use subdiagnostic for sugesting add let

compiler/rustc_error_messages/locales/en-US/infer.ftl

@@ -171,3 +171,4 @@ infer_msl_introduces_static = introduces a `'static` lifetime requirement
 infer_msl_unmet_req = because this has an unmet lifetime requirement
 infer_msl_trait_note = this has an implicit `'static` lifetime requirement
 infer_msl_trait_sugg = consider relaxing the implicit `'static` requirement
+infer_suggest_add_let_for_letchains = consider adding `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(
+                err.subdiagnostic(SuggAddLetForLetChains{span: span.shrink_to_lo()});
-                    span.shrink_to_lo(),
-                    "consider adding `let`",
-                    "let ".to_string(),
-                    Applicability::MachineApplicable,
-                );
             }
         }
     }

compiler/rustc_parse/src/errors.rs

@@ -425,7 +425,7 @@ pub(crate) struct ExpectedExpressionFoundLet {
 pub(crate) struct ExpectedEqForLetExpr {
     #[primary_span]
     pub span: Span,
-    #[suggestion_verbose(applicability = "maybe-incorrect", code = "=")]
+    #[suggestion(applicability = "maybe-incorrect", code = "=", style = "verbose")]
     pub sugg_span: Span,
 }
 

src/test/ui/did_you_mean/issue-103909.stderr

@@ -14,6 +14,11 @@ error[E0308]: mismatched types
    |
 LL |     if Err(err) = File::open("hello.txt") {
    |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `bool`, found `()`
+   |
+help: consider adding `let`
+   |
+LL |     if let Err(err) = File::open("hello.txt") {
+   |        +++
 
 error: aborting due to 2 previous errors