Let lint_dropping_references give the suggestion if possible.

compiler/rustc_lint/src/drop_forget_useless.rs

@@ -5,7 +5,7 @@ use rustc_span::sym;
 
 use crate::{
     lints::{
-        DropCopyDiag, DropCopySuggestion, DropRefDiag, ForgetCopyDiag, ForgetRefDiag,
+        DropCopyDiag, DropRefDiag, ForgetCopyDiag, ForgetRefDiag, IgnoreDropSuggestion,
         UndroppedManuallyDropsDiag, UndroppedManuallyDropsSuggestion,
     },
     LateContext, LateLintPass, LintContext,
@@ -148,12 +148,24 @@ impl<'tcx> LateLintPass<'tcx> for DropForgetUseless {
             let arg_ty = cx.typeck_results().expr_ty(arg);
             let is_copy = arg_ty.is_copy_modulo_regions(cx.tcx, cx.param_env);
             let drop_is_single_call_in_arm = is_single_call_in_arm(cx, arg, expr);
+            let sugg = if let Some((_, node)) = cx.tcx.hir().parent_iter(expr.hir_id).nth(0)
+                && let Node::Stmt(stmt) = node
+                && let StmtKind::Semi(e) = stmt.kind
+                && e.hir_id == expr.hir_id
+            {
+                IgnoreDropSuggestion::Suggestion {
+                    start_span: expr.span.shrink_to_lo().until(arg.span),
+                    end_span: arg.span.shrink_to_hi().until(expr.span.shrink_to_hi()),
+                }
+            } else {
+                IgnoreDropSuggestion::Note
+            };
             match fn_name {
                 sym::mem_drop if arg_ty.is_ref() && !drop_is_single_call_in_arm => {
                     cx.emit_span_lint(
                         DROPPING_REFERENCES,
                         expr.span,
-                        DropRefDiag { arg_ty, label: arg.span },
+                        DropRefDiag { arg_ty, label: arg.span, sugg },
                     );
                 }
                 sym::mem_forget if arg_ty.is_ref() => {

compiler/rustc_lint/src/lints.rs

@@ -656,14 +656,28 @@ pub struct ForLoopsOverFalliblesSuggestion<'a> {
     pub end_span: Span,
 }
 
+#[derive(Subdiagnostic)]
+pub enum IgnoreDropSuggestion {
+    #[note(lint_note)]
+    Note,
+    #[multipart_suggestion(lint_suggestion, style = "verbose", applicability = "maybe-incorrect")]
+    Suggestion {
+        #[suggestion_part(code = "let _ = ")]
+        start_span: Span,
+        #[suggestion_part(code = "")]
+        end_span: Span,
+    },
+}
+
 // drop_forget_useless.rs
 #[derive(LintDiagnostic)]
 #[diag(lint_dropping_references)]
-#[note]
 pub struct DropRefDiag<'a> {
     pub arg_ty: Ty<'a>,
     #[label]
     pub label: Span,
+    #[subdiagnostic]
+    pub sugg: IgnoreDropSuggestion,
 }
 
 #[derive(LintDiagnostic)]