rustdoc: Fix empty doc comment with backline ICE

compiler/rustc_ast/src/util/comments.rs

@@ -52,7 +52,10 @@ pub fn beautify_doc_string(data: Symbol, kind: CommentKind) -> Symbol {
         // 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() {

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 = "
+"]
+}