When picking a candidate, consider the unstable ones last.

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

12 files changed, 317 insertions, 41 deletions

diff --git a/src/librustc/lint/builtin.rs b/src/librustc/lint/builtin.rs
index b4ed9c269bd..97cfcf0f607 100644
--- a/src/librustc/lint/builtin.rs
+++ b/src/librustc/lint/builtin.rs
@@ -260,6 +260,12 @@ declare_lint! {
     "floating-point literals cannot be used in patterns"
 }
 
+declare_lint! {
+    pub UNSTABLE_NAME_COLLISION,
+    Warn,
+    "detects name collision with an existing but unstable method"
+}
+
 /// Does nothing as a lint pass, but registers some `Lint`s
 /// which are used by other parts of the compiler.
 #[derive(Copy, Clone)]
@@ -307,7 +313,8 @@ impl LintPass for HardwiredLints {
             SINGLE_USE_LIFETIME,
             TYVAR_BEHIND_RAW_POINTER,
             ELIDED_LIFETIME_IN_PATH,
-            BARE_TRAIT_OBJECT
+            BARE_TRAIT_OBJECT,
+            UNSTABLE_NAME_COLLISION,
         )
     }
 }
diff --git a/src/librustc/middle/stability.rs b/src/librustc/middle/stability.rs
index 16c33d6bd83..9193dc086fd 100644
--- a/src/librustc/middle/stability.rs
+++ b/src/librustc/middle/stability.rs
@@ -474,6 +474,22 @@ struct Checker<'a, 'tcx: 'a> {
     tcx: TyCtxt<'a, 'tcx, 'tcx>,
 }
 
+/// Result of `TyCtxt::eval_stability`.
+pub enum EvalResult {
+    /// We can use the item because it is stable or we provided the
+    /// corresponding feature gate.
+    Allow,
+    /// We cannot use the item because it is unstable and we did not provide the
+    /// corresponding feature gate.
+    Deny {
+        feature: Symbol,
+        reason: Option<Symbol>,
+        issue: u32,
+    },
+    /// The item does not have the `#[stable]` or `#[unstable]` marker assigned.
+    Unmarked,
+}
+
 impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
     // (See issue #38412)
     fn skip_stability_check_due_to_privacy(self, mut def_id: DefId) -> bool {
@@ -509,11 +525,16 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         }
     }
 
-    pub fn check_stability(self, def_id: DefId, id: NodeId, span: Span) {
+    /// Evaluates the stability of an item.
+    ///
+    /// Returns `None` if the item is stable, or unstable but the corresponding `#![feature]` has
+    /// been provided. Returns the tuple `Some((feature, reason, issue))` of the offending unstable
+    /// feature otherwise.
+    pub fn eval_stability(self, def_id: DefId, id: NodeId, span: Span) -> EvalResult {
         if span.allows_unstable() {
             debug!("stability: \
                     skipping span={:?} since it is internal", span);
-            return;
+            return EvalResult::Allow;
         }
 
         let lint_deprecated = |def_id: DefId, note: Option<Symbol>| {
@@ -549,7 +570,7 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
             ..def_id
         }).is_some();
         if !is_staged_api {
-            return;
+            return EvalResult::Allow;
         }
 
         let stability = self.lookup_stability(def_id);
@@ -566,18 +587,18 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         // Only the cross-crate scenario matters when checking unstable APIs
         let cross_crate = !def_id.is_local();
         if !cross_crate {
-            return
+            return EvalResult::Allow;
         }
 
         // Issue 38412: private items lack stability markers.
         if self.skip_stability_check_due_to_privacy(def_id) {
-            return
+            return EvalResult::Allow;
         }
 
         match stability {
-            Some(&Stability { level: attr::Unstable {ref reason, issue}, ref feature, .. }) => {
-                if self.stability().active_features.contains(feature) {
-                    return
+            Some(&Stability { level: attr::Unstable { reason, issue }, feature, .. }) => {
+                if self.stability().active_features.contains(&feature) {
+                    return EvalResult::Allow;
                 }
 
                 // When we're compiling the compiler itself we may pull in
@@ -589,19 +610,34 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
                 // the `-Z force-unstable-if-unmarked` flag present (we're
                 // compiling a compiler crate), then let this missing feature
                 // annotation slide.
-                if *feature == "rustc_private" && issue == 27812 {
+                if feature == "rustc_private" && issue == 27812 {
                     if self.sess.opts.debugging_opts.force_unstable_if_unmarked {
-                        return
+                        return EvalResult::Allow;
                     }
                 }
 
-                let msg = match *reason {
-                    Some(ref r) => format!("use of unstable library feature '{}': {}",
-                                           feature.as_str(), &r),
+                EvalResult::Deny { feature, reason, issue }
+            }
+            Some(_) => {
+                // Stable APIs are always ok to call and deprecated APIs are
+                // handled by the lint emitting logic above.
+                EvalResult::Allow
+            }
+            None => {
+                EvalResult::Unmarked
+            }
+        }
+    }
+
+    pub fn check_stability(self, def_id: DefId, id: NodeId, span: Span) {
+        match self.eval_stability(def_id, id, span) {
+            EvalResult::Allow => {}
+            EvalResult::Deny { feature, reason, issue } => {
+                let msg = match reason {
+                    Some(r) => format!("use of unstable library feature '{}': {}", feature, r),
                     None => format!("use of unstable library feature '{}'", &feature)
                 };
 
-
                 let msp: MultiSpan = span.into();
                 let cm = &self.sess.parse_sess.codemap();
                 let span_key = msp.primary_span().and_then(|sp: Span|
@@ -624,12 +660,8 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
                                      GateIssue::Library(Some(issue)), &msg);
                 }
             }
-            Some(_) => {
-                // Stable APIs are always ok to call and deprecated APIs are
-                // handled by the lint emitting logic above.
-            }
-            None => {
-                span_bug!(span, "encountered unmarked API");
+            EvalResult::Unmarked => {
+                span_bug!(span, "encountered unmarked API: {:?}", def_id);
             }
         }
     }
diff --git a/src/librustc_lint/lib.rs b/src/librustc_lint/lib.rs
index 901d76edc80..fad21ecef4d 100644
--- a/src/librustc_lint/lib.rs
+++ b/src/librustc_lint/lib.rs
@@ -273,7 +273,15 @@ pub fn register_builtins(store: &mut lint::LintStore, sess: Option<&Session>) {
             id: LintId::of(TYVAR_BEHIND_RAW_POINTER),
             reference: "issue #46906 <https://github.com/rust-lang/rust/issues/46906>",
             edition: Some(Edition::Edition2018),
-        }
+        },
+        FutureIncompatibleInfo {
+            id: LintId::of(UNSTABLE_NAME_COLLISION),
+            reference: "pr #48552 <https://github.com/rust-lang/rust/pull/48552>",
+            edition: None,
+            // FIXME: create a proper tracking issue.
+            // Note: this item represents future incompatibility of all unstable functions in the
+            //       standard library, and thus should never be removed or changed to an error.
+        },
         ]);
 
     // Register renamed and removed lints
diff --git a/src/librustc_typeck/check/method/probe.rs b/src/librustc_typeck/check/method/probe.rs
index c7921d2bd45..738e1ee87c7 100644
--- a/src/librustc_typeck/check/method/probe.rs
+++ b/src/librustc_typeck/check/method/probe.rs
@@ -23,9 +23,10 @@ use rustc::ty::{self, Ty, ToPolyTraitRef, ToPredicate, TraitRef, TypeFoldable};
 use rustc::infer::type_variable::TypeVariableOrigin;
 use rustc::util::nodemap::FxHashSet;
 use rustc::infer::{self, InferOk};
+use rustc::middle::stability;
 use syntax::ast;
 use syntax::util::lev_distance::{lev_distance, find_best_match_for_name};
-use syntax_pos::Span;
+use syntax_pos::{Span, symbol::Symbol};
 use rustc::hir;
 use rustc::lint;
 use std::mem;
@@ -937,30 +938,59 @@ impl<'a, 'gcx, 'tcx> ProbeContext<'a, 'gcx, 'tcx> {
         debug!("pick_method(self_ty={})", self.ty_to_string(self_ty));
 
         let mut possibly_unsatisfied_predicates = Vec::new();
-
-        debug!("searching inherent candidates");
-        if let Some(pick) = self.consider_candidates(self_ty,
-                                                     &self.inherent_candidates,
-                                                     &mut possibly_unsatisfied_predicates) {
-            return Some(pick);
+        let mut unstable_candidates = Vec::new();
+
+        for (kind, candidates) in &[
+            ("inherent", &self.inherent_candidates),
+            ("extension", &self.extension_candidates),
+        ] {
+            debug!("searching {} candidates", kind);
+            let res = self.consider_candidates(
+                self_ty,
+                candidates.iter(),
+                &mut possibly_unsatisfied_predicates,
+                Some(&mut unstable_candidates),
+            );
+            if let Some(pick) = res {
+                if !unstable_candidates.is_empty() && !self_ty.is_ty_var() {
+                    if let Ok(p) = &pick {
+                        // Emit a lint if there are unstable candidates alongside the stable ones.
+                        //
+                        // Note, we suppress warning if `self_ty` is TyVar (`_`), since every
+                        // possible candidates of every type will be considered, which leads to
+                        // bogus ambiguity like `str::rsplit` vs `[_]::rsplit`. This condition is
+                        // seen in `src/test/compile-fail/occurs-check-2.rs`.
+                        self.emit_unstable_name_collision_hint(p, &unstable_candidates);
+                    }
+                }
+                return Some(pick);
+            }
         }
 
-        debug!("searching extension candidates");
-        let res = self.consider_candidates(self_ty,
-                                           &self.extension_candidates,
-                                           &mut possibly_unsatisfied_predicates);
-        if let None = res {
+        debug!("searching unstable candidates");
+        let res = self.consider_candidates(
+            self_ty,
+            unstable_candidates.into_iter().map(|(c, _)| c),
+            &mut possibly_unsatisfied_predicates,
+            None,
+        );
+        if res.is_none() {
             self.unsatisfied_predicates.extend(possibly_unsatisfied_predicates);
         }
         res
     }
 
-    fn consider_candidates(&self,
-                           self_ty: Ty<'tcx>,
-                           probes: &[Candidate<'tcx>],
-                           possibly_unsatisfied_predicates: &mut Vec<TraitRef<'tcx>>)
-                           -> Option<PickResult<'tcx>> {
-        let mut applicable_candidates: Vec<_> = probes.iter()
+    fn consider_candidates<'b, ProbesIter>(
+        &self,
+        self_ty: Ty<'tcx>,
+        probes: ProbesIter,
+        possibly_unsatisfied_predicates: &mut Vec<TraitRef<'tcx>>,
+        unstable_candidates: Option<&mut Vec<(&'b Candidate<'tcx>, Symbol)>>,
+    ) -> Option<PickResult<'tcx>>
+    where
+        ProbesIter: Iterator<Item = &'b Candidate<'tcx>> + Clone,
+    {
+        let mut applicable_candidates: Vec<_> = probes.clone()
             .map(|probe| {
                 (probe, self.consider_probe(self_ty, probe, possibly_unsatisfied_predicates))
             })
@@ -975,8 +1005,20 @@ impl<'a, 'gcx, 'tcx> ProbeContext<'a, 'gcx, 'tcx> {
             }
         }
 
+        if let Some(uc) = unstable_candidates {
+            applicable_candidates.retain(|&(p, _)| {
+                if let stability::EvalResult::Deny { feature, .. } =
+                    self.tcx.eval_stability(p.item.def_id, ast::DUMMY_NODE_ID, self.span)
+                {
+                    uc.push((p, feature));
+                    return false;
+                }
+                true
+            });
+        }
+
         if applicable_candidates.len() > 1 {
-            let sources = probes.iter()
+            let sources = probes
                 .map(|p| self.candidate_source(p, self_ty))
                 .collect();
             return Some(Err(MethodError::Ambiguity(sources)));
@@ -991,6 +1033,39 @@ impl<'a, 'gcx, 'tcx> ProbeContext<'a, 'gcx, 'tcx> {
         })
     }
 
+    fn emit_unstable_name_collision_hint(
+        &self,
+        stable_pick: &Pick,
+        unstable_candidates: &[(&Candidate<'tcx>, Symbol)],
+    ) {
+        let mut diag = self.tcx.struct_span_lint_node(
+            lint::builtin::UNSTABLE_NAME_COLLISION,
+            self.fcx.body_id,
+            self.span,
+            "a method with this name will be added to the standard library in the future",
+        );
+
+        // FIXME: This should be a `span_suggestion` instead of `help`. However `self.span` only
+        // highlights the method name, so we can't use it. Also consider reusing the code from
+        // `report_method_error()`.
+        diag.help(&format!(
+            "call with fully qualified syntax `{}(...)` to keep using the current method",
+            self.tcx.item_path_str(stable_pick.item.def_id),
+        ));
+
+        if ::rustc::session::config::nightly_options::is_nightly_build() {
+            for (candidate, feature) in unstable_candidates {
+                diag.note(&format!(
+                    "add #![feature({})] to the crate attributes to enable `{}`",
+                    feature,
+                    self.tcx.item_path_str(candidate.item.def_id),
+                ));
+            }
+        }
+
+        diag.emit();
+    }
+
     fn select_trait_candidate(&self, trait_ref: ty::TraitRef<'tcx>)
                               -> traits::SelectionResult<'tcx, traits::Selection<'tcx>>
     {
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/inference_unstable.rs b/src/test/ui/inference_unstable.rs
new file mode 100644
index 00000000000..525fda33955
--- /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 will be added to the standard library in the future
+    //~^^ WARN it will become a hard error in a future release
+}
diff --git a/src/test/ui/inference_unstable.stderr b/src/test/ui/inference_unstable.stderr
new file mode 100644
index 00000000000..a5cf4d6dff3
--- /dev/null
+++ b/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`
+
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`.