Fix removed box_syntax diagnostic if source isn't available

compiler/rustc_parse/src/errors.rs

@@ -2725,15 +2725,24 @@ impl HelpUseLatestEdition {
 
 #[derive(Diagnostic)]
 #[diag(parse_box_syntax_removed)]
-pub struct BoxSyntaxRemoved<'a> {
+pub struct BoxSyntaxRemoved {
     #[primary_span]
-    #[suggestion(
-        code = "Box::new({code})",
-        applicability = "machine-applicable",
-        style = "verbose"
-    )]
     pub span: Span,
-    pub code: &'a str,
+    #[subdiagnostic]
+    pub sugg: AddBoxNew,
+}
+
+#[derive(Subdiagnostic)]
+#[multipart_suggestion(
+    parse_box_syntax_removed_suggestion,
+    applicability = "machine-applicable",
+    style = "verbose"
+)]
+pub struct AddBoxNew {
+    #[suggestion_part(code = "Box::new(")]
+    pub box_kw_and_lo: Span,
+    #[suggestion_part(code = ")")]
+    pub hi: Span,
 }
 
 #[derive(Diagnostic)]

compiler/rustc_parse/src/parser/expr.rs

@@ -618,10 +618,12 @@ impl<'a> Parser<'a> {
     /// Parse `box expr` - this syntax has been removed, but we still parse this
     /// for now to provide a more useful error
     fn parse_expr_box(&mut self, box_kw: Span) -> PResult<'a, (Span, ExprKind)> {
-        let (span, _) = self.parse_expr_prefix_common(box_kw)?;
-        let inner_span = span.with_lo(box_kw.hi());
-        let code = self.psess.source_map().span_to_snippet(inner_span).unwrap();
-        let guar = self.dcx().emit_err(errors::BoxSyntaxRemoved { span: span, code: code.trim() });
+        let (span, expr) = self.parse_expr_prefix_common(box_kw)?;
+        // Make a multipart suggestion instead of `span_to_snippet` in case source isn't available
+        let box_kw_and_lo = box_kw.until(self.interpolated_or_expr_span(&expr));
+        let hi = span.shrink_to_hi();
+        let sugg = errors::AddBoxNew { box_kw_and_lo, hi };
+        let guar = self.dcx().emit_err(errors::BoxSyntaxRemoved { span, sugg });
         Ok((span, ExprKind::Err(guar)))
     }