Fix invalid array access in `beautify_doc_string`
diff --git a/compiler/rustc_ast/src/util/comments.rs b/compiler/rustc_ast/src/util/comments.rs
index f51b0086..8730aeb 100644
--- a/compiler/rustc_ast/src/util/comments.rs
+++ b/compiler/rustc_ast/src/util/comments.rs
@@ -52,7 +52,10 @@ fn get_horizontal_trim<'a>(lines: &'a [&str], kind: CommentKind) -> Option<Strin
// when we try to compute the "horizontal trim".
let lines = if kind == CommentKind::Block {
// Whatever happens, we skip the first line.
- let mut i = if lines[0].trim_start().starts_with('*') { 0 } else { 1 };
+ let mut i = lines
+ .get(0)
+ .map(|l| if l.trim_start().starts_with('*') { 0 } else { 1 })
+ .unwrap_or(0);
let mut j = lines.len();
while i < j && lines[i].trim().is_empty() {
