Add std/core to prelude if extern_prelude enabled

Fixes #50605

src/librustc_resolve/lib.rs

@@ -1610,6 +1610,16 @@ impl<'a> Resolver<'a> {
         DefCollector::new(&mut definitions, Mark::root())
             .collect_root(crate_name, session.local_crate_disambiguator());
 
+        let mut extern_prelude: FxHashSet<Name> =
+            session.opts.externs.iter().map(|kv| Symbol::intern(kv.0)).collect();
+        if !attr::contains_name(&krate.attrs, "no_core") {
+            if !attr::contains_name(&krate.attrs, "no_std") {
+                extern_prelude.insert(Symbol::intern("std"));
+            } else {
+                extern_prelude.insert(Symbol::intern("core"));
+            }
+        }
+
         let mut invocations = FxHashMap();
         invocations.insert(Mark::root(),
                            arenas.alloc_invocation_data(InvocationData::root(graph_root)));
@@ -1630,7 +1640,7 @@ impl<'a> Resolver<'a> {
             // AST.
             graph_root,
             prelude: None,
-            extern_prelude: session.opts.externs.iter().map(|kv| Symbol::intern(kv.0)).collect(),
+            extern_prelude,
 
             has_self: FxHashSet(),
             field_names: FxHashMap(),

src/test/run-pass/extern-prelude-core.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.
+
+#![feature(extern_prelude, lang_items, start, alloc)]
+#![no_std]
+
+extern crate std as other;
+
+mod foo {
+    pub fn test() {
+        let x = core::cmp::min(2, 3);
+        assert_eq!(x, 2);
+    }
+}
+
+#[start]
+fn start(_argc: isize, _argv: *const *const u8) -> isize {
+    foo::test();
+    0
+}

src/test/run-pass/extern-prelude-std.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.
+
+#![feature(extern_prelude)]
+
+mod foo {
+    pub fn test() {
+        let x = std::cmp::min(2, 3);
+        assert_eq!(x, 2);
+    }
+}
+
+fn main() {
+    foo::test();
+}