Add exmaple/test for quote! hygiene.

Author: jseyfried

src/test/run-pass-fulldeps/proc-macro/auxiliary/hygiene_example.rs

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+// Copyright 2017 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.
+
+#![feature(proc_macro)]
+
+extern crate hygiene_example_codegen;
+
+pub use hygiene_example_codegen::hello;
+
+pub fn print(string: &str) {
+    println!("{}", string);
+}

src/test/run-pass-fulldeps/proc-macro/auxiliary/hygiene_example_codegen.rs

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+// Copyright 2017 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.
+
+// no-prefer-dynamic
+
+#![feature(proc_macro)]
+#![crate_type = "proc-macro"]
+
+extern crate proc_macro as proc_macro_renamed; // This does not break `quote!`
+
+use proc_macro_renamed::{TokenStream, quote};
+
+#[proc_macro]
+pub fn hello(input: TokenStream) -> TokenStream {
+    quote!(hello_helper!($input))
+    //^ `hello_helper!` always resolves to the following proc macro,
+    //| no matter where `hello!` is used.
+}
+
+#[proc_macro]
+pub fn hello_helper(input: TokenStream) -> TokenStream {
+    quote! {
+        extern crate hygiene_example; // This is never a conflict error
+        let string = format!("hello {}", $input);
+        //^ `format!` always resolves to the prelude macro,
+        //| even if a different `format!` is in scope where `hello!` is used.
+        hygiene_example::print(&string)
+    }
+}

src/test/run-pass-fulldeps/proc-macro/hygiene_example.rs

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+// Copyright 2017 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:hygiene_example_codegen.rs
+// aux-build:hygiene_example.rs
+
+#![feature(proc_macro)]
+
+extern crate hygiene_example;
+use hygiene_example::hello;
+
+fn main() {
+    mod hygiene_example {} // no conflict with `extern crate hygiene_example;` from the proc macro
+    macro_rules! format { () => {} } // does not interfere with `format!` from the proc macro
+    macro_rules! hello_helper { () => {} } // similarly does not intefere with the proc macro
+
+    let string = "world"; // no conflict with `string` from the proc macro
+    hello!(string);
+    hello!(string);
+}