Only emit useless_vec suggestion if the macro does not contain code comments (rust-lang#13911)

Fixes rust-lang#13692.

If the `vec!` macro call contains comments, we should not provide
suggestions and let users handle it however they see fit.

changelog: Only emit `useless_vec` suggestion if the macro does not
contain code comments

Author: xFrednet

clippy_lints/src/vec.rs

@@ -8,7 +8,7 @@ use clippy_utils::msrvs::{self, Msrv};
 use clippy_utils::source::SpanRangeExt;
 use clippy_utils::ty::is_copy;
 use clippy_utils::visitors::for_each_local_use_after_expr;
-use clippy_utils::{get_parent_expr, higher, is_in_test, is_trait_method};
+use clippy_utils::{get_parent_expr, higher, is_in_test, is_trait_method, span_contains_comment};
 use rustc_errors::Applicability;
 use rustc_hir::{BorrowKind, Expr, ExprKind, HirId, LetStmt, Mutability, Node, Pat, PatKind};
 use rustc_lint::{LateContext, LateLintPass};
@@ -132,9 +132,19 @@ impl<'tcx> LateLintPass<'tcx> for UselessVec {
     fn check_crate_post(&mut self, cx: &LateContext<'tcx>) {
         for (span, lint_opt) in &self.span_to_lint_map {
             if let Some((hir_id, suggest_slice, snippet, applicability)) = lint_opt {
-                let help_msg = format!("you can use {} directly", suggest_slice.desc(),);
+                let help_msg = format!("you can use {} directly", suggest_slice.desc());
                 span_lint_hir_and_then(cx, USELESS_VEC, *hir_id, *span, "useless use of `vec!`", |diag| {
-                    diag.span_suggestion(*span, help_msg, snippet, *applicability);
+                    // If the `vec!` macro contains comment, better not make the suggestion machine
+                    // applicable as it would remove them.
+                    let applicability = if *applicability != Applicability::Unspecified
+                        && let source_map = cx.tcx.sess.source_map()
+                        && span_contains_comment(source_map, *span)
+                    {
+                        Applicability::Unspecified
+                    } else {
+                        *applicability
+                    };
+                    diag.span_suggestion(*span, help_msg, snippet, applicability);
                 });
             }
         }

tests/ui/useless_vec.rs

@@ -0,0 +1,15 @@
+//@no-rustfix: no suggestions
+
+#![warn(clippy::useless_vec)]
+
+// Regression test for <https://github.com/rust-lang/rust-clippy/issues/13692>.
+fn foo() {
+    // There should be no suggestion in this case.
+    let _some_variable = vec![
+        //~^ useless_vec
+        1, 2, // i'm here to stay
+        3, 4, // but this one going away ;-;
+    ]; // that is life anyways
+}
+
+fn main() {}

tests/ui/useless_vec.stderr

@@ -0,0 +1,21 @@
+error: useless use of `vec!`
+  --> tests/ui/useless_vec.rs:8:26
+   |
+LL |       let _some_variable = vec![
+   |  __________________________^
+LL | |
+LL | |         1, 2, // i'm here to stay
+LL | |         3, 4, // but this one going away ;-;
+LL | |     ]; // that is life anyways
+   | |_____^
+   |
+   = note: `-D clippy::useless-vec` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::useless_vec)]`
+help: you can use an array directly
+   |
+LL ~     let _some_variable = [1, 2, // i'm here to stay
+LL ~         3, 4]; // that is life anyways
+   |
+
+error: aborting due to 1 previous error
+