From 43d0497824d3d1a6f0b15d865a00715537673af2 Mon Sep 17 00:00:00 2001
From: Guillaume Gomez <guillaume.gomez@huawei.com>
Date: Fri, 8 Apr 2022 15:30:37 +0200
Subject: [PATCH 1/2] Fix invalid array access in `beautify_doc_string`

---
 compiler/rustc_ast/src/util/comments.rs | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/compiler/rustc_ast/src/util/comments.rs b/compiler/rustc_ast/src/util/comments.rs
index f51b0086dc8b..8730aeb0f3bf 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() {

From 5e8bd9bbaaaa842e96a3ddef3a75208d0c96ffe7 Mon Sep 17 00:00:00 2001
From: Guillaume Gomez <guillaume.gomez@huawei.com>
Date: Fri, 8 Apr 2022 15:30:53 +0200
Subject: [PATCH 2/2] Add test for empty doc comments with a backline

---
 src/test/rustdoc/empty-doc-comment.rs | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)
 create mode 100644 src/test/rustdoc/empty-doc-comment.rs

diff --git a/src/test/rustdoc/empty-doc-comment.rs b/src/test/rustdoc/empty-doc-comment.rs
new file mode 100644
index 000000000000..b1dae930e066
--- /dev/null
+++ b/src/test/rustdoc/empty-doc-comment.rs
@@ -0,0 +1,22 @@
+// Ensure that empty doc comments don't panic.
+
+/*!
+*/
+
+///
+///
+pub struct Foo;
+
+#[doc = "
+"]
+pub mod Mod {
+   //!
+   //!
+}
+
+/**
+*/
+pub mod Another {
+   #![doc = "
+"]
+}