don't ask what edition we are in; ask what edition a span is in

src/librustc/lint/mod.rs

@@ -89,7 +89,13 @@ impl Lint {
 
     pub fn default_level(&self, session: &Session) -> Level {
         if let Some(edition_deny) = self.edition_deny {
-            if session.edition() >= edition_deny {
+            // Ideally, we would get the edition for the actual span,
+            // but that is kind of a pain in the neck to do right
+            // now. Also, lints are not breaking things anyway (due to
+            // `-Acap-lints`), and the lint itself should probably be
+            // checking the span to see if the code was injected via
+            // macro etc, so for now we'll just use the local level.
+            if session.local_edition() >= edition_deny {
                 return Level::Deny
             }
         }

src/librustc/session/mod.rs

@@ -948,12 +948,14 @@ impl Session {
         self.opts.debugging_opts.teach && self.parse_sess.span_diagnostic.must_teach(code)
     }
 
-    /// Are we allowed to use features from the Rust 2018 edition?
-    pub fn rust_2018(&self) -> bool {
-        self.opts.edition >= Edition::Edition2018
-    }
-
-    pub fn edition(&self) -> Edition {
+    /// What is the edition of the "local crate" being compiled?
+    ///
+    /// You should not call this except as a last resort. It is better
+    /// to do `span.edition()` instead, which gives the edition for a
+    /// particular span: that way, when you are looking at code
+    /// creating a macro from a Rust 2015 crate, you will use the Rust
+    /// 2015 Edition rules.
+    pub fn local_edition(&self) -> Edition {
         self.opts.edition
     }
 }

src/librustc_driver/driver.rs

@@ -736,7 +736,7 @@ where
         krate,
         &sess.parse_sess,
         sess.opts.test,
-        sess.edition(),
+        sess.local_edition(),
     );
     // these need to be set "early" so that expansion sees `quote` if enabled.
     sess.init_features(features);

src/librustc_resolve/lib.rs

@@ -3276,7 +3276,7 @@ impl<'a> Resolver<'a> {
                     if prev_name == keywords::Extern.name() ||
                        prev_name == keywords::CrateRoot.name() &&
                        self.session.features_untracked().extern_absolute_paths &&
-                       self.session.rust_2018() {
+                       path_span.edition().rust_2018() {
                         // `::extern_crate::a::b`
                         let crate_id = self.crate_loader.process_path_extern(name, ident.span);
                         let crate_root =
@@ -3446,7 +3446,7 @@ impl<'a> Resolver<'a> {
 
     fn lint_path_starts_with_module(&self, id: NodeId, span: Span) {
         // In the 2018 edition this lint is a hard error, so nothing to do
-        if self.session.rust_2018() {
+        if span.edition().rust_2018() {
             return
         }
         // In the 2015 edition there's no use in emitting lints unless the

src/librustc_resolve/resolve_imports.rs

@@ -677,7 +677,7 @@ impl<'a, 'b:'a> ImportResolver<'a, 'b> {
                                       module_path[0].name == keywords::Extern.name()) {
             let is_extern = module_path[0].name == keywords::Extern.name() ||
                             (self.session.features_untracked().extern_absolute_paths &&
-                             self.session.rust_2018());
+                             span.edition().rust_2018());
             match directive.subclass {
                 GlobImport { .. } if is_extern => {
                     return Some((directive.span,

src/librustc_typeck/check/method/probe.rs

@@ -334,10 +334,10 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                         // this case used to be allowed by the compiler,
                         // so we do a future-compat lint here for the 2015 edition
                         // (see https://github.com/rust-lang/rust/issues/46906)
-                        if self.tcx.sess.rust_2018() {
-                          span_err!(self.tcx.sess, span, E0908,
-                                    "the type of this value must be known \
-                                     to call a method on a raw pointer on it");
+                        if span.edition().rust_2018() {
+                            span_err!(self.tcx.sess, span, E0908,
+                                      "the type of this value must be known \
+                                       to call a method on a raw pointer on it");
                         } else {
                             self.tcx.lint_node(
                                 lint::builtin::TYVAR_BEHIND_RAW_POINTER,

src/libsyntax_pos/edition.rs

@@ -68,6 +68,11 @@ impl Edition {
             Edition::Edition2018 => false,
         }
     }
+
+    /// Is this at least 2018?
+    pub fn rust_2018(self) -> bool {
+        self >= Edition::Edition2018
+    }
 }
 
 impl FromStr for Edition {

src/test/ui/rust-2018/auxiliary/inject-2015-use-root-module-lib.rs

@@ -0,0 +1,29 @@
+// 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.
+
+// this is a rust 2015 crate
+
+#[macro_export]
+macro_rules! inject_me_at_the_root {
+    ($name1:ident, $name2:ident) => {
+        mod $name1 {
+            pub(crate) const THE_CONSTANT: u32 = 22;
+        }
+
+        fn $name2() -> u32 {
+            // Key point: this `use` statement -- in Rust 2018 --
+            // would be an error. But because this crate is in Rust
+            // 2015, it works, even when executed from a Rust 2018
+            // environment.
+            use $name1::THE_CONSTANT;
+            THE_CONSTANT
+        }
+    }
+}

src/test/ui/rust-2018/inject-2015-use-root-module.rs

@@ -0,0 +1,27 @@
+// 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.
+
+// compile-flags:--edition 2018
+// aux-build:inject-2015-use-root-module-lib.rs
+// run-pass
+
+// The macro `inject_me_at_the_root!` generates some code that uses
+// `use x::y` to name the global item `x`. In Rust 2018, that should
+// be `use crate::x::y`, but we test here that we still accept it,
+// as `inject_2015_lib` is in the 2015 edition.
+
+#[macro_use]
+extern crate inject_2015_use_root_module_lib;
+
+inject_me_at_the_root!(x, y);
+
+fn main() {
+    println!("Hello, world: {}", y());
+}