Handle block comment before Traits in derive attribute (issue 4984)

When a block comment comes before the traits of a derive attribute
the string "#[derive(" is included as part of the the pre snippet,
and as a result gets duplicated.

For example:

    #[derive(/*Debug, */Clone)] -> #[derive(#[derive(/*Debug, */ Clone)]

Now the string "#[derive(" is trimmed from the start of the pre snippet
before looking for the pre comment.

src/lists.rs

@@ -570,7 +570,9 @@ where
 }
 
 pub(crate) fn extract_pre_comment(pre_snippet: &str) -> (Option<String>, ListItemCommentStyle) {
-    let trimmed_pre_snippet = pre_snippet.trim();
+    // "#[derive(" is included as part of the pre snippet if followed by a block comment
+    // see https://github.com/rust-lang/rustfmt/issues/4984
+    let trimmed_pre_snippet = pre_snippet.trim_start_matches("#[derive(").trim();
     // Both start and end are checked to support keeping a block comment inline with
     // the item, even if there are preceeding line comments, while still supporting
     // a snippet that starts with a block comment but also contains one or more

tests/source/issue-4984/minimum_example.rs

@@ -0,0 +1,2 @@
+#[derive(/*Debug, */Clone)]
+struct Foo;

tests/source/issue-4984/multi_line_derive.rs

@@ -0,0 +1,20 @@
+#[derive(
+/* ---------- Some really important comment that just had to go inside the derive --------- */
+Debug, Clone, Eq, PartialEq,
+)]
+struct Foo {
+    a: i32,
+    b: T,
+}
+
+#[derive(
+/*
+    Some really important comment that just had to go inside the derive.
+    Also had to be put over multiple lines
+*/
+Debug, Clone, Eq, PartialEq,
+)]
+struct Bar {
+    a: i32,
+    b: T,
+}

tests/target/issue-4984/minimum_example.rs

@@ -0,0 +1,2 @@
+#[derive(/*Debug, */ Clone)]
+struct Foo;

tests/target/issue-4984/multi_line_derive.rs

@@ -0,0 +1,26 @@
+#[derive(
+    /* ---------- Some really important comment that just had to go inside the derive --------- */
+    Debug,
+    Clone,
+    Eq,
+    PartialEq,
+)]
+struct Foo {
+    a: i32,
+    b: T,
+}
+
+#[derive(
+    /*
+        Some really important comment that just had to go inside the derive.
+        Also had to be put over multiple lines
+    */
+    Debug,
+    Clone,
+    Eq,
+    PartialEq,
+)]
+struct Bar {
+    a: i32,
+    b: T,
+}