui/invalid-syntax.stderr

@@ -1,3 +1,4 @@
+Output from rustc:
 error: unknown start of token: /
  --> <stdin>:1:1
   |