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

We now track the edition of each span. Using that info when gating the
Rust 2018 interpretation means that macros from Rust 2015 crates "just
work" when used in Rust 2018 crates (at least in the case of `use`
paths).

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());
+}