Auto merge of #48552 - kennytm:lower-unstable-priority, r=nikomatsakis

Lower the priority of unstable methods when picking a candidate.

Previously, when searching for the impl of a method, we do not consider the stability of the impl. This leads to lots of insta-inference-regressions due to method ambiguity when a popular name is chosen. This has happened multiple times in Rust's history e.g.

* `f64::from_bits` #40470
* `Ord::{min, max}` #42496
* `Ord::clamp` #44095 (eventually got reverted due to these breakages)
* `Iterator::flatten` #48115 (recently added)

This PR changes the probing order so that unstable items are considered last. If a stable item is found, the unstable items will not be considered (but a future-incompatible warning will still be emitted), thus allowing stable code continue to function without using qualified names.

Once the unstable feature is stabilized, the ambiguity error will still be emitted, but the user can also use newly stable std methods, while the current situation is that downstream user is forced to update the code without any immediate benefit.

(I hope that we could bring back `Ord::clamp` if this PR is merged.)

10 files changed, 157 insertions, 9 deletions

diff --git a/src/test/ui/auxiliary/inference_unstable_iterator.rs b/src/test/ui/auxiliary/inference_unstable_iterator.rs
new file mode 100644
index 00000000000..b73346e6332
--- /dev/null
+++ b/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 {}
diff --git a/src/test/ui/auxiliary/inference_unstable_itertools.rs b/src/test/ui/auxiliary/inference_unstable_itertools.rs
new file mode 100644
index 00000000000..2ad264ee3d8
--- /dev/null
+++ b/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 {}
diff --git a/src/test/ui/did_you_mean/recursion_limit_deref.rs b/src/test/ui/did_you_mean/recursion_limit_deref.rs
index 3e261ec636c..f5e75f40fca 100644
--- a/src/test/ui/did_you_mean/recursion_limit_deref.rs
+++ b/src/test/ui/did_you_mean/recursion_limit_deref.rs
@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-//~^^^^^^^^^^ ERROR reached the recursion limit
-
 // Test that the recursion limit can be changed and that the compiler
 // suggests a fix. In this case, we have a long chain of Deref impls
 // which will cause an overflow during the autoderef loop.
diff --git a/src/test/ui/did_you_mean/recursion_limit_deref.stderr b/src/test/ui/did_you_mean/recursion_limit_deref.stderr
index 2c803961557..20a94f7aac1 100644
--- a/src/test/ui/did_you_mean/recursion_limit_deref.stderr
+++ b/src/test/ui/did_you_mean/recursion_limit_deref.stderr
@@ -1,17 +1,13 @@
 error[E0055]: reached the recursion limit while auto-dereferencing I
-  --> $DIR/recursion_limit_deref.rs:62:22
+  --> $DIR/recursion_limit_deref.rs:60:22
    |
 LL |     let x: &Bottom = &t; //~ ERROR mismatched types
    |                      ^^ deref recursion limit reached
    |
    = help: consider adding a `#![recursion_limit="20"]` attribute to your crate
 
-error[E0055]: reached the recursion limit while auto-dereferencing I
-   |
-   = help: consider adding a `#![recursion_limit="20"]` attribute to your crate
-
 error[E0308]: mismatched types
-  --> $DIR/recursion_limit_deref.rs:62:22
+  --> $DIR/recursion_limit_deref.rs:60:22
    |
 LL |     let x: &Bottom = &t; //~ ERROR mismatched types
    |                      ^^ expected struct `Bottom`, found struct `Top`
@@ -19,7 +15,7 @@ LL |     let x: &Bottom = &t; //~ ERROR mismatched types
    = note: expected type `&Bottom`
               found type `&Top`
 
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors
 
 Some errors occurred: E0055, E0308.
 For more information about an error, try `rustc --explain E0055`.
diff --git a/src/test/ui/inference_unstable.rs b/src/test/ui/inference_unstable.rs
new file mode 100644
index 00000000000..816c443a06c
--- /dev/null
+++ b/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 may be added to the standard library in the future
+    //~^^ WARN once this method is added to the standard library, there will be ambiguity here
+}
diff --git a/src/test/ui/inference_unstable.stderr b/src/test/ui/inference_unstable.stderr
new file mode 100644
index 00000000000..9c614d659d3
--- /dev/null
+++ b/src/test/ui/inference_unstable.stderr
@@ -0,0 +1,12 @@
+warning: a method with this name may 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: once this method is added to the standard library, there will be ambiguity here, which will cause a hard error!
+   = note: for more information, see issue #48919 <https://github.com/rust-lang/rust/issues/48919>
+   = 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`
+
diff --git a/src/test/ui/inference_unstable_featured.rs b/src/test/ui/inference_unstable_featured.rs
new file mode 100644
index 00000000000..f5c49bedc71
--- /dev/null
+++ b/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
+}
diff --git a/src/test/ui/inference_unstable_featured.stderr b/src/test/ui/inference_unstable_featured.stderr
new file mode 100644
index 00000000000..cb5f3623291
--- /dev/null
+++ b/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`.
diff --git a/src/test/ui/inference_unstable_forced.rs b/src/test/ui/inference_unstable_forced.rs
new file mode 100644
index 00000000000..82ce4034ce2
--- /dev/null
+++ b/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
+}
diff --git a/src/test/ui/inference_unstable_forced.stderr b/src/test/ui/inference_unstable_forced.stderr
new file mode 100644
index 00000000000..00eb81cd9a2
--- /dev/null
+++ b/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`.