Auto merge of #46551 - jseyfried:improve_legacy_modern_macro_interaction, r=nrc

macros: improve 1.0/2.0 interaction

This PR supports using unhygienic macros from hygienic macros without breaking the latter's hygiene.
```rust
// crate A:
#[macro_export]
macro_rules! m1 { () => {
    f(); // unhygienic: this macro needs `f` in its environment
    fn g() {} // (1) unhygienic: `g` is usable outside the macro definition
} }

// crate B:
#![feature(decl_macro)]
extern crate A;
use A::m1;

macro m2() {
    fn f() {} // (2)
    m1!(); // After this PR, `f()` in the expansion resolves to (2), not (3)
    g(); // After this PR, this resolves to `fn g() {}` from the above expansion.
         // Today, it is a resolution error.
}

fn test() {
    fn f() {} // (3)
    m2!(); // Today, `m2!()` can see (3) even though it should be hygienic.
    fn g() {} // Today, this conflicts with `fn g() {}` from the expansion, even though it should be hygienic.
}
```

Once this PR lands, you can make an existing unhygienic macro hygienic by wrapping it in a hygienic macro. There is an [example](https://github.com/rust-lang/rust/pull/46551/commits/b766fa887dc0e4b923a38751fe4d570e35a75710) of this in the tests.

r? @nrc

src/test/run-pass/hygiene/auxiliary/legacy_interaction.rs

@@ -0,0 +1,19 @@
+// 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
+
+#[macro_export]
+macro_rules! m {
+    () => {
+        fn f() {} // (2)
+        g(); // (1)
+    }
+}

src/test/run-pass/hygiene/auxiliary/my_crate.rs

@@ -0,0 +1,11 @@
+// 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

@@ -0,0 +1,37 @@
+// 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/legacy_interaction.rs

@@ -0,0 +1,50 @@
+// 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:legacy_interaction.rs
+
+#![feature(decl_macro)]
+#[allow(unused)]
+
+extern crate legacy_interaction;
+// ^ defines
+// ```rust
+//  macro_rules! m {
+//     () => {
+//         fn f() // (1)
+//         g() // (2)
+//     }
+// }
+// ```rust
+
+mod def_site {
+    // Unless this macro opts out of hygiene, it should resolve the same wherever it is invoked.
+    pub macro m2() {
+        ::legacy_interaction::m!();
+        f(); // This should resolve to (1)
+        fn g() {} // We want (2) resolve to this, not to (4)
+    }
+}
+
+mod use_site {
+    fn test() {
+        fn f() -> bool { true } // (3)
+        fn g() -> bool { true } // (4)
+
+        ::def_site::m2!();
+
+        let _: bool = f(); // This should resolve to (3)
+        let _: bool = g(); // This should resolve to (4)
+    }
+}
+
+fn main() {}

src/test/run-pass/hygiene/wrap_unhygienic_example.rs

@@ -0,0 +1,43 @@
+// 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() {}