fix: handle empty /**/ in block_to_line assist

block_to_line sliced &text[prefix.len()..text.len()-2], which panicked
on `/**/` since its prefix matches the entire comment (len 4) and
text.len() - 2 = 2, producing the invalid range [4..2].

use strip_prefix/strip_suffix and emit a plain `//` when the body is
empty after trimming.

fixes #22071

Author: Albab-Hasan

crates/ide-assists/src/handlers/convert_comment_block.rs

@@ -46,23 +46,26 @@ fn block_to_line(acc: &mut Assists, comment: ast::Comment) -> Option<()> {
             let line_prefix = CommentKind { shape: CommentShape::Line, ..comment.kind() }.prefix();
 
             let text = comment.text();
-            let text = &text[comment.prefix().len()..(text.len() - "*/".len())].trim();
-
-            let lines = text.lines().peekable();
+            let text = text.strip_prefix(comment.prefix()).unwrap_or(text);
+            let text = text.strip_suffix("*/").unwrap_or(text).trim();
 
             let indent_spaces = indentation.to_string();
-            let output = lines
-                .map(|line| {
-                    let line = line.trim_start_matches(&indent_spaces);
-
-                    // Don't introduce trailing whitespace
-                    if line.is_empty() {
-                        line_prefix.to_owned()
-                    } else {
-                        format!("{line_prefix} {line}")
-                    }
-                })
-                .join(&format!("\n{indent_spaces}"));
+            let output = if text.is_empty() {
+                line_prefix.to_owned()
+            } else {
+                text.lines()
+                    .map(|line| {
+                        let line = line.trim_start_matches(&indent_spaces);
+
+                        // Don't introduce trailing whitespace
+                        if line.is_empty() {
+                            line_prefix.to_owned()
+                        } else {
+                            format!("{line_prefix} {line}")
+                        }
+                    })
+                    .join(&format!("\n{indent_spaces}"))
+            };
 
             edit.replace(target, output)
         },
@@ -381,6 +384,25 @@ fn main() {
         );
     }
 
+    #[test]
+    fn empty_block_to_line() {
+        check_assist(
+            convert_comment_block,
+            r#"
+fn main() {
+    /**/$0
+    foo();
+}
+"#,
+            r#"
+fn main() {
+    //
+    foo();
+}
+"#,
+        );
+    }
+
     #[test]
     fn end_of_line_block_to_line() {
         check_assist_not_applicable(