Improve let_underscore_lock

- lint if the lock was in a nested pattern
- lint if the lock is inside a `Result<Lock, _>`

compiler/rustc_lint/src/lints.rs

@@ -930,8 +930,7 @@ pub enum NonBindingLet {
 
 pub struct NonBindingLetSub {
     pub suggestion: Span,
-    pub multi_suggestion_start: Span,
-    pub multi_suggestion_end: Span,
+    pub drop_fn_start_end: Option<(Span, Span)>,
     pub is_assign_desugar: bool,
 }
 
@@ -940,21 +939,31 @@ impl AddToDiagnostic for NonBindingLetSub {
     where
         F: Fn(&mut Diagnostic, SubdiagnosticMessage) -> SubdiagnosticMessage,
     {
-        let prefix = if self.is_assign_desugar { "let " } else { "" };
-        diag.span_suggestion_verbose(
-            self.suggestion,
-            fluent::lint_non_binding_let_suggestion,
-            format!("{prefix}_unused"),
-            Applicability::MachineApplicable,
-        );
-        diag.multipart_suggestion(
-            fluent::lint_non_binding_let_multi_suggestion,
-            vec![
-                (self.multi_suggestion_start, "drop(".to_string()),
-                (self.multi_suggestion_end, ")".to_string()),
-            ],
-            Applicability::MachineApplicable,
-        );
+        let can_suggest_binding = self.drop_fn_start_end.is_some() || !self.is_assign_desugar;
+
+        if can_suggest_binding {
+            let prefix = if self.is_assign_desugar { "let " } else { "" };
+            diag.span_suggestion_verbose(
+                self.suggestion,
+                fluent::lint_non_binding_let_suggestion,
+                format!("{prefix}_unused"),
+                Applicability::MachineApplicable,
+            );
+        } else {
+            diag.span_help(self.suggestion, fluent::lint_non_binding_let_suggestion);
+        }
+        if let Some(drop_fn_start_end) = self.drop_fn_start_end {
+            diag.multipart_suggestion(
+                fluent::lint_non_binding_let_multi_suggestion,
+                vec![
+                    (drop_fn_start_end.0, "drop(".to_string()),
+                    (drop_fn_start_end.1, ")".to_string()),
+                ],
+                Applicability::MachineApplicable,
+            );
+        } else {
+            diag.help(fluent::lint_non_binding_let_multi_drop_fn);
+        }
     }
 }