fixes rust-lang#5234

Author: andrewgazelka

src/lib.rs

@@ -396,37 +396,40 @@ fn format_code_block(
         .unwrap_or_else(|| formatted.snippet.len());
     let mut is_indented = true;
     let indent_str = Indent::from_width(config, config.tab_spaces()).to_string(config);
-    for (kind, ref line) in LineClasses::new(&formatted.snippet[FN_MAIN_PREFIX.len()..block_len]) {
-        if !is_first {
-            result.push('\n');
-        } else {
-            is_first = false;
-        }
-        let trimmed_line = if !is_indented {
-            line
-        } else if line.len() > config.max_width() {
-            // If there are lines that are larger than max width, we cannot tell
-            // whether we have succeeded but have some comments or strings that
-            // are too long, or we have failed to format code block. We will be
-            // conservative and just return `None` in this case.
-            return None;
-        } else if line.len() > indent_str.len() {
-            // Make sure that the line has leading whitespaces.
-            if line.starts_with(indent_str.as_ref()) {
-                let offset = if config.hard_tabs() {
-                    1
+
+    if let Some(to_format) = formatted.snippet.get(FN_MAIN_PREFIX.len()..block_len) {
+        for (kind, ref line) in LineClasses::new(to_format) {
+            if !is_first {
+                result.push('\n');
+            } else {
+                is_first = false;
+            }
+            let trimmed_line = if !is_indented {
+                line
+            } else if line.len() > config.max_width() {
+                // If there are lines that are larger than max width, we cannot tell
+                // whether we have succeeded but have some comments or strings that
+                // are too long, or we have failed to format code block. We will be
+                // conservative and just return `None` in this case.
+                return None;
+            } else if line.len() > indent_str.len() {
+                // Make sure that the line has leading whitespaces.
+                if line.starts_with(indent_str.as_ref()) {
+                    let offset = if config.hard_tabs() {
+                        1
+                    } else {
+                        config.tab_spaces()
+                    };
+                    &line[offset..]
                 } else {
-                    config.tab_spaces()
-                };
-                &line[offset..]
+                    line
+                }
             } else {
                 line
-            }
-        } else {
-            line
-        };
-        result.push_str(trimmed_line);
-        is_indented = indent_next_line(kind, line, config);
+            };
+            result.push_str(trimmed_line);
+            is_indented = indent_next_line(kind, line, config);
+        }
     }
     Some(FormattedSnippet {
         snippet: result,

tests/target/issue-5234.rs

@@ -0,0 +1,14 @@
+// rustfmt-format_code_in_doc_comments: true
+
+/// ```
+/// ```
+fn a() {}
+
+/// ```
+/// ```
+fn a() {}
+
+/// ```rust
+/// k1 == k2 ⇒ hash(k1) == hash(k2)
+/// ```
+pub struct a;