Switch pretty-printer roundtrip test to better parser

tests/ui-fulldeps/pprust-expr-roundtrip.rs

@@ -22,6 +22,7 @@
 extern crate rustc_ast;
 extern crate rustc_ast_pretty;
 extern crate rustc_data_structures;
+extern crate rustc_errors;
 extern crate rustc_parse;
 extern crate rustc_session;
 extern crate rustc_span;
@@ -32,27 +33,20 @@ extern crate thin_vec;
 #[allow(unused_extern_crates)]
 extern crate rustc_driver;
 
+#[path = "auxiliary/parser.rs"]
+mod parser;
+
 use rustc_ast::mut_visit::{visit_clobber, MutVisitor};
 use rustc_ast::ptr::P;
 use rustc_ast::*;
 use rustc_ast_pretty::pprust;
-use rustc_parse::{new_parser_from_source_str, unwrap_or_emit_fatal};
 use rustc_session::parse::ParseSess;
 use rustc_span::source_map::Spanned;
 use rustc_span::symbol::Ident;
-use rustc_span::{FileName, DUMMY_SP};
+use rustc_span::DUMMY_SP;
 use thin_vec::{thin_vec, ThinVec};
 
-fn parse_expr(psess: &ParseSess, src: &str) -> Option<P<Expr>> {
-    let src_as_string = src.to_string();
-
-    let mut p = unwrap_or_emit_fatal(new_parser_from_source_str(
-        psess,
-        FileName::Custom(src_as_string.clone()),
-        src_as_string,
-    ));
-    p.parse_expr().map_err(|e| e.cancel()).ok()
-}
+use crate::parser::parse_expr;
 
 // Helper functions for building exprs
 fn expr(kind: ExprKind) -> P<Expr> {