rustc_resolve: always include core, std and meta in the extern prelude.

src/test/ui/rfc-2126-extern-absolute-paths/meta.rs

@@ -0,0 +1,17 @@
+// 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
+
+// Tests that `meta` is whitelisted, even if the crate doesn't exist
+// yet (i.e. it causes a different error than `not-whitelisted.rs`).
+use meta; //~ ERROR can't find crate for `meta`
+
+fn main() {}

src/test/ui/rfc-2126-extern-absolute-paths/meta.stderr

@@ -0,0 +1,9 @@
+error[E0463]: can't find crate for `meta`
+  --> $DIR/meta.rs:15:5
+   |
+LL | use meta; //~ ERROR can't find crate for `meta`
+   |     ^^^^ can't find crate
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0463`.

src/test/ui/rfc-2126-extern-absolute-paths/not-whitelisted.rs

@@ -0,0 +1,19 @@
+// 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
+
+// Tests that arbitrary crates (other than `core`, `std` and `meta`)
+// aren't allowed without `--extern`, even if they're in the sysroot.
+use alloc; //~ ERROR unresolved import `alloc`
+use test; //~ ERROR unresolved import `test`
+use proc_macro; //~ ERROR unresolved import `proc_macro`
+
+fn main() {}

src/test/ui/rfc-2126-extern-absolute-paths/not-whitelisted.stderr

@@ -0,0 +1,21 @@
+error[E0432]: unresolved import `alloc`
+  --> $DIR/not-whitelisted.rs:15:5
+   |
+LL | use alloc; //~ ERROR unresolved import `alloc`
+   |     ^^^^^ no `alloc` external crate
+
+error[E0432]: unresolved import `test`
+  --> $DIR/not-whitelisted.rs:16:5
+   |
+LL | use test; //~ ERROR unresolved import `test`
+   |     ^^^^ no `test` external crate
+
+error[E0432]: unresolved import `proc_macro`
+  --> $DIR/not-whitelisted.rs:17:5
+   |
+LL | use proc_macro; //~ ERROR unresolved import `proc_macro`
+   |     ^^^^^^^^^^ no `proc_macro` external crate
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0432`.

src/test/ui/run-pass/rfcs/rfc-2126-extern-absolute-paths/whitelisted.rs

@@ -0,0 +1,24 @@
+// 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.
+
+// run-pass
+// edition:2018
+
+// Tests that `core` and `std` are always available.
+use core::iter;
+use std::io;
+// FIXME(eddyb) Add a `meta` crate to the distribution.
+// use meta;
+
+fn main() {
+    for _ in iter::once(()) {
+        io::stdout();
+    }
+}

src/test/ui/rust-2018/remove-extern-crate.fixed

@@ -12,7 +12,7 @@
 // edition:2018
 // compile-pass
 // aux-build:remove-extern-crate.rs
-// compile-flags:--extern remove_extern_crate --extern core
+// compile-flags:--extern remove_extern_crate
 
 #![warn(rust_2018_idioms)]
 

src/test/ui/rust-2018/remove-extern-crate.rs

@@ -12,7 +12,7 @@
 // edition:2018
 // compile-pass
 // aux-build:remove-extern-crate.rs
-// compile-flags:--extern remove_extern_crate --extern core
+// compile-flags:--extern remove_extern_crate
 
 #![warn(rust_2018_idioms)]