Add example of making an unhygienic macro hygienic by wrapping it in a declarative macro.

Author: jseyfried

src/test/run-pass/hygiene/auxiliary/my_crate.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.
+
+pub fn f() {}

src/test/run-pass/hygiene/auxiliary/unhygienic_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.
+
+#![crate_type = "lib"]
+
+extern crate my_crate;
+
+pub fn g() {} // (a)
+
+#[macro_export]
+macro_rules! unhygienic_macro {
+    () => {
+        // (1) unhygienic: depends on `my_crate` in the crate root at the invocation site.
+        ::my_crate::f();
+
+        // (2) unhygienic: defines `f` at the invocation site (in addition to the above point).
+        use my_crate::f;
+        f();
+
+        g(); // (3) unhygienic: `g` needs to be in scope at use site.
+
+        $crate::g(); // (4) hygienic: this always resolves to (a)
+    }
+}
+
+#[allow(unused)]
+fn test_unhygienic() {
+    unhygienic_macro!();
+    f(); // `f` was defined at the use site
+}

src/test/run-pass/hygiene/wrap_unhygienic_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.
+
+// ignore-pretty pretty-printing is unhygienic
+
+// aux-build:my_crate.rs
+// aux-build:unhygienic_example.rs
+
+#![feature(decl_macro)]
+
+extern crate unhygienic_example;
+extern crate my_crate; // (b)
+
+// Hygienic version of `unhygienic_macro`.
+pub macro hygienic_macro() {
+    fn g() {} // (c)
+    ::unhygienic_example::unhygienic_macro!();
+    // ^ Even though we invoke an unhygienic macro, `hygienic_macro` remains hygienic.
+    // In the above expansion:
+    // (1) `my_crate` always resolves to (b) regardless of invocation site.
+    // (2) The defined function `f` is only usable inside this macro definition.
+    // (3) `g` always resolves to (c) regardless of invocation site.
+    // (4) `$crate::g` remains hygienic and continues to resolve to (a).
+
+    f();
+}
+
+#[allow(unused)]
+fn test_hygienic_macro() {
+    hygienic_macro!();
+
+    fn f() {} // (d) no conflict
+    f(); // resolves to (d)
+}
+
+fn main() {}