When picking a candidate, consider the unstable ones last.

If there is potential ambiguity after stabilizing those candidates, a
warning will be emitted.

src/test/ui/auxiliary/inference_unstable_iterator.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.
+
+#![feature(staged_api)]
+
+#![stable(feature = "ipu_iterator", since = "1.0.0")]
+
+#[stable(feature = "ipu_iterator", since = "1.0.0")]
+pub trait IpuIterator {
+    #[unstable(feature = "ipu_flatten", issue = "99999")]
+    fn ipu_flatten(&self) -> u32 {
+        0
+    }
+}
+
+#[stable(feature = "ipu_iterator", since = "1.0.0")]
+impl IpuIterator for char {}

src/test/ui/auxiliary/inference_unstable_itertools.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.
+
+pub trait IpuItertools {
+    fn ipu_flatten(&self) -> u32 {
+        1
+    }
+}
+
+impl IpuItertools for char {}

src/test/ui/inference_unstable.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.
+
+// Ensures #[unstable] functions without opting in the corresponding #![feature]
+// will not break inference.
+
+// aux-build:inference_unstable_iterator.rs
+// aux-build:inference_unstable_itertools.rs
+// run-pass
+
+extern crate inference_unstable_iterator;
+extern crate inference_unstable_itertools;
+
+#[allow(unused_imports)]
+use inference_unstable_iterator::IpuIterator;
+use inference_unstable_itertools::IpuItertools;
+
+fn main() {
+    assert_eq!('x'.ipu_flatten(), 1);
+    //~^ WARN a method with this name will be added to the standard library in the future
+    //~^^ WARN it will become a hard error in a future release
+}

src/test/ui/inference_unstable.stderr

@@ -0,0 +1,12 @@
+warning: a method with this name will be added to the standard library in the future
+  --> $DIR/inference_unstable.rs:26:20
+   |
+LL |     assert_eq!('x'.ipu_flatten(), 1);
+   |                    ^^^^^^^^^^^
+   |
+   = note: #[warn(unstable_name_collision)] on by default
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see pr #48552 <https://github.com/rust-lang/rust/pull/48552>
+   = help: call with fully qualified syntax `inference_unstable_itertools::IpuItertools::ipu_flatten(...)` to keep using the current method
+   = note: add #![feature(ipu_flatten)] to the crate attributes to enable `inference_unstable_iterator::IpuIterator::ipu_flatten`
+

src/test/ui/inference_unstable_featured.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.
+
+// There should be E0034 "multiple applicable items in scope" if we opt-in for
+// the feature.
+
+// aux-build:inference_unstable_iterator.rs
+// aux-build:inference_unstable_itertools.rs
+
+#![feature(ipu_flatten)]
+
+extern crate inference_unstable_iterator;
+extern crate inference_unstable_itertools;
+
+use inference_unstable_iterator::IpuIterator;
+use inference_unstable_itertools::IpuItertools;
+
+fn main() {
+    assert_eq!('x'.ipu_flatten(), 0);   //~ ERROR E0034
+}

src/test/ui/inference_unstable_featured.stderr

@@ -0,0 +1,12 @@
+error[E0034]: multiple applicable items in scope
+  --> $DIR/inference_unstable_featured.rs:26:20
+   |
+LL |     assert_eq!('x'.ipu_flatten(), 0);   //~ ERROR E0034
+   |                    ^^^^^^^^^^^ multiple `ipu_flatten` found
+   |
+   = note: candidate #1 is defined in an impl of the trait `inference_unstable_iterator::IpuIterator` for the type `char`
+   = note: candidate #2 is defined in an impl of the trait `inference_unstable_itertools::IpuItertools` for the type `char`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0034`.

src/test/ui/inference_unstable_forced.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.
+
+// If the unstable API is the only possible solution,
+// still emit E0658 "use of unstable library feature".
+
+// aux-build:inference_unstable_iterator.rs
+
+extern crate inference_unstable_iterator;
+
+use inference_unstable_iterator::IpuIterator;
+
+fn main() {
+    assert_eq!('x'.ipu_flatten(), 0);   //~ ERROR E0658
+}

src/test/ui/inference_unstable_forced.stderr

@@ -0,0 +1,11 @@
+error[E0658]: use of unstable library feature 'ipu_flatten' (see issue #99999)
+  --> $DIR/inference_unstable_forced.rs:21:20
+   |
+LL |     assert_eq!('x'.ipu_flatten(), 0);   //~ ERROR E0658
+   |                    ^^^^^^^^^^^
+   |
+   = help: add #![feature(ipu_flatten)] to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.