move sugg to derive session diagnostic

Author: chenyukang

Diff for: compiler/rustc_parse/messages.ftl

@@ -339,6 +339,7 @@ parse_expected_identifier = expected identifier
 parse_sugg_escape_identifier = escape `{$ident_name}` to use it as an identifier
 
 parse_sugg_remove_comma = remove this comma
+parse_sugg_add_let_for_stmt = you might have meant to introduce a new binding
 
 parse_expected_semi_found_reserved_identifier_str = expected `;`, found reserved identifier `{$token}`
 parse_expected_semi_found_keyword_str = expected `;`, found keyword `{$token}`

Diff for: compiler/rustc_parse/src/errors.rs

@@ -906,6 +906,18 @@ pub(crate) struct SuggRemoveComma {
     pub span: Span,
 }
 
+#[derive(Subdiagnostic)]
+#[suggestion(
+    parse_sugg_add_let_for_stmt,
+    style = "verbose",
+    applicability = "maybe-incorrect",
+    code = "let "
+)]
+pub(crate) struct SuggAddMissingLetStmt {
+    #[primary_span]
+    pub span: Span,
+}
+
 #[derive(Subdiagnostic)]
 pub(crate) enum ExpectedIdentifierFound {
     #[label(parse_expected_identifier_found_reserved_identifier)]

Diff for: compiler/rustc_parse/src/parser/diagnostics.rs

@@ -13,7 +13,7 @@ use crate::errors::{
     IncorrectUseOfAwait, ParenthesesInForHead, ParenthesesInForHeadSugg,
     PatternMethodParamWithoutBody, QuestionMarkInType, QuestionMarkInTypeSugg, SelfParamNotFirst,
     StructLiteralBodyWithoutPath, StructLiteralBodyWithoutPathSugg, StructLiteralNeedingParens,
-    StructLiteralNeedingParensSugg, SuggEscapeIdentifier, SuggRemoveComma,
+    StructLiteralNeedingParensSugg, SuggAddMissingLetStmt, SuggEscapeIdentifier, SuggRemoveComma,
     UnexpectedConstInGenericParam, UnexpectedConstParamDeclaration,
     UnexpectedConstParamDeclarationSugg, UnmatchedAngleBrackets, UseEqInstead,
 };
@@ -32,8 +32,8 @@ use rustc_ast::{
 use rustc_ast_pretty::pprust;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::{
-    pluralize, Applicability, Diagnostic, DiagnosticBuilder, DiagnosticMessage, ErrorGuaranteed,
-    FatalError, Handler, IntoDiagnostic, MultiSpan, PResult,
+    pluralize, AddToDiagnostic, Applicability, Diagnostic, DiagnosticBuilder, DiagnosticMessage,
+    ErrorGuaranteed, FatalError, Handler, IntoDiagnostic, MultiSpan, PResult,
 };
 use rustc_session::errors::ExprParenthesesNeeded;
 use rustc_span::source_map::Spanned;
@@ -1019,12 +1019,8 @@ impl<'a> Parser<'a> {
             match self.parse_ty() {
                 Ok(_) => {
                     if self.token == token::Eq {
-                        err.span_suggestion_verbose(
-                            prev_span,
-                            "you might have meant to introduce a new binding",
-                            "let ".to_string(),
-                            Applicability::MaybeIncorrect,
-                        );
+                        let sugg = SuggAddMissingLetStmt { span: prev_span };
+                        sugg.add_to_diagnostic(err);
                     }
                 }
                 Err(e) => {