Specialize future-incompatibility warning for UNSTABLE_NAME_COLLISION.

src/librustc/lint/mod.rs

@@ -498,15 +498,21 @@ pub fn struct_lint_level<'a>(sess: &'a Session,
 
     // Check for future incompatibility lints and issue a stronger warning.
     let lints = sess.lint_store.borrow();
-    if let Some(future_incompatible) = lints.future_incompatible(LintId::of(lint)) {
-        let future = if let Some(edition) = future_incompatible.edition {
-            format!("the {} edition", edition)
+    let lint_id = LintId::of(lint);
+    if let Some(future_incompatible) = lints.future_incompatible(lint_id) {
+        const STANDARD_MESSAGE: &str =
+            "this was previously accepted by the compiler but is being phased out; \
+             it will become a hard error";
+
+        let explanation = if lint_id == LintId::of(::lint::builtin::UNSTABLE_NAME_COLLISION) {
+            "once this method is added to the standard library, \
+             there will be ambiguity here, which will cause a hard error!"
+                .to_owned()
+        } else if let Some(edition) = future_incompatible.edition {
+            format!("{} in the {} edition!", STANDARD_MESSAGE, edition)
         } else {
-            "a future release".to_owned()
+            format!("{} in a future release!", STANDARD_MESSAGE)
         };
-        let explanation = format!("this was previously accepted by the compiler \
-                                   but is being phased out; \
-                                   it will become a hard error in {}!", future);
         let citation = format!("for more information, see {}",
                                future_incompatible.reference);
         err.warn(&explanation);

src/librustc_typeck/check/method/probe.rs

@@ -1042,7 +1042,7 @@ impl<'a, 'gcx, 'tcx> ProbeContext<'a, 'gcx, 'tcx> {
             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",
+            "a method with this name may be added to the standard library in the future",
         );
 
         // FIXME: This should be a `span_suggestion` instead of `help`. However `self.span` only

src/test/ui/inference_unstable.rs

@@ -24,6 +24,6 @@ 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
+    //~^ 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
 }

src/test/ui/inference_unstable.stderr

@@ -1,11 +1,11 @@
-warning: a method with this name will be added to the standard library in the future
+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: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = 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 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`