Improve error display for codeblocks in rustdoc

src/librustdoc/html/highlight.rs

@@ -44,9 +44,21 @@ pub fn render_with_highlighting(src: &str, class: Option<&str>,
     }
     write_header(class, &mut out).unwrap();
 
-    let mut classifier = Classifier::new(lexer::StringReader::new(&sess, fm, None),
-                                         sess.source_map());
+    let lexer = match lexer::StringReader::new_without_err(&sess, fm, None) {
+        Ok(l) => l,
+        Err(_) => {
+            let first_line = src.lines().next().unwrap_or_else(|| "");
+            let mut err = sess.span_diagnostic
+                              .struct_warn(&format!("Invalid doc comment starting with: `{}`\n\
+                                                     (Ignoring this codeblock)",
+                                                    first_line));
+            err.emit();
+            return String::new();
+        }
+    };
+    let mut classifier = Classifier::new(lexer, sess.source_map());
     if classifier.write_source(&mut out).is_err() {
+        classifier.lexer.emit_fatal_errors();
         return format!("<pre>{}</pre>", src);
     }
 
@@ -162,11 +174,10 @@ impl<'a> Classifier<'a> {
         match self.lexer.try_next_token() {
             Ok(tas) => Ok(tas),
             Err(_) => {
-                self.lexer.emit_fatal_errors();
-                self.lexer.sess.span_diagnostic
-                    .struct_warn("Backing out of syntax highlighting")
-                    .note("You probably did not intend to render this as a rust code-block")
-                    .emit();
+                let mut err = self.lexer.sess.span_diagnostic
+                                  .struct_warn("Backing out of syntax highlighting");
+                err.note("You probably did not intend to render this as a rust code-block");
+                err.emit();
                 Err(io::Error::new(io::ErrorKind::Other, ""))
             }
         }

src/libsyntax/parse/lexer/mod.rs

@@ -238,6 +238,17 @@ impl<'a> StringReader<'a> {
         sr
     }
 
+    pub fn new_without_err(sess: &'a ParseSess,
+                           source_file: Lrc<syntax_pos::SourceFile>,
+                           override_span: Option<Span>) -> Result<Self, ()> {
+        let mut sr = StringReader::new_raw(sess, source_file, override_span);
+        if sr.advance_token().is_err() {
+            sr.emit_fatal_errors();
+            return Err(());
+        }
+        Ok(sr)
+    }
+
     pub fn retokenize(sess: &'a ParseSess, mut span: Span) -> Self {
         let begin = sess.source_map().lookup_byte_offset(span.lo());
         let end = sess.source_map().lookup_byte_offset(span.hi());

src/test/rustdoc-ui/invalid-syntax.rs

@@ -0,0 +1,17 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-pass
+// compile-flags: --error-format=human
+
+/// ```
+/// \__________pkt->size___________/          \_result->size_/ \__pkt->size__/
+/// ```
+pub fn foo() {}

src/test/rustdoc-ui/invalid-syntax.stderr

@@ -0,0 +1,9 @@
+error: unknown start of token: /
+ --> <stdin>:1:1
+  |
+1 | /__________pkt->size___________/          /_result->size_/ /__pkt->size__/
+  | ^
+
+warning: Invalid doc comment starting with: `/__________pkt->size___________/          /_result->size_/ /__pkt->size__/`
+(Ignoring this codeblock)
+