Guard against overflow in codemap::span_to_lines.

Make `span_to_lines` to return a `Result`.
(This is better than just asserting internally, since it allows caller
to decide if they can recover from the problem.)

Added type alias for `FileLinesResult` returned by `span_to_lines`.

Update embedded unit test to reflect `span_to_lines` signature change.

In diagnostic, catch `Err` from `span_to_lines` and print
`"(internal compiler error: unprintable span)"` instead.

----

There a number of recent issues that report the bug here.  See
e.g. #24761 and #24954.

This change *might* fix them. However, that is not its main goal.
The main goals are:

 1. Make it possible for callers to recover from an error here, and

 2. Insert a more conservative check, in that we are
    also checking that the files match up.

src/libsyntax/codemap.rs

@@ -667,9 +667,22 @@ impl CodeMap {
         self.lookup_char_pos(sp.lo).file.name.to_string()
     }
 
-    pub fn span_to_lines(&self, sp: Span) -> FileLines {
+    pub fn span_to_lines(&self, sp: Span) -> FileLinesResult {
+        if sp.lo > sp.hi {
+            return Err(SpanLinesError::IllFormedSpan(sp));
+        }
+
         let lo = self.lookup_char_pos(sp.lo);
         let hi = self.lookup_char_pos(sp.hi);
+
+        if lo.file.start_pos != hi.file.start_pos {
+            return Err(SpanLinesError::DistinctSources(DistinctSources {
+                begin: (lo.file.name.clone(), lo.file.start_pos),
+                end: (hi.file.name.clone(), hi.file.start_pos),
+            }));
+        }
+        assert!(hi.line >= lo.line);
+
         let mut lines = Vec::with_capacity(hi.line - lo.line + 1);
 
         // The span starts partway through the first line,
@@ -693,7 +706,7 @@ impl CodeMap {
                               start_col: start_col,
                               end_col: hi.col });
 
-        FileLines {file: lo.file, lines: lines}
+        Ok(FileLines {file: lo.file, lines: lines})
     }
 
     pub fn span_to_snippet(&self, sp: Span) -> Result<String, SpanSnippetError> {
@@ -918,9 +931,17 @@ impl CodeMap {
 }
 
 // _____________________________________________________________________________
-// SpanSnippetError, DistinctSources, MalformedCodemapPositions
+// SpanLinesError, SpanSnippetError, DistinctSources, MalformedCodemapPositions
 //
 
+pub type FileLinesResult = Result<FileLines, SpanLinesError>;
+
+#[derive(Clone, PartialEq, Eq, Debug)]
+pub enum SpanLinesError {
+    IllFormedSpan(Span),
+    DistinctSources(DistinctSources),
+}
+
 #[derive(Clone, PartialEq, Eq, Debug)]
 pub enum SpanSnippetError {
     IllFormedSpan(Span),
@@ -1086,7 +1107,7 @@ mod tests {
         // Test span_to_lines for a span ending at the end of filemap
         let cm = init_code_map();
         let span = Span {lo: BytePos(12), hi: BytePos(23), expn_id: NO_EXPANSION};
-        let file_lines = cm.span_to_lines(span);
+        let file_lines = cm.span_to_lines(span).unwrap();
 
         assert_eq!(file_lines.file.name, "blork.rs");
         assert_eq!(file_lines.lines.len(), 1);
@@ -1131,7 +1152,7 @@ mod tests {
         assert_eq!(&cm.span_to_snippet(span).unwrap(), "BB\nCCC\nDDDDD");
 
         // check that span_to_lines gives us the complete result with the lines/cols we expected
-        let lines = cm.span_to_lines(span);
+        let lines = cm.span_to_lines(span).unwrap();
         let expected = vec![
             LineInfo { line_index: 1, start_col: CharPos(4), end_col: CharPos(6) },
             LineInfo { line_index: 2, start_col: CharPos(0), end_col: CharPos(3) },

src/libsyntax/diagnostic.rs

@@ -522,7 +522,7 @@ fn highlight_suggestion(err: &mut EmitterWriter,
                         suggestion: &str)
                         -> io::Result<()>
 {
-    let lines = cm.span_to_lines(sp);
+    let lines = cm.span_to_lines(sp).unwrap();
     assert!(!lines.lines.is_empty());
 
     // To build up the result, we want to take the snippet from the first
@@ -567,9 +567,17 @@ fn highlight_lines(err: &mut EmitterWriter,
                    cm: &codemap::CodeMap,
                    sp: Span,
                    lvl: Level,
-                   lines: codemap::FileLines)
+                   lines: codemap::FileLinesResult)
                    -> io::Result<()>
 {
+    let lines = match lines {
+        Ok(lines) => lines,
+        Err(_) => {
+            try!(write!(&mut err.dst, "(internal compiler error: unprintable span)\n"));
+            return Ok(());
+        }
+    };
+
     let fm = &*lines.file;
 
     let line_strings: Option<Vec<&str>> =
@@ -690,8 +698,16 @@ fn end_highlight_lines(w: &mut EmitterWriter,
                           cm: &codemap::CodeMap,
                           sp: Span,
                           lvl: Level,
-                          lines: codemap::FileLines)
+                          lines: codemap::FileLinesResult)
                           -> io::Result<()> {
+    let lines = match lines {
+        Ok(lines) => lines,
+        Err(_) => {
+            try!(write!(&mut w.dst, "(internal compiler error: unprintable span)\n"));
+            return Ok(());
+        }
+    };
+
     let fm = &*lines.file;
 
     let lines = &lines.lines[..];