Improve ternary operator recovery

compiler/rustc_parse/src/errors.rs

@@ -436,10 +436,28 @@ pub(crate) enum IfExpressionMissingThenBlockSub {
 
 #[derive(Diagnostic)]
 #[diag(parse_ternary_operator)]
-#[help]
 pub(crate) struct TernaryOperator {
     #[primary_span]
     pub span: Span,
+    /// If we have a span for the condition expression, suggest the if/else
+    #[subdiagnostic]
+    pub sugg: Option<TernaryOperatorSuggestion>,
+    /// Otherwise, just print the suggestion message
+    #[help(parse_use_if_else)]
+    pub no_sugg: bool,
+}
+
+#[derive(Subdiagnostic, Copy, Clone)]
+#[multipart_suggestion(parse_use_if_else, applicability = "maybe-incorrect", style = "verbose")]
+pub(crate) struct TernaryOperatorSuggestion {
+    #[suggestion_part(code = "if ")]
+    pub before_cond: Span,
+    #[suggestion_part(code = "{{")]
+    pub question: Span,
+    #[suggestion_part(code = "}} else {{")]
+    pub colon: Span,
+    #[suggestion_part(code = " }}")]
+    pub end: Span,
 }
 
 #[derive(Subdiagnostic)]