use subdiagnostic for sugesting add let

compiler/rustc_infer/src/errors/mod.rs

@@ -180,6 +180,18 @@ pub enum SourceKindMultiSuggestion<'a> {
     },
 }
 
+#[derive(Subdiagnostic)]
+#[suggestion(
+    infer_suggest_add_let_for_letchains,
+    style = "verbose",
+    applicability = "machine-applicable",
+    code = "let "
+)]
+pub(crate) struct SuggAddLetForLetChains {
+    #[primary_span]
+    pub span: Span,
+}
+
 impl<'a> SourceKindMultiSuggestion<'a> {
     pub fn new_fully_qualified(
         span: Span,

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -59,6 +59,7 @@ use crate::traits::{
     StatementAsExpression,
 };
 
+use crate::errors::SuggAddLetForLetChains;
 use hir::intravisit::{walk_expr, walk_stmt};
 use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
 use rustc_errors::{pluralize, struct_span_err, Diagnostic, ErrorGuaranteed, IntoDiagnosticArg};
@@ -2421,12 +2422,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         let mut visitor = IfVisitor { err_span: span, found_if: false, result: false };
         visitor.visit_body(&body);
         if visitor.result {
-                err.span_suggestion_verbose(
-                    span.shrink_to_lo(),
-                    "consider adding `let`",
-                    "let ".to_string(),
-                    Applicability::MachineApplicable,
-                );
+                err.subdiagnostic(SuggAddLetForLetChains{span: span.shrink_to_lo()});
             }
         }
     }