rustdoc: do not panic all the way when lexing a source snippet fails

Currently, the libsyntax lexer panics on invalid source, which
makes rustdoc panic when trying to highlight it.

I assume there are efforts underway to make the lexer panic-free,
but until this is done this should be an acceptable workaround.

Note that the panic is still printed like normal as "thread X panicked,
run with RUST_BACKTRACE=1 ...", so I added the printout below to
make it seem less like a fatal error.

I didn't touch `render_inner_with_highlighting` below, as it is
currently unused.  It returns a Result whose Err type would have to
be changed if it was to support lexer errors.

Fixes: rust-lang#30032

Author: birkenfeld

src/librustdoc/html/highlight.rs

@@ -24,6 +24,7 @@ use html::escape::Escape;
 
 use std::fmt::Display;
 use std::io;
+use std::panic::catch_unwind;
 use std::io::prelude::*;
 
 use syntax::codemap::CodeMap;
@@ -35,20 +36,28 @@ use syntax_pos::Span;
 /// Highlights `src`, returning the HTML output.
 pub fn render_with_highlighting(src: &str, class: Option<&str>, id: Option<&str>) -> String {
     debug!("highlighting: ================\n{}\n==============", src);
-    let sess = parse::ParseSess::new();
-    let fm = sess.codemap().new_filemap("<stdin>".to_string(), None, src.to_string());
-
-    let mut out = Vec::new();
-    write_header(class, id, &mut out).unwrap();
 
-    let mut classifier = Classifier::new(lexer::StringReader::new(&sess.span_diagnostic, fm),
-                                         sess.codemap());
-    if let Err(_) = classifier.write_source(&mut out) {
-        return format!("<pre>{}</pre>", src);
+    // As long as the lexer panics...
+    let result = catch_unwind(|| {
+        let sess = parse::ParseSess::new();
+        let fm = sess.codemap().new_filemap("<stdin>".to_string(), None, src.to_string());
+        let mut out = Vec::new();
+        write_header(class, id, &mut out).unwrap();
+        let mut classifier = Classifier::new(lexer::StringReader::new(&sess.span_diagnostic, fm),
+                                             sess.codemap());
+        classifier.write_source(&mut out).unwrap();
+        write_footer(&mut out).unwrap();
+        out
+    });
+    match result {
+        // catches both panics from the lexer and the unwrap()s
+        Err(_) => {
+            println!("warning: error during source highlighting (see above), \
+                      passing source through unhighlighted");
+            format!("<pre>{}</pre>", src)
+        },
+        Ok(out) => String::from_utf8_lossy(&out[..]).into_owned(),
     }
-
-    write_footer(&mut out).unwrap();
-    String::from_utf8_lossy(&out[..]).into_owned()
 }
 
 /// Highlights `src`, returning the HTML output. Returns only the inner html to

src/test/rustdoc/issue-30032.rs

@@ -0,0 +1,18 @@
+// Copyright 2016 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.
+
+// This used to panic due to the invalid source, so just checking for
+// existence of the output file is enough.
+
+// @has issue_30032/fn.main.html
+/// ```ignore
+/// let invalid = 'abc';
+/// ```
+pub fn main() {}