rustc: Disallow machine applicability in foreign macros

Recent changes to lints disallowed lints from being emitted against code located
in foreign macros, except for future-incompatible lints. For a future
incompatible lint, however, the automatic suggestions may not be applicable!

This commit updates this code path to force all applicability suggestions made
to foreign macros to never be `MachineApplicable`. This should avoid rustfix
actually attempting fixing these suggestions, causing non-compiling code to be
produced.

Closes rust-lang/cargo#5799

src/test/ui-fulldeps/rust-2018/auxiliary/suggestions-not-always-applicable.rs

@@ -0,0 +1,22 @@
+// Copyright 2018 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
+
+#![crate_type = "proc-macro"]
+
+extern crate proc_macro;
+
+use proc_macro::*;
+
+#[proc_macro_attribute]
+pub fn foo(_attr: TokenStream, _f: TokenStream) -> TokenStream {
+    "pub fn foo() -> ::Foo { ::Foo }".parse().unwrap()
+}

src/test/ui-fulldeps/rust-2018/suggestions-not-always-applicable.fixed

@@ -0,0 +1,37 @@
+// Copyright 2018 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:suggestions-not-always-applicable.rs
+// compile-flags: --edition 2015
+// run-rustfix
+// rustfix-only-machine-applicable
+// compile-pass
+
+#![feature(rust_2018_preview)]
+#![warn(rust_2018_compatibility)]
+
+extern crate suggestions_not_always_applicable as foo;
+
+pub struct Foo;
+
+mod test {
+    use crate::foo::foo;
+
+    #[foo] //~ WARN: absolute paths must start with
+    //~| WARN: previously accepted
+    //~| WARN: absolute paths
+    //~| WARN: previously accepted
+    fn main() {
+    }
+}
+
+fn main() {
+    test::foo();
+}

src/test/ui-fulldeps/rust-2018/suggestions-not-always-applicable.rs

@@ -0,0 +1,37 @@
+// Copyright 2018 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:suggestions-not-always-applicable.rs
+// compile-flags: --edition 2015
+// run-rustfix
+// rustfix-only-machine-applicable
+// compile-pass
+
+#![feature(rust_2018_preview)]
+#![warn(rust_2018_compatibility)]
+
+extern crate suggestions_not_always_applicable as foo;
+
+pub struct Foo;
+
+mod test {
+    use crate::foo::foo;
+
+    #[foo] //~ WARN: absolute paths must start with
+    //~| WARN: previously accepted
+    //~| WARN: absolute paths
+    //~| WARN: previously accepted
+    fn main() {
+    }
+}
+
+fn main() {
+    test::foo();
+}

src/test/ui-fulldeps/rust-2018/suggestions-not-always-applicable.stderr

@@ -0,0 +1,24 @@
+warning: absolute paths must start with `self`, `super`, `crate`, or an external crate name in the 2018 edition
+  --> $DIR/suggestions-not-always-applicable.rs:27:5
+   |
+LL |     #[foo] //~ WARN: absolute paths must start with
+   |     ^^^^^^
+   |
+note: lint level defined here
+  --> $DIR/suggestions-not-always-applicable.rs:18:9
+   |
+LL | #![warn(rust_2018_compatibility)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^
+   = note: #[warn(absolute_paths_not_starting_with_crate)] implied by #[warn(rust_2018_compatibility)]
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in the 2018 edition!
+   = note: for more information, see issue TBD
+
+warning: absolute paths must start with `self`, `super`, `crate`, or an external crate name in the 2018 edition
+  --> $DIR/suggestions-not-always-applicable.rs:27:5
+   |
+LL |     #[foo] //~ WARN: absolute paths must start with
+   |     ^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in the 2018 edition!
+   = note: for more information, see issue TBD
+