Followup to PR #16477: a run-pass regression test for Issue #15750.

Author: pnkfelix

src/test/auxiliary/macro_crate_test.rs

@@ -19,6 +19,7 @@ use syntax::ast::{TokenTree, Item, MetaItem};
 use syntax::codemap::Span;
 use syntax::ext::base::*;
 use syntax::parse::token;
+use syntax::parse;
 use rustc::plugin::Registry;
 
 use std::gc::{Gc, GC};
@@ -32,6 +33,7 @@ macro_rules! unexported_macro (() => (3i))
 pub fn plugin_registrar(reg: &mut Registry) {
     reg.register_macro("make_a_1", expand_make_a_1);
     reg.register_macro("forged_ident", expand_forged_ident);
+    reg.register_macro("identity", expand_identity);
     reg.register_syntax_extension(
         token::intern("into_foo"),
         ItemModifier(expand_into_foo));
@@ -45,6 +47,16 @@ fn expand_make_a_1(cx: &mut ExtCtxt, sp: Span, tts: &[TokenTree])
     MacExpr::new(quote_expr!(cx, 1i))
 }
 
+// See Issue #15750
+fn expand_identity(cx: &mut ExtCtxt, _span: Span, tts: &[TokenTree])
+                   -> Box<MacResult> {
+    // Parse an expression and emit it unchanged.
+    let mut parser = parse::new_parser_from_tts(cx.parse_sess(),
+        cx.cfg(), Vec::from_slice(tts));
+    let expr = parser.parse_expr();
+    MacExpr::new(quote_expr!(&mut *cx, $expr))
+}
+
 fn expand_into_foo(cx: &mut ExtCtxt, sp: Span, attr: Gc<MetaItem>, it: Gc<Item>)
                    -> Gc<Item> {
     box(GC) Item {

src/test/run-pass-fulldeps/macro-crate-does-hygiene-work.rs

@@ -0,0 +1,28 @@
+// Copyright 2013-2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// aux-build:macro_crate_test.rs
+// ignore-stage1
+
+// Issue #15750: a macro that internally parses its input and then
+// uses `quote_expr!` to rearrange it should be hygiene-preserving.
+
+#![feature(phase)]
+
+#[phase(plugin)]
+extern crate macro_crate_test;
+
+fn main() {
+    let x = 3i;
+    assert_eq!(3, identity!(x));
+    assert_eq!(6, identity!(x+x));
+    let x = 4i;
+    assert_eq!(4, identity!(x));
+}