A small diagnostic improvement for dropping_copy_types

fixes #125189

compiler/rustc_lint/messages.ftl

@@ -197,6 +197,7 @@ lint_drop_trait_constraints =
 lint_dropping_copy_types = calls to `std::mem::drop` with a value that implements `Copy` does nothing
     .label = argument has type `{$arg_ty}`
     .note = use `let _ = ...` to ignore the expression or result
+    .suggestion = use `let _ = ...` to ignore the expression or result
 
 lint_dropping_references = calls to `std::mem::drop` with a reference instead of an owned value does nothing
     .label = argument has type `{$arg_ty}`

compiler/rustc_lint/src/drop_forget_useless.rs

@@ -1,11 +1,11 @@
-use rustc_hir::{Arm, Expr, ExprKind, Node};
+use rustc_hir::{Arm, Expr, ExprKind, Node, StmtKind};
 use rustc_middle::ty;
 use rustc_span::sym;
 
 use crate::{
     lints::{
-        DropCopyDiag, DropRefDiag, ForgetCopyDiag, ForgetRefDiag, UndroppedManuallyDropsDiag,
-        UndroppedManuallyDropsSuggestion,
+        DropCopyDiag, DropCopySuggestion, DropRefDiag, ForgetCopyDiag, ForgetRefDiag,
+        UndroppedManuallyDropsDiag, UndroppedManuallyDropsSuggestion,
     },
     LateContext, LateLintPass, LintContext,
 };
@@ -163,10 +163,23 @@ impl<'tcx> LateLintPass<'tcx> for DropForgetUseless {
                     );
                 }
                 sym::mem_drop if is_copy && !drop_is_single_call_in_arm => {
+                    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
+                    {
+                        DropCopySuggestion::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 {
+                        DropCopySuggestion::Note
+                    };
+
                     cx.emit_span_lint(
                         DROPPING_COPY_TYPES,
                         expr.span,
-                        DropCopyDiag { arg_ty, label: arg.span },
+                        DropCopyDiag { arg_ty, label: arg.span, sugg },
                     );
                 }
                 sym::mem_forget if is_copy => {

compiler/rustc_lint/src/lints.rs

@@ -669,11 +669,25 @@ pub struct DropRefDiag<'a> {
 
 #[derive(LintDiagnostic)]
 #[diag(lint_dropping_copy_types)]
-#[note]
 pub struct DropCopyDiag<'a> {
     pub arg_ty: Ty<'a>,
     #[label]
     pub label: Span,
+    #[subdiagnostic]
+    pub sugg: DropCopySuggestion,
+}
+
+#[derive(Subdiagnostic)]
+pub enum DropCopySuggestion {
+    #[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,
+    },
 }
 
 #[derive(LintDiagnostic)]