Fix underflow panic when doctests are at top of file

crates/ra_ide/src/snapshots/highlight_doctest.html

@@ -32,7 +32,10 @@
 .keyword.unsafe     { color: #BC8383; font-weight: bold; }
 .control            { font-style: italic; }
 </style>
-<pre><code><span class="keyword">struct</span> <span class="struct declaration">Foo</span> {
+<pre><code><span class="comment documentation">/// ```</span>
+<span class="comment documentation">/// </span><span class="keyword">let</span> _ = <span class="string_literal">"early doctests should not go boom"</span>;
+<span class="comment documentation">/// ```</span>
+<span class="keyword">struct</span> <span class="struct declaration">Foo</span> {
     <span class="field declaration">bar</span>: <span class="builtin_type">bool</span>,
 }
 

crates/ra_ide/src/syntax_highlighting/injection.rs

@@ -155,17 +155,21 @@ pub(super) fn highlight_doc_comment(
         let mut start_offset = None;
         let mut end_offset = None;
         for (line_start, orig_line_start) in range_mapping.range(..h.range.end()).rev() {
+            // It's possible for orig_line_start - line_start to be negative. Add h.range.start()
+            // here and remove it from the end range after the loop below so that the values are
+            // always non-negative.
+            let offset = h.range.start() + orig_line_start - line_start;
             if line_start <= &h.range.start() {
-                start_offset.get_or_insert(orig_line_start - line_start);
+                start_offset.get_or_insert(offset);
                 break;
             } else {
-                end_offset.get_or_insert(orig_line_start - line_start);
+                end_offset.get_or_insert(offset);
             }
         }
         if let Some(start_offset) = start_offset {
             h.range = TextRange::new(
-                h.range.start() + start_offset,
-                h.range.end() + end_offset.unwrap_or(start_offset),
+                start_offset,
+                h.range.end() + end_offset.unwrap_or(start_offset) - h.range.start(),
             );
 
             stack.add(h);

crates/ra_ide/src/syntax_highlighting/tests.rs

@@ -291,6 +291,9 @@ fn main() {
 fn test_highlight_doctest() {
     check_highlighting(
         r#"
+/// ```
+/// let _ = "early doctests should not go boom";
+/// ```
 struct Foo {
     bar: bool,
 }