auto-fix slow_vector_initialization in some cases (#13947)

changelog: [`slow_vector_initialization`]: auto-fix when appropriate

I made a change for `slow_vector_initialization` lint suggestion to use
`vec!` with size and remove the unneeded `resize` (or similar one) call
in #13912, while only the former one was suggested in the previous
implementation. Now, I think this lint can be automatically fixed with
no unnecessary code in some cases. I wrote “in some cases” because if
there are comments between vector declaration and `resize`, Clippy
shouldn't apply auto-fix because the comment may informational.

clippy_lints/src/slow_vector_initialization.rs

@@ -3,6 +3,7 @@ use clippy_utils::macros::matching_root_macro_call;
 use clippy_utils::sugg::Sugg;
 use clippy_utils::{
     SpanlessEq, get_enclosing_block, is_integer_literal, is_path_diagnostic_item, path_to_local, path_to_local_id,
+    span_contains_comment,
 };
 use rustc_errors::Applicability;
 use rustc_hir::intravisit::{Visitor, walk_block, walk_expr, walk_stmt};
@@ -206,6 +207,14 @@ impl SlowVectorInit {
         let span_to_replace = slow_fill
             .span
             .with_lo(vec_alloc.allocation_expr.span.source_callsite().lo());
+
+        // If there is no comment in `span_to_replace`, Clippy can automatically fix the code.
+        let app = if span_contains_comment(cx.tcx.sess.source_map(), span_to_replace) {
+            Applicability::Unspecified
+        } else {
+            Applicability::MachineApplicable
+        };
+
         span_lint_and_sugg(
             cx,
             SLOW_VECTOR_INITIALIZATION,
@@ -213,7 +222,7 @@ impl SlowVectorInit {
             msg,
             "consider replacing this with",
             format!("vec![0; {len_expr}]"),
-            Applicability::Unspecified,
+            app,
         );
     }
 }