rustc_resolve: inject `uniform_paths` canaries regardless of the feature-gate, on Rust 2018.

src/librustc_resolve/build_reduced_graph.rs

@@ -194,7 +194,7 @@ fn build_reduced_graph_for_use_tree(
         // ergonomically unacceptable.
         let emit_uniform_paths_canary =
             !uniform_paths_canary_emitted &&
-            uniform_paths &&
+            self.session.rust_2018() &&
             starts_with_non_keyword;
         if emit_uniform_paths_canary {
             let source = prefix_start.unwrap();

src/librustc_resolve/resolve_imports.rs

@@ -705,6 +705,7 @@ struct UniformPathsCanaryResult {
             }
         }
 
+        let uniform_paths_feature = self.session.features_untracked().uniform_paths;
         for ((span, _), (name, results)) in uniform_paths_canaries {
             self.per_ns(|this, ns| {
                 let results = &results[ns];
@@ -736,15 +737,24 @@ struct UniformPathsCanaryResult {
                         suggestion_choices.push_str(" or ");
                     }
                     write!(suggestion_choices, "`self::{}`", name);
-                    err.span_label(span,
-                        format!("can refer to `self::{}`", name));
+                    if uniform_paths_feature {
+                        err.span_label(span,
+                            format!("can refer to `self::{}`", name));
+                    } else {
+                        err.span_label(span,
+                            format!("may refer to `self::{}` in the future", name));
+                    }
                 }
                 for &span in &results.block_scopes {
                     err.span_label(span,
                         format!("shadowed by block-scoped `{}`", name));
                 }
                 err.help(&format!("write {} explicitly instead", suggestion_choices));
-                err.note("relative `use` paths enabled by `#![feature(uniform_paths)]`");
+                if uniform_paths_feature {
+                    err.note("relative `use` paths enabled by `#![feature(uniform_paths)]`");
+                } else {
+                    err.note("in the future, `#![feature(uniform_paths)]` may become the default");
+                }
                 err.emit();
             });
         }
@@ -930,11 +940,15 @@ fn finalize_import(&mut self, directive: &'b ImportDirective<'b>) -> Option<(Spa
             _ => unreachable!(),
         };
 
+        // Do not record uses from canaries, to avoid interfering with other
+        // diagnostics or suggestions that rely on some items not being used.
+        let record_used = !directive.is_uniform_paths_canary;
+
         let mut all_ns_err = true;
         self.per_ns(|this, ns| if !type_ns_only || ns == TypeNS {
             if let Ok(binding) = result[ns].get() {
                 all_ns_err = false;
-                if this.record_use(ident, ns, binding) {
+                if record_used && this.record_use(ident, ns, binding) {
                     if let ModuleOrUniformRoot::Module(module) = module {
                         this.resolution(module, ident, ns).borrow_mut().binding =
                             Some(this.dummy_binding);
@@ -946,7 +960,7 @@ fn finalize_import(&mut self, directive: &'b ImportDirective<'b>) -> Option<(Spa
         if all_ns_err {
             let mut all_ns_failed = true;
             self.per_ns(|this, ns| if !type_ns_only || ns == TypeNS {
-                match this.resolve_ident_in_module(module, ident, ns, true, span) {
+                match this.resolve_ident_in_module(module, ident, ns, record_used, span) {
                     Ok(_) => all_ns_failed = false,
                     _ => {}
                 }

src/test/ui/rust-2018/uniform-paths-forward-compat/ambiguity-macros-nested.rs

+// 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.
+
+// edition:2018
+
+// This test is similar to `ambiguity-macros.rs`, but nested in a module.
+
+mod foo {
+    pub use std::io;
+    //~^ ERROR `std` import is ambiguous
+
+    macro_rules! m {
+        () => {
+            mod std {
+                pub struct io;
+            }
+        }
+    }
+    m!();
+}
+
+fn main() {}

src/test/ui/rust-2018/uniform-paths-forward-compat/ambiguity-macros-nested.stderr

+error: `std` import is ambiguous
+  --> $DIR/ambiguity-macros-nested.rs:16:13
+   |
+LL |       pub use std::io;
+   |               ^^^ can refer to external crate `::std`
+...
+LL | /             mod std {
+LL | |                 pub struct io;
+LL | |             }
+   | |_____________- may refer to `self::std` in the future
+   |
+   = help: write `::std` or `self::std` explicitly instead
+   = note: in the future, `#![feature(uniform_paths)]` may become the default
+
+error: aborting due to previous error
+

src/test/ui/rust-2018/uniform-paths-forward-compat/ambiguity-macros.rs

+// 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.
+
+// edition:2018
+
+// This test is similar to `ambiguity.rs`, but with macros defining local items.
+
+use std::io;
+//~^ ERROR `std` import is ambiguous
+
+macro_rules! m {
+    () => {
+        mod std {
+            pub struct io;
+        }
+    }
+}
+m!();
+
+fn main() {}

src/test/ui/rust-2018/uniform-paths-forward-compat/ambiguity-macros.stderr

+error: `std` import is ambiguous
+  --> $DIR/ambiguity-macros.rs:15:5
+   |
+LL |   use std::io;
+   |       ^^^ can refer to external crate `::std`
+...
+LL | /         mod std {
+LL | |             pub struct io;
+LL | |         }
+   | |_________- may refer to `self::std` in the future
+   |
+   = help: write `::std` or `self::std` explicitly instead
+   = note: in the future, `#![feature(uniform_paths)]` may become the default
+
+error: aborting due to previous error
+

src/test/ui/rust-2018/uniform-paths-forward-compat/ambiguity-nested.rs

+// 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.
+
+// edition:2018
+
+// This test is similar to `ambiguity.rs`, but nested in a module.
+
+mod foo {
+    pub use std::io;
+    //~^ ERROR `std` import is ambiguous
+
+    mod std {
+        pub struct io;
+    }
+}
+
+fn main() {}

src/test/ui/rust-2018/uniform-paths-forward-compat/ambiguity-nested.stderr

+error: `std` import is ambiguous
+  --> $DIR/ambiguity-nested.rs:16:13
+   |
+LL |       pub use std::io;
+   |               ^^^ can refer to external crate `::std`
+...
+LL | /     mod std {
+LL | |         pub struct io;
+LL | |     }
+   | |_____- may refer to `self::std` in the future
+   |
+   = help: write `::std` or `self::std` explicitly instead
+   = note: in the future, `#![feature(uniform_paths)]` may become the default
+
+error: aborting due to previous error
+

src/test/ui/rust-2018/uniform-paths-forward-compat/ambiguity.rs

+// 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.
+
+// edition:2018
+
+use std::io;
+//~^ ERROR `std` import is ambiguous
+
+mod std {
+    pub struct io;
+}
+
+fn main() {}

src/test/ui/rust-2018/uniform-paths-forward-compat/ambiguity.stderr

+error: `std` import is ambiguous
+  --> $DIR/ambiguity.rs:13:5
+   |
+LL |   use std::io;
+   |       ^^^ can refer to external crate `::std`
+...
+LL | / mod std {
+LL | |     pub struct io;
+LL | | }
+   | |_- may refer to `self::std` in the future
+   |
+   = help: write `::std` or `self::std` explicitly instead
+   = note: in the future, `#![feature(uniform_paths)]` may become the default
+
+error: aborting due to previous error
+

src/test/ui/rust-2018/uniform-paths-forward-compat/block-scoped-shadow.rs

+// 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.
+
+// edition:2018
+
+struct std;
+
+fn main() {
+    fn std() {}
+    enum std {}
+    use std as foo;
+    //~^ ERROR `std` import is ambiguous
+    //~| ERROR `std` import is ambiguous
+}

src/test/ui/rust-2018/uniform-paths-forward-compat/block-scoped-shadow.stderr

+error: `std` import is ambiguous
+  --> $DIR/block-scoped-shadow.rs:18:9
+   |
+LL | struct std;
+   | ----------- may refer to `self::std` in the future
+...
+LL |     enum std {}
+   |     ----------- shadowed by block-scoped `std`
+LL |     use std as foo;
+   |         ^^^ can refer to external crate `::std`
+   |
+   = help: write `::std` or `self::std` explicitly instead
+   = note: in the future, `#![feature(uniform_paths)]` may become the default
+
+error: `std` import is ambiguous
+  --> $DIR/block-scoped-shadow.rs:18:9
+   |
+LL | struct std;
+   | ----------- may refer to `self::std` in the future
+...
+LL |     fn std() {}
+   |     ----------- shadowed by block-scoped `std`
+LL |     enum std {}
+LL |     use std as foo;
+   |         ^^^
+   |
+   = help: write `self::std` explicitly instead
+   = note: in the future, `#![feature(uniform_paths)]` may become the default
+
+error: aborting due to 2 previous errors
+

rls @ fa922de1

-Subproject commit 5b5cd9d45719414196e254ec17baa598acc8cd25
+Subproject commit fa922de1e5e1f02b576b7a5aa6ded16935693ec5