Add rustc_hir_pretty::item_to_string function

Author: GuillaumeGomez

Diff for: compiler/rustc_hir_pretty/src/lib.rs

@@ -325,6 +325,10 @@ pub fn expr_to_string(ann: &dyn PpAnn, pat: &hir::Expr<'_>) -> String {
     to_string(ann, |s| s.print_expr(pat))
 }
 
+pub fn item_to_string(ann: &dyn PpAnn, pat: &hir::Item<'_>) -> String {
+    to_string(ann, |s| s.print_item(pat))
+}
+
 impl<'a> State<'a> {
     fn bclose_maybe_open(&mut self, span: rustc_span::Span, close_box: bool) {
         self.maybe_print_comment(span.hi());