warn against 'specialization' feature

107 files changed, 635 insertions, 92 deletions

diff --git a/src/librustc_feature/active.rs b/src/librustc_feature/active.rs
index d186f35a12b..7aadf58243f 100644
--- a/src/librustc_feature/active.rs
+++ b/src/librustc_feature/active.rs
@@ -596,4 +596,5 @@ pub const INCOMPLETE_FEATURES: &[Symbol] = &[
     sym::raw_dylib,
     sym::const_trait_impl,
     sym::const_trait_bound_opt_out,
+    sym::specialization,
 ];
diff --git a/src/test/ui/associated-types/defaults-specialization.rs b/src/test/ui/associated-types/defaults-specialization.rs
index d0ed718b839..553705b2a4f 100644
--- a/src/test/ui/associated-types/defaults-specialization.rs
+++ b/src/test/ui/associated-types/defaults-specialization.rs
@@ -1,6 +1,7 @@
 //! Tests the interaction of associated type defaults and specialization.
 
 #![feature(associated_type_defaults, specialization)]
+//~^ WARN the feature `specialization` is incomplete
 
 trait Tr {
     type Ty = u8;
diff --git a/src/test/ui/associated-types/defaults-specialization.stderr b/src/test/ui/associated-types/defaults-specialization.stderr
index 37a4d9b60fd..d71b8045352 100644
--- a/src/test/ui/associated-types/defaults-specialization.stderr
+++ b/src/test/ui/associated-types/defaults-specialization.stderr
@@ -1,5 +1,14 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/defaults-specialization.rs:3:38
+   |
+LL | #![feature(associated_type_defaults, specialization)]
+   |                                      ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
 error[E0053]: method `make` has an incompatible type for trait
-  --> $DIR/defaults-specialization.rs:18:18
+  --> $DIR/defaults-specialization.rs:19:18
    |
 LL |     fn make() -> Self::Ty {
    |                  -------- type in trait
@@ -11,7 +20,7 @@ LL |     fn make() -> u8 { 0 }
               found fn pointer `fn() -> u8`
 
 error[E0053]: method `make` has an incompatible type for trait
-  --> $DIR/defaults-specialization.rs:34:18
+  --> $DIR/defaults-specialization.rs:35:18
    |
 LL |     fn make() -> Self::Ty {
    |                  -------- type in trait
@@ -26,7 +35,7 @@ LL |     fn make() -> bool { true }
               found fn pointer `fn() -> bool`
 
 error[E0308]: mismatched types
-  --> $DIR/defaults-specialization.rs:9:9
+  --> $DIR/defaults-specialization.rs:10:9
    |
 LL |     type Ty = u8;
    |     ------------- associated type defaults can't be assumed inside the trait defining them
@@ -40,7 +49,7 @@ LL |         0u8
                          found type `u8`
 
 error[E0308]: mismatched types
-  --> $DIR/defaults-specialization.rs:25:29
+  --> $DIR/defaults-specialization.rs:26:29
    |
 LL |     fn make() -> Self::Ty { 0u8 }
    |                  --------   ^^^ expected associated type, found `u8`
@@ -53,7 +62,7 @@ LL |     fn make() -> Self::Ty { 0u8 }
    = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
 
 error[E0308]: mismatched types
-  --> $DIR/defaults-specialization.rs:43:29
+  --> $DIR/defaults-specialization.rs:44:29
    |
 LL |     default type Ty = bool;
    |     ----------------------- expected this associated type
@@ -67,7 +76,7 @@ LL |     fn make() -> Self::Ty { true }
                          found type `bool`
 
 error[E0308]: mismatched types
-  --> $DIR/defaults-specialization.rs:86:32
+  --> $DIR/defaults-specialization.rs:87:32
    |
 LL |     let _: <B<()> as Tr>::Ty = 0u8;
    |            -----------------   ^^^ expected associated type, found `u8`
@@ -77,13 +86,13 @@ LL |     let _: <B<()> as Tr>::Ty = 0u8;
    = note: expected associated type `<B<()> as Tr>::Ty`
                          found type `u8`
 help: a method is available that returns `<B<()> as Tr>::Ty`
-  --> $DIR/defaults-specialization.rs:8:5
+  --> $DIR/defaults-specialization.rs:9:5
    |
 LL |     fn make() -> Self::Ty {
    |     ^^^^^^^^^^^^^^^^^^^^^ consider calling `Tr::make`
 
 error[E0308]: mismatched types
-  --> $DIR/defaults-specialization.rs:87:32
+  --> $DIR/defaults-specialization.rs:88:32
    |
 LL |     let _: <B<()> as Tr>::Ty = true;
    |            -----------------   ^^^^ expected associated type, found `bool`
@@ -93,13 +102,13 @@ LL |     let _: <B<()> as Tr>::Ty = true;
    = note: expected associated type `<B<()> as Tr>::Ty`
                          found type `bool`
 help: a method is available that returns `<B<()> as Tr>::Ty`
-  --> $DIR/defaults-specialization.rs:8:5
+  --> $DIR/defaults-specialization.rs:9:5
    |
 LL |     fn make() -> Self::Ty {
    |     ^^^^^^^^^^^^^^^^^^^^^ consider calling `Tr::make`
 
 error[E0308]: mismatched types
-  --> $DIR/defaults-specialization.rs:88:33
+  --> $DIR/defaults-specialization.rs:89:33
    |
 LL |     let _: <B2<()> as Tr>::Ty = 0u8;
    |            ------------------   ^^^ expected associated type, found `u8`
@@ -109,13 +118,13 @@ LL |     let _: <B2<()> as Tr>::Ty = 0u8;
    = note: expected associated type `<B2<()> as Tr>::Ty`
                          found type `u8`
 help: a method is available that returns `<B2<()> as Tr>::Ty`
-  --> $DIR/defaults-specialization.rs:8:5
+  --> $DIR/defaults-specialization.rs:9:5
    |
 LL |     fn make() -> Self::Ty {
    |     ^^^^^^^^^^^^^^^^^^^^^ consider calling `Tr::make`
 
 error[E0308]: mismatched types
-  --> $DIR/defaults-specialization.rs:89:33
+  --> $DIR/defaults-specialization.rs:90:33
    |
 LL |     let _: <B2<()> as Tr>::Ty = true;
    |            ------------------   ^^^^ expected associated type, found `bool`
@@ -125,12 +134,12 @@ LL |     let _: <B2<()> as Tr>::Ty = true;
    = note: expected associated type `<B2<()> as Tr>::Ty`
                          found type `bool`
 help: a method is available that returns `<B2<()> as Tr>::Ty`
-  --> $DIR/defaults-specialization.rs:8:5
+  --> $DIR/defaults-specialization.rs:9:5
    |
 LL |     fn make() -> Self::Ty {
    |     ^^^^^^^^^^^^^^^^^^^^^ consider calling `Tr::make`
 
-error: aborting due to 9 previous errors
+error: aborting due to 9 previous errors; 1 warning emitted
 
 Some errors have detailed explanations: E0053, E0308.
 For more information about an error, try `rustc --explain E0053`.
diff --git a/src/test/ui/consts/trait_specialization.rs b/src/test/ui/consts/trait_specialization.rs
index 8010d2fe1ae..3adbbb53046 100644
--- a/src/test/ui/consts/trait_specialization.rs
+++ b/src/test/ui/consts/trait_specialization.rs
@@ -5,7 +5,7 @@
 // Tests that specialization does not cause optimizations running on polymorphic MIR to resolve
 // to a `default` implementation.
 
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 trait Marker {}
 
diff --git a/src/test/ui/consts/trait_specialization.stderr b/src/test/ui/consts/trait_specialization.stderr
new file mode 100644
index 00000000000..03da7d512e5
--- /dev/null
+++ b/src/test/ui/consts/trait_specialization.stderr
@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/trait_specialization.rs:8:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/impl-trait/equality-rpass.rs b/src/test/ui/impl-trait/equality-rpass.rs
index 05c9e4173b0..607b4a49661 100644
--- a/src/test/ui/impl-trait/equality-rpass.rs
+++ b/src/test/ui/impl-trait/equality-rpass.rs
@@ -1,6 +1,6 @@
 // run-pass
 
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 trait Foo: std::fmt::Debug + Eq {}
 
diff --git a/src/test/ui/impl-trait/equality-rpass.stderr b/src/test/ui/impl-trait/equality-rpass.stderr
new file mode 100644
index 00000000000..1abf05dca82
--- /dev/null
+++ b/src/test/ui/impl-trait/equality-rpass.stderr
@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/equality-rpass.rs:3:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/impl-trait/equality.rs b/src/test/ui/impl-trait/equality.rs
index 14b0eeb739a..828b5aac896 100644
--- a/src/test/ui/impl-trait/equality.rs
+++ b/src/test/ui/impl-trait/equality.rs
@@ -1,4 +1,4 @@
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 trait Foo: Copy + ToString {}
 
diff --git a/src/test/ui/impl-trait/equality.stderr b/src/test/ui/impl-trait/equality.stderr
index 9178358b60a..628dfb13d4c 100644
--- a/src/test/ui/impl-trait/equality.stderr
+++ b/src/test/ui/impl-trait/equality.stderr
@@ -1,3 +1,12 @@
+warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/equality.rs:1:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
 error[E0308]: mismatched types
   --> $DIR/equality.rs:15:5
    |
@@ -24,7 +33,7 @@ LL |         n + sum_to(n - 1)
    |
    = help: the trait `std::ops::Add<impl Foo>` is not implemented for `u32`
 
-error: aborting due to 2 previous errors
+error: aborting due to 2 previous errors; 1 warning emitted
 
 Some errors have detailed explanations: E0277, E0308.
 For more information about an error, try `rustc --explain E0277`.
diff --git a/src/test/ui/impl-trait/equality2.rs b/src/test/ui/impl-trait/equality2.rs
index abce8c8c204..2e325867da8 100644
--- a/src/test/ui/impl-trait/equality2.rs
+++ b/src/test/ui/impl-trait/equality2.rs
@@ -1,4 +1,4 @@
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 trait Foo: Copy + ToString {}
 
diff --git a/src/test/ui/impl-trait/equality2.stderr b/src/test/ui/impl-trait/equality2.stderr
index 2454c218ffc..1780931efc5 100644
--- a/src/test/ui/impl-trait/equality2.stderr
+++ b/src/test/ui/impl-trait/equality2.stderr
@@ -1,3 +1,12 @@
+warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/equality2.rs:1:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
 error[E0308]: mismatched types
   --> $DIR/equality2.rs:25:18
    |
@@ -58,6 +67,6 @@ LL |          x.0);
    = note: expected opaque type `impl Foo` (`i32`)
               found opaque type `impl Foo` (`u32`)
 
-error: aborting due to 4 previous errors
+error: aborting due to 4 previous errors; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/overlap-doesnt-conflict-with-specialization.rs b/src/test/ui/overlap-doesnt-conflict-with-specialization.rs
index dd09d68367e..1e413120a37 100644
--- a/src/test/ui/overlap-doesnt-conflict-with-specialization.rs
+++ b/src/test/ui/overlap-doesnt-conflict-with-specialization.rs
@@ -1,7 +1,7 @@
 // run-pass
 
 #![feature(marker_trait_attr)]
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 #[marker]
 trait MyMarker {}
diff --git a/src/test/ui/overlap-doesnt-conflict-with-specialization.stderr b/src/test/ui/overlap-doesnt-conflict-with-specialization.stderr
new file mode 100644
index 00000000000..d49e97bc09c
--- /dev/null
+++ b/src/test/ui/overlap-doesnt-conflict-with-specialization.stderr
@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/overlap-doesnt-conflict-with-specialization.rs:4:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/parser/trait-item-with-defaultness-fail-semantic.rs b/src/test/ui/parser/trait-item-with-defaultness-fail-semantic.rs
index 09f967f161e..34aee7f6935 100644
--- a/src/test/ui/parser/trait-item-with-defaultness-fail-semantic.rs
+++ b/src/test/ui/parser/trait-item-with-defaultness-fail-semantic.rs
@@ -1,4 +1,4 @@
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 fn main() {}
 
diff --git a/src/test/ui/parser/trait-item-with-defaultness-fail-semantic.stderr b/src/test/ui/parser/trait-item-with-defaultness-fail-semantic.stderr
index 6bb946d5b64..e8ff93f6323 100644
--- a/src/test/ui/parser/trait-item-with-defaultness-fail-semantic.stderr
+++ b/src/test/ui/parser/trait-item-with-defaultness-fail-semantic.stderr
@@ -46,5 +46,14 @@ LL |     default fn f2() {}
    |     |
    |     `default` because of this
 
-error: aborting due to 6 previous errors
+warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/trait-item-with-defaultness-fail-semantic.rs:1:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+error: aborting due to 6 previous errors; 1 warning emitted
 
diff --git a/src/test/ui/specialization/assoc-ty-graph-cycle.rs b/src/test/ui/specialization/assoc-ty-graph-cycle.rs
index 54d51492ab3..fc39b553a61 100644
--- a/src/test/ui/specialization/assoc-ty-graph-cycle.rs
+++ b/src/test/ui/specialization/assoc-ty-graph-cycle.rs
@@ -2,7 +2,7 @@
 
 // Make sure we don't crash with a cycle error during coherence.
 
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 trait Trait<T> {
     type Assoc;
diff --git a/src/test/ui/specialization/assoc-ty-graph-cycle.stderr b/src/test/ui/specialization/assoc-ty-graph-cycle.stderr
new file mode 100644
index 00000000000..a7bd37e8259
--- /dev/null
+++ b/src/test/ui/specialization/assoc-ty-graph-cycle.stderr
@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/assoc-ty-graph-cycle.rs:5:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/specialization/cross-crate-defaults.rs b/src/test/ui/specialization/cross-crate-defaults.rs
index 79cb6594397..fc28d0c815e 100644
--- a/src/test/ui/specialization/cross-crate-defaults.rs
+++ b/src/test/ui/specialization/cross-crate-defaults.rs
@@ -2,7 +2,7 @@
 
 // aux-build:cross_crates_defaults.rs
 
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 extern crate cross_crates_defaults;
 
diff --git a/src/test/ui/specialization/cross-crate-defaults.stderr b/src/test/ui/specialization/cross-crate-defaults.stderr
new file mode 100644
index 00000000000..6ab4db3e7e1
--- /dev/null
+++ b/src/test/ui/specialization/cross-crate-defaults.stderr
@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/cross-crate-defaults.rs:5:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/specialization/defaultimpl/allowed-cross-crate.rs b/src/test/ui/specialization/defaultimpl/allowed-cross-crate.rs
index 15550bcce2a..5d67160eb96 100644
--- a/src/test/ui/specialization/defaultimpl/allowed-cross-crate.rs
+++ b/src/test/ui/specialization/defaultimpl/allowed-cross-crate.rs
@@ -5,7 +5,7 @@
 
 // aux-build:go_trait.rs
 
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 extern crate go_trait;
 
diff --git a/src/test/ui/specialization/defaultimpl/allowed-cross-crate.stderr b/src/test/ui/specialization/defaultimpl/allowed-cross-crate.stderr
new file mode 100644
index 00000000000..0966de8394a
--- /dev/null
+++ b/src/test/ui/specialization/defaultimpl/allowed-cross-crate.stderr
@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/allowed-cross-crate.rs:8:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/specialization/defaultimpl/out-of-order.rs b/src/test/ui/specialization/defaultimpl/out-of-order.rs
index f9c73a19cfa..13258ac8c9f 100644
--- a/src/test/ui/specialization/defaultimpl/out-of-order.rs
+++ b/src/test/ui/specialization/defaultimpl/out-of-order.rs
@@ -2,7 +2,7 @@
 
 // Test that you can list the more specific impl before the more general one.
 
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 trait Foo {
     type Out;
diff --git a/src/test/ui/specialization/defaultimpl/out-of-order.stderr b/src/test/ui/specialization/defaultimpl/out-of-order.stderr
new file mode 100644
index 00000000000..2901272b39b
--- /dev/null
+++ b/src/test/ui/specialization/defaultimpl/out-of-order.stderr
@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/out-of-order.rs:5:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/specialization/defaultimpl/overlap-projection.rs b/src/test/ui/specialization/defaultimpl/overlap-projection.rs
index ed38bb3fc3a..0add4d5516c 100644
--- a/src/test/ui/specialization/defaultimpl/overlap-projection.rs
+++ b/src/test/ui/specialization/defaultimpl/overlap-projection.rs
@@ -4,7 +4,7 @@
 // projections involve specialization, so long as the associated type is
 // provided by the most specialized impl.
 
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 trait Assoc {
     type Output;
diff --git a/src/test/ui/specialization/defaultimpl/overlap-projection.stderr b/src/test/ui/specialization/defaultimpl/overlap-projection.stderr
new file mode 100644
index 00000000000..245300923cd
--- /dev/null
+++ b/src/test/ui/specialization/defaultimpl/overlap-projection.stderr
@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/overlap-projection.rs:7:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/specialization/defaultimpl/projection.rs b/src/test/ui/specialization/defaultimpl/projection.rs
index 897a7aade2f..4a914096932 100644
--- a/src/test/ui/specialization/defaultimpl/projection.rs
+++ b/src/test/ui/specialization/defaultimpl/projection.rs
@@ -1,7 +1,7 @@
 // run-pass
 #![allow(dead_code)]
 
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 // Make sure we *can* project non-defaulted associated types
 // cf compile-fail/specialization-default-projection.rs
diff --git a/src/test/ui/specialization/defaultimpl/projection.stderr b/src/test/ui/specialization/defaultimpl/projection.stderr
new file mode 100644
index 00000000000..0fd5ba0e03d
--- /dev/null
+++ b/src/test/ui/specialization/defaultimpl/projection.stderr
@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/projection.rs:4:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/specialization/defaultimpl/specialization-no-default.rs b/src/test/ui/specialization/defaultimpl/specialization-no-default.rs
index 37005f839d4..661724eef8a 100644
--- a/src/test/ui/specialization/defaultimpl/specialization-no-default.rs
+++ b/src/test/ui/specialization/defaultimpl/specialization-no-default.rs
@@ -1,4 +1,4 @@
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 // Check a number of scenarios in which one impl tries to override another,
 // without correctly using `default`.
diff --git a/src/test/ui/specialization/defaultimpl/specialization-no-default.stderr b/src/test/ui/specialization/defaultimpl/specialization-no-default.stderr
index 13636b28b12..6cbaae7edbc 100644
--- a/src/test/ui/specialization/defaultimpl/specialization-no-default.stderr
+++ b/src/test/ui/specialization/defaultimpl/specialization-no-default.stderr
@@ -1,3 +1,12 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/specialization-no-default.rs:1:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
 error[E0520]: `foo` specializes an item from a parent `impl`, but that item is not marked `default`
   --> $DIR/specialization-no-default.rs:20:5
    |
@@ -65,6 +74,6 @@ LL |       fn redundant(&self) {}
    |
    = note: to specialize, `redundant` in the parent `impl` must be marked `default`
 
-error: aborting due to 5 previous errors
+error: aborting due to 5 previous errors; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0520`.
diff --git a/src/test/ui/specialization/defaultimpl/specialization-trait-item-not-implemented-rpass.rs b/src/test/ui/specialization/defaultimpl/specialization-trait-item-not-implemented-rpass.rs
index 2b8ca6bb1dd..89fef5b5ef9 100644
--- a/src/test/ui/specialization/defaultimpl/specialization-trait-item-not-implemented-rpass.rs
+++ b/src/test/ui/specialization/defaultimpl/specialization-trait-item-not-implemented-rpass.rs
@@ -3,7 +3,7 @@
 // Tests that we can combine a default impl that supplies one method with a
 // full impl that supplies the other, and they can invoke one another.
 
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 trait Foo {
     fn foo_one(&self) -> &'static str;
diff --git a/src/test/ui/specialization/defaultimpl/specialization-trait-item-not-implemented-rpass.stderr b/src/test/ui/specialization/defaultimpl/specialization-trait-item-not-implemented-rpass.stderr
new file mode 100644
index 00000000000..dc377dd10c8
--- /dev/null
+++ b/src/test/ui/specialization/defaultimpl/specialization-trait-item-not-implemented-rpass.stderr
@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/specialization-trait-item-not-implemented-rpass.rs:6:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/specialization/defaultimpl/specialization-trait-item-not-implemented.rs b/src/test/ui/specialization/defaultimpl/specialization-trait-item-not-implemented.rs
index 2a121e61aaa..3c5414469fa 100644
--- a/src/test/ui/specialization/defaultimpl/specialization-trait-item-not-implemented.rs
+++ b/src/test/ui/specialization/defaultimpl/specialization-trait-item-not-implemented.rs
@@ -1,6 +1,6 @@
 // Tests that default impls do not have to supply all items but regular impls do.
 
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 trait Foo {
     fn foo_one(&self) -> &'static str;
diff --git a/src/test/ui/specialization/defaultimpl/specialization-trait-item-not-implemented.stderr b/src/test/ui/specialization/defaultimpl/specialization-trait-item-not-implemented.stderr
index b862a937066..9d1eca1d6af 100644
--- a/src/test/ui/specialization/defaultimpl/specialization-trait-item-not-implemented.stderr
+++ b/src/test/ui/specialization/defaultimpl/specialization-trait-item-not-implemented.stderr
@@ -1,3 +1,12 @@
+warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/specialization-trait-item-not-implemented.rs:3:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
 error[E0046]: not all trait items implemented, missing: `foo_two`
   --> $DIR/specialization-trait-item-not-implemented.rs:18:1
    |
@@ -7,6 +16,6 @@ LL |     fn foo_two(&self) -> &'static str;
 LL | impl Foo for MyStruct {}
    | ^^^^^^^^^^^^^^^^^^^^^ missing `foo_two` in implementation
 
-error: aborting due to previous error
+error: aborting due to previous error; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0046`.
diff --git a/src/test/ui/specialization/defaultimpl/specialization-trait-not-implemented.rs b/src/test/ui/specialization/defaultimpl/specialization-trait-not-implemented.rs
index 5c104449fe9..35e3b8725a8 100644
--- a/src/test/ui/specialization/defaultimpl/specialization-trait-not-implemented.rs
+++ b/src/test/ui/specialization/defaultimpl/specialization-trait-not-implemented.rs
@@ -2,7 +2,7 @@
 // - default impls do not have to supply all items and
 // - a default impl does not count as an impl (in this case, an incomplete default impl).
 
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 trait Foo {
     fn foo_one(&self) -> &'static str;
diff --git a/src/test/ui/specialization/defaultimpl/specialization-trait-not-implemented.stderr b/src/test/ui/specialization/defaultimpl/specialization-trait-not-implemented.stderr
index a55d79ee035..6b8e559bc36 100644
--- a/src/test/ui/specialization/defaultimpl/specialization-trait-not-implemented.stderr
+++ b/src/test/ui/specialization/defaultimpl/specialization-trait-not-implemented.stderr
@@ -1,3 +1,12 @@
+warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/specialization-trait-not-implemented.rs:5:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
 error[E0599]: no method named `foo_one` found for struct `MyStruct` in the current scope
   --> $DIR/specialization-trait-not-implemented.rs:22:29
    |
@@ -19,6 +28,6 @@ note: `Foo` defines an item `foo_one`, perhaps you need to implement it
 LL | trait Foo {
    | ^^^^^^^^^
 
-error: aborting due to previous error
+error: aborting due to previous error; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0599`.
diff --git a/src/test/ui/specialization/defaultimpl/specialization-wfcheck.rs b/src/test/ui/specialization/defaultimpl/specialization-wfcheck.rs
index 232338d9d4d..afd634725e3 100644
--- a/src/test/ui/specialization/defaultimpl/specialization-wfcheck.rs
+++ b/src/test/ui/specialization/defaultimpl/specialization-wfcheck.rs
@@ -1,6 +1,6 @@
 // Tests that a default impl still has to have a WF trait ref.
 
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 trait Foo<'a, T: Eq + 'a> { }
 
diff --git a/src/test/ui/specialization/defaultimpl/specialization-wfcheck.stderr b/src/test/ui/specialization/defaultimpl/specialization-wfcheck.stderr
index f499c1f5698..5eeb07f992e 100644
--- a/src/test/ui/specialization/defaultimpl/specialization-wfcheck.stderr
+++ b/src/test/ui/specialization/defaultimpl/specialization-wfcheck.stderr
@@ -1,3 +1,12 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/specialization-wfcheck.rs:3:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
 error[E0277]: the trait bound `U: std::cmp::Eq` is not satisfied
   --> $DIR/specialization-wfcheck.rs:7:17
    |
@@ -12,6 +21,6 @@ help: consider restricting type parameter `U`
 LL | default impl<U: std::cmp::Eq> Foo<'static, U> for () {}
    |               ^^^^^^^^^^^^^^
 
-error: aborting due to previous error
+error: aborting due to previous error; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0277`.
diff --git a/src/test/ui/specialization/defaultimpl/validation.rs b/src/test/ui/specialization/defaultimpl/validation.rs
index 8134333c58f..8558a1efb82 100644
--- a/src/test/ui/specialization/defaultimpl/validation.rs
+++ b/src/test/ui/specialization/defaultimpl/validation.rs
@@ -1,5 +1,5 @@
 #![feature(negative_impls)]
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 struct S;
 struct Z;
diff --git a/src/test/ui/specialization/defaultimpl/validation.stderr b/src/test/ui/specialization/defaultimpl/validation.stderr
index 254eaf51a64..0752614674d 100644
--- a/src/test/ui/specialization/defaultimpl/validation.stderr
+++ b/src/test/ui/specialization/defaultimpl/validation.stderr
@@ -8,6 +8,15 @@ LL | default impl S {}
    |
    = note: only trait implementations may be annotated with `default`
 
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/validation.rs:2:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
 error: impls of auto traits cannot be default
   --> $DIR/validation.rs:9:21
    |
@@ -36,6 +45,6 @@ error[E0750]: negative impls cannot be default impls
 LL | default impl !Tr for S {}
    | ^^^^^^^      ^
 
-error: aborting due to 5 previous errors
+error: aborting due to 5 previous errors; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0750`.
diff --git a/src/test/ui/specialization/issue-36804.rs b/src/test/ui/specialization/issue-36804.rs
index 9546a5dd5f5..89350602f36 100644
--- a/src/test/ui/specialization/issue-36804.rs
+++ b/src/test/ui/specialization/issue-36804.rs
@@ -1,5 +1,5 @@
 // check-pass
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 pub struct Cloned<I>(I);
 
diff --git a/src/test/ui/specialization/issue-36804.stderr b/src/test/ui/specialization/issue-36804.stderr
new file mode 100644
index 00000000000..9be2888592c
--- /dev/null
+++ b/src/test/ui/specialization/issue-36804.stderr
@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/issue-36804.rs:2:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/specialization/issue-39448.rs b/src/test/ui/specialization/issue-39448.rs
index 8ac6d8e9311..9dd47a4a17e 100644
--- a/src/test/ui/specialization/issue-39448.rs
+++ b/src/test/ui/specialization/issue-39448.rs
@@ -1,4 +1,4 @@
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 // Regression test for a specialization-related ICE (#39448).
 
diff --git a/src/test/ui/specialization/issue-39448.stderr b/src/test/ui/specialization/issue-39448.stderr
index 861a1d9e8fc..886b2e1e8d4 100644
--- a/src/test/ui/specialization/issue-39448.stderr
+++ b/src/test/ui/specialization/issue-39448.stderr
@@ -1,3 +1,12 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/issue-39448.rs:1:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
 error[E0275]: overflow evaluating the requirement `T: FromA<U>`
   --> $DIR/issue-39448.rs:45:13
    |
@@ -7,6 +16,6 @@ LL |     x.foo(y.to()).to()
    = note: required because of the requirements on the impl of `FromA<U>` for `T`
    = note: required because of the requirements on the impl of `ToA<T>` for `U`
 
-error: aborting due to previous error
+error: aborting due to previous error; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0275`.
diff --git a/src/test/ui/specialization/issue-39618.rs b/src/test/ui/specialization/issue-39618.rs
index 20e81e4359b..72630ee9c70 100644
--- a/src/test/ui/specialization/issue-39618.rs
+++ b/src/test/ui/specialization/issue-39618.rs
@@ -4,7 +4,7 @@
 
 // check-pass
 
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 trait Foo {
     fn foo(&self);
diff --git a/src/test/ui/specialization/issue-39618.stderr b/src/test/ui/specialization/issue-39618.stderr
new file mode 100644
index 00000000000..1712b4afbca
--- /dev/null
+++ b/src/test/ui/specialization/issue-39618.stderr
@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/issue-39618.rs:7:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/specialization/issue-50452.rs b/src/test/ui/specialization/issue-50452.rs
index 93f081d9558..29fc12066e8 100644
--- a/src/test/ui/specialization/issue-50452.rs
+++ b/src/test/ui/specialization/issue-50452.rs
@@ -1,6 +1,6 @@
 // run-pass
 
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 pub trait Foo {
     fn foo();
diff --git a/src/test/ui/specialization/issue-50452.stderr b/src/test/ui/specialization/issue-50452.stderr
new file mode 100644
index 00000000000..efae48d0ed4
--- /dev/null
+++ b/src/test/ui/specialization/issue-50452.stderr
@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/issue-50452.rs:3:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/specialization/issue-52050.rs b/src/test/ui/specialization/issue-52050.rs
index 1e1bfe9cf07..80465870206 100644
--- a/src/test/ui/specialization/issue-52050.rs
+++ b/src/test/ui/specialization/issue-52050.rs
@@ -1,4 +1,4 @@
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 // Regression test for #52050: when inserting the blanket impl `I`
 // into the tree, we had to replace the child node for `Foo`, which
diff --git a/src/test/ui/specialization/issue-52050.stderr b/src/test/ui/specialization/issue-52050.stderr
index 36f96b01198..bf95b0a3faa 100644
--- a/src/test/ui/specialization/issue-52050.stderr
+++ b/src/test/ui/specialization/issue-52050.stderr
@@ -1,3 +1,12 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/issue-52050.rs:1:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
 error[E0119]: conflicting implementations of trait `IntoPyDictPointer` for type `()`:
   --> $DIR/issue-52050.rs:28:1
    |
@@ -13,6 +22,6 @@ LL |   impl IntoPyDictPointer for ()
    |
    = note: upstream crates may add a new impl of trait `std::iter::Iterator` for type `()` in future versions
 
-error: aborting due to previous error
+error: aborting due to previous error; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0119`.
diff --git a/src/test/ui/specialization/issue-63716-parse-async.rs b/src/test/ui/specialization/issue-63716-parse-async.rs
index c3764ffaab8..10f185c3351 100644
--- a/src/test/ui/specialization/issue-63716-parse-async.rs
+++ b/src/test/ui/specialization/issue-63716-parse-async.rs
@@ -4,7 +4,7 @@
 // check-pass
 // edition:2018
 
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 fn main() {}
 
diff --git a/src/test/ui/specialization/issue-63716-parse-async.stderr b/src/test/ui/specialization/issue-63716-parse-async.stderr
new file mode 100644
index 00000000000..4c2bf69590e
--- /dev/null
+++ b/src/test/ui/specialization/issue-63716-parse-async.stderr
@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/issue-63716-parse-async.rs:7:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/specialization/issue-70442.rs b/src/test/ui/specialization/issue-70442.rs
index 4371dd2e167..d41b5355c2c 100644
--- a/src/test/ui/specialization/issue-70442.rs
+++ b/src/test/ui/specialization/issue-70442.rs
@@ -1,4 +1,4 @@
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 // check-pass
 
diff --git a/src/test/ui/specialization/issue-70442.stderr b/src/test/ui/specialization/issue-70442.stderr
new file mode 100644
index 00000000000..1f846cc449d
--- /dev/null
+++ b/src/test/ui/specialization/issue-70442.stderr
@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/issue-70442.rs:1:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/specialization/non-defaulted-item-fail.rs b/src/test/ui/specialization/non-defaulted-item-fail.rs
index 403f718d7dd..b7d6ac829dd 100644
--- a/src/test/ui/specialization/non-defaulted-item-fail.rs
+++ b/src/test/ui/specialization/non-defaulted-item-fail.rs
@@ -1,4 +1,5 @@
 #![feature(specialization, associated_type_defaults)]
+//~^ WARN the feature `specialization` is incomplete
 
 // Test that attempting to override a non-default method or one not in the
 // parent impl causes an error.
diff --git a/src/test/ui/specialization/non-defaulted-item-fail.stderr b/src/test/ui/specialization/non-defaulted-item-fail.stderr
index e6c5fc1441b..084266205b6 100644
--- a/src/test/ui/specialization/non-defaulted-item-fail.stderr
+++ b/src/test/ui/specialization/non-defaulted-item-fail.stderr
@@ -1,5 +1,14 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/non-defaulted-item-fail.rs:1:12
+   |
+LL | #![feature(specialization, associated_type_defaults)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
 error[E0520]: `Ty` specializes an item from a parent `impl`, but that item is not marked `default`
-  --> $DIR/non-defaulted-item-fail.rs:29:5
+  --> $DIR/non-defaulted-item-fail.rs:30:5
    |
 LL | / impl<T> Foo for Box<T> {
 LL | |     type Ty = bool;
@@ -14,7 +23,7 @@ LL |       type Ty = Vec<()>;
    = note: to specialize, `Ty` in the parent `impl` must be marked `default`
 
 error[E0520]: `CONST` specializes an item from a parent `impl`, but that item is not marked `default`
-  --> $DIR/non-defaulted-item-fail.rs:31:5
+  --> $DIR/non-defaulted-item-fail.rs:32:5
    |
 LL | / impl<T> Foo for Box<T> {
 LL | |     type Ty = bool;
@@ -29,7 +38,7 @@ LL |       const CONST: u8 = 42;
    = note: to specialize, `CONST` in the parent `impl` must be marked `default`
 
 error[E0520]: `foo` specializes an item from a parent `impl`, but that item is not marked `default`
-  --> $DIR/non-defaulted-item-fail.rs:33:5
+  --> $DIR/non-defaulted-item-fail.rs:34:5
    |
 LL | / impl<T> Foo for Box<T> {
 LL | |     type Ty = bool;
@@ -44,7 +53,7 @@ LL |       fn foo(&self) -> bool { true }
    = note: to specialize, `foo` in the parent `impl` must be marked `default`
 
 error[E0520]: `Ty` specializes an item from a parent `impl`, but that item is not marked `default`
-  --> $DIR/non-defaulted-item-fail.rs:45:5
+  --> $DIR/non-defaulted-item-fail.rs:46:5
    |
 LL | impl<T> Foo for Vec<T> {}
    | ------------------------- parent `impl` is here
@@ -55,7 +64,7 @@ LL |     type Ty = Vec<()>;
    = note: to specialize, `Ty` in the parent `impl` must be marked `default`
 
 error[E0520]: `CONST` specializes an item from a parent `impl`, but that item is not marked `default`
-  --> $DIR/non-defaulted-item-fail.rs:47:5
+  --> $DIR/non-defaulted-item-fail.rs:48:5
    |
 LL | impl<T> Foo for Vec<T> {}
    | ------------------------- parent `impl` is here
@@ -66,7 +75,7 @@ LL |     const CONST: u8 = 42;
    = note: to specialize, `CONST` in the parent `impl` must be marked `default`
 
 error[E0520]: `foo` specializes an item from a parent `impl`, but that item is not marked `default`
-  --> $DIR/non-defaulted-item-fail.rs:49:5
+  --> $DIR/non-defaulted-item-fail.rs:50:5
    |
 LL | impl<T> Foo for Vec<T> {}
    | ------------------------- parent `impl` is here
@@ -76,6 +85,6 @@ LL |     fn foo(&self) -> bool { true }
    |
    = note: to specialize, `foo` in the parent `impl` must be marked `default`
 
-error: aborting due to 6 previous errors
+error: aborting due to 6 previous errors; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0520`.
diff --git a/src/test/ui/specialization/specialization-allowed-cross-crate.rs b/src/test/ui/specialization/specialization-allowed-cross-crate.rs
index 15550bcce2a..5d67160eb96 100644
--- a/src/test/ui/specialization/specialization-allowed-cross-crate.rs
+++ b/src/test/ui/specialization/specialization-allowed-cross-crate.rs
@@ -5,7 +5,7 @@
 
 // aux-build:go_trait.rs
 
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 extern crate go_trait;
 
diff --git a/src/test/ui/specialization/specialization-allowed-cross-crate.stderr b/src/test/ui/specialization/specialization-allowed-cross-crate.stderr
new file mode 100644
index 00000000000..734e90baed1
--- /dev/null
+++ b/src/test/ui/specialization/specialization-allowed-cross-crate.stderr
@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/specialization-allowed-cross-crate.rs:8:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/specialization/specialization-assoc-fns.rs b/src/test/ui/specialization/specialization-assoc-fns.rs
index b6a7a48972a..cbfcb4719f6 100644
--- a/src/test/ui/specialization/specialization-assoc-fns.rs
+++ b/src/test/ui/specialization/specialization-assoc-fns.rs
@@ -2,7 +2,7 @@
 
 // Test that non-method associated functions can be specialized
 
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 trait Foo {
     fn mk() -> Self;
diff --git a/src/test/ui/specialization/specialization-assoc-fns.stderr b/src/test/ui/specialization/specialization-assoc-fns.stderr
new file mode 100644
index 00000000000..1ad3ea8ffd3
--- /dev/null
+++ b/src/test/ui/specialization/specialization-assoc-fns.stderr
@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/specialization-assoc-fns.rs:5:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/specialization/specialization-basics.rs b/src/test/ui/specialization/specialization-basics.rs
index 6c359e51bc2..721c934dbfa 100644
--- a/src/test/ui/specialization/specialization-basics.rs
+++ b/src/test/ui/specialization/specialization-basics.rs
@@ -1,6 +1,6 @@
 // run-pass
 
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 // Tests a variety of basic specialization scenarios and method
 // dispatch for them.
diff --git a/src/test/ui/specialization/specialization-basics.stderr b/src/test/ui/specialization/specialization-basics.stderr
new file mode 100644
index 00000000000..37b586f4df7
--- /dev/null
+++ b/src/test/ui/specialization/specialization-basics.stderr
@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/specialization-basics.rs:3:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/specialization/specialization-cross-crate.rs b/src/test/ui/specialization/specialization-cross-crate.rs
index fa63c866329..4171505aa37 100644
--- a/src/test/ui/specialization/specialization-cross-crate.rs
+++ b/src/test/ui/specialization/specialization-cross-crate.rs
@@ -2,7 +2,7 @@
 
 // aux-build:specialization_cross_crate.rs
 
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 extern crate specialization_cross_crate;
 
diff --git a/src/test/ui/specialization/specialization-cross-crate.stderr b/src/test/ui/specialization/specialization-cross-crate.stderr
new file mode 100644
index 00000000000..fd853db4323
--- /dev/null
+++ b/src/test/ui/specialization/specialization-cross-crate.stderr
@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/specialization-cross-crate.rs:5:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/specialization/specialization-default-methods.rs b/src/test/ui/specialization/specialization-default-methods.rs
index 9ae3d1e9f39..dcf68afa945 100644
--- a/src/test/ui/specialization/specialization-default-methods.rs
+++ b/src/test/ui/specialization/specialization-default-methods.rs
@@ -1,6 +1,6 @@
 // run-pass
 
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 // Test that default methods are cascaded correctly
 
diff --git a/src/test/ui/specialization/specialization-default-methods.stderr b/src/test/ui/specialization/specialization-default-methods.stderr
new file mode 100644
index 00000000000..4e069e858fa
--- /dev/null
+++ b/src/test/ui/specialization/specialization-default-methods.stderr
@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/specialization-default-methods.rs:3:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/specialization/specialization-default-projection.rs b/src/test/ui/specialization/specialization-default-projection.rs
index e9343f23601..7f3ae951287 100644
--- a/src/test/ui/specialization/specialization-default-projection.rs
+++ b/src/test/ui/specialization/specialization-default-projection.rs
@@ -1,4 +1,4 @@
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 // Make sure we can't project defaulted associated types
 
diff --git a/src/test/ui/specialization/specialization-default-projection.stderr b/src/test/ui/specialization/specialization-default-projection.stderr
index ac15ab0681a..695102a1878 100644
--- a/src/test/ui/specialization/specialization-default-projection.stderr
+++ b/src/test/ui/specialization/specialization-default-projection.stderr
@@ -1,3 +1,12 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/specialization-default-projection.rs:1:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
 error[E0308]: mismatched types
   --> $DIR/specialization-default-projection.rs:21:5
    |
@@ -28,6 +37,6 @@ LL |     generic::<()>()
    = help: consider constraining the associated type `<() as Foo>::Assoc` to `()`
    = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
 
-error: aborting due to 2 previous errors
+error: aborting due to 2 previous errors; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/specialization/specialization-default-types.rs b/src/test/ui/specialization/specialization-default-types.rs
index acb86d889d4..346471f11e4 100644
--- a/src/test/ui/specialization/specialization-default-types.rs
+++ b/src/test/ui/specialization/specialization-default-types.rs
@@ -2,7 +2,7 @@
 // associated type in the impl defining it -- otherwise, what happens
 // if it's overridden?
 
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 trait Example {
     type Output;
diff --git a/src/test/ui/specialization/specialization-default-types.stderr b/src/test/ui/specialization/specialization-default-types.stderr
index 7233387eba1..7fe2366f781 100644
--- a/src/test/ui/specialization/specialization-default-types.stderr
+++ b/src/test/ui/specialization/specialization-default-types.stderr
@@ -1,3 +1,12 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/specialization-default-types.rs:5:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
 error[E0308]: mismatched types
   --> $DIR/specialization-default-types.rs:15:9
    |
@@ -24,6 +33,6 @@ LL |     Example::generate(t)
    = help: consider constraining the associated type `<T as Example>::Output` to `std::boxed::Box<T>`
    = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
 
-error: aborting due to 2 previous errors
+error: aborting due to 2 previous errors; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/specialization/specialization-no-default.rs b/src/test/ui/specialization/specialization-no-default.rs
index 57346b26d24..ae739b2358d 100644
--- a/src/test/ui/specialization/specialization-no-default.rs
+++ b/src/test/ui/specialization/specialization-no-default.rs
@@ -1,4 +1,4 @@
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 // Check a number of scenarios in which one impl tries to override another,
 // without correctly using `default`.
diff --git a/src/test/ui/specialization/specialization-no-default.stderr b/src/test/ui/specialization/specialization-no-default.stderr
index 992e9abbd4c..0222cba2ba6 100644
--- a/src/test/ui/specialization/specialization-no-default.stderr
+++ b/src/test/ui/specialization/specialization-no-default.stderr
@@ -1,3 +1,12 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/specialization-no-default.rs:1:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
 error[E0520]: `foo` specializes an item from a parent `impl`, but that item is not marked `default`
   --> $DIR/specialization-no-default.rs:20:5
    |
@@ -65,6 +74,6 @@ LL |       default fn redundant(&self) {}
    |
    = note: to specialize, `redundant` in the parent `impl` must be marked `default`
 
-error: aborting due to 5 previous errors
+error: aborting due to 5 previous errors; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0520`.
diff --git a/src/test/ui/specialization/specialization-on-projection.rs b/src/test/ui/specialization/specialization-on-projection.rs
index 5606eaea307..be8dcc4232e 100644
--- a/src/test/ui/specialization/specialization-on-projection.rs
+++ b/src/test/ui/specialization/specialization-on-projection.rs
@@ -1,7 +1,7 @@
 // run-pass
 #![allow(dead_code)]
 
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 // Ensure that specialization works for impls defined directly on a projection
 
diff --git a/src/test/ui/specialization/specialization-on-projection.stderr b/src/test/ui/specialization/specialization-on-projection.stderr
new file mode 100644
index 00000000000..173f9eee8f9
--- /dev/null
+++ b/src/test/ui/specialization/specialization-on-projection.stderr
@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/specialization-on-projection.rs:4:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/specialization/specialization-out-of-order.rs b/src/test/ui/specialization/specialization-out-of-order.rs
index 94e764f7636..cb7563e2760 100644
--- a/src/test/ui/specialization/specialization-out-of-order.rs
+++ b/src/test/ui/specialization/specialization-out-of-order.rs
@@ -2,7 +2,7 @@
 
 // Test that you can list the more specific impl before the more general one.
 
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 trait Foo {
     type Out;
diff --git a/src/test/ui/specialization/specialization-out-of-order.stderr b/src/test/ui/specialization/specialization-out-of-order.stderr
new file mode 100644
index 00000000000..27d5d851f52
--- /dev/null
+++ b/src/test/ui/specialization/specialization-out-of-order.stderr
@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/specialization-out-of-order.rs:5:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/specialization/specialization-overlap-negative.rs b/src/test/ui/specialization/specialization-overlap-negative.rs
index 90dbef3075b..550d3708295 100644
--- a/src/test/ui/specialization/specialization-overlap-negative.rs
+++ b/src/test/ui/specialization/specialization-overlap-negative.rs
@@ -1,5 +1,5 @@
 #![feature(negative_impls)]
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 trait MyTrait {}
 
diff --git a/src/test/ui/specialization/specialization-overlap-negative.stderr b/src/test/ui/specialization/specialization-overlap-negative.stderr
index e2616534d20..a28f961c03c 100644
--- a/src/test/ui/specialization/specialization-overlap-negative.stderr
+++ b/src/test/ui/specialization/specialization-overlap-negative.stderr
@@ -1,3 +1,12 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/specialization-overlap-negative.rs:2:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
 error[E0751]: found both positive and negative implementation of trait `std::marker::Send` for type `TestType<_>`:
   --> $DIR/specialization-overlap-negative.rs:9:1
    |
@@ -6,6 +15,6 @@ LL | unsafe impl<T: Clone> Send for TestType<T> {}
 LL | impl<T: MyTrait> !Send for TestType<T> {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ negative implementation here
 
-error: aborting due to previous error
+error: aborting due to previous error; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0751`.
diff --git a/src/test/ui/specialization/specialization-overlap-projection.rs b/src/test/ui/specialization/specialization-overlap-projection.rs
index 00b83c7e7a1..b07efb2a5c1 100644
--- a/src/test/ui/specialization/specialization-overlap-projection.rs
+++ b/src/test/ui/specialization/specialization-overlap-projection.rs
@@ -4,7 +4,7 @@
 // projections involve specialization, so long as the associated type is
 // provided by the most specialized impl.
 
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 trait Assoc {
     type Output;
diff --git a/src/test/ui/specialization/specialization-overlap-projection.stderr b/src/test/ui/specialization/specialization-overlap-projection.stderr
new file mode 100644
index 00000000000..10952ede91a
--- /dev/null
+++ b/src/test/ui/specialization/specialization-overlap-projection.stderr
@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/specialization-overlap-projection.rs:7:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/specialization/specialization-overlap.rs b/src/test/ui/specialization/specialization-overlap.rs
index c8ef8d61c1e..6bee22ceb8b 100644
--- a/src/test/ui/specialization/specialization-overlap.rs
+++ b/src/test/ui/specialization/specialization-overlap.rs
@@ -1,4 +1,4 @@
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 trait Foo { fn foo() {} }
 impl<T: Clone> Foo for T {}
diff --git a/src/test/ui/specialization/specialization-overlap.stderr b/src/test/ui/specialization/specialization-overlap.stderr
index 4275e7bdd85..7cc865b62ad 100644
--- a/src/test/ui/specialization/specialization-overlap.stderr
+++ b/src/test/ui/specialization/specialization-overlap.stderr
@@ -1,3 +1,12 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/specialization-overlap.rs:1:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
 error[E0119]: conflicting implementations of trait `Foo` for type `std::vec::Vec<_>`:
   --> $DIR/specialization-overlap.rs:5:1
    |
@@ -30,6 +39,6 @@ LL | impl<T: Clone> Qux for T {}
 LL | impl<T: Eq> Qux for T {}
    | ^^^^^^^^^^^^^^^^^^^^^ conflicting implementation
 
-error: aborting due to 4 previous errors
+error: aborting due to 4 previous errors; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0119`.
diff --git a/src/test/ui/specialization/specialization-polarity.rs b/src/test/ui/specialization/specialization-polarity.rs
index e78035f1710..17897d8b803 100644
--- a/src/test/ui/specialization/specialization-polarity.rs
+++ b/src/test/ui/specialization/specialization-polarity.rs
@@ -2,7 +2,7 @@
 
 #![feature(optin_builtin_traits)]
 #![feature(negative_impls)]
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 auto trait Foo {}
 
diff --git a/src/test/ui/specialization/specialization-polarity.stderr b/src/test/ui/specialization/specialization-polarity.stderr
index 44e60cad67a..7d269fe3293 100644
--- a/src/test/ui/specialization/specialization-polarity.stderr
+++ b/src/test/ui/specialization/specialization-polarity.stderr
@@ -1,3 +1,12 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/specialization-polarity.rs:5:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
 error[E0751]: found both positive and negative implementation of trait `Foo` for type `u8`:
   --> $DIR/specialization-polarity.rs:10:1
    |
@@ -14,6 +23,6 @@ LL | impl<T> !Bar for T {}
 LL | impl Bar for u8 {}
    | ^^^^^^^^^^^^^^^ positive implementation here
 
-error: aborting due to 2 previous errors
+error: aborting due to 2 previous errors; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0751`.
diff --git a/src/test/ui/specialization/specialization-projection-alias.rs b/src/test/ui/specialization/specialization-projection-alias.rs
index 0081ed455c9..f1f0b47bb65 100644
--- a/src/test/ui/specialization/specialization-projection-alias.rs
+++ b/src/test/ui/specialization/specialization-projection-alias.rs
@@ -2,7 +2,7 @@
 #![allow(dead_code)]
 #![allow(unused_variables)]
 
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 // Regression test for ICE when combining specialized associated types and type
 // aliases
diff --git a/src/test/ui/specialization/specialization-projection-alias.stderr b/src/test/ui/specialization/specialization-projection-alias.stderr
new file mode 100644
index 00000000000..385c4e2df91
--- /dev/null
+++ b/src/test/ui/specialization/specialization-projection-alias.stderr
@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/specialization-projection-alias.rs:5:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/specialization/specialization-projection.rs b/src/test/ui/specialization/specialization-projection.rs
index 86cdccf131e..700975e3b82 100644
--- a/src/test/ui/specialization/specialization-projection.rs
+++ b/src/test/ui/specialization/specialization-projection.rs
@@ -1,7 +1,7 @@
 // run-pass
 #![allow(dead_code)]
 
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 // Make sure we *can* project non-defaulted associated types
 // cf compile-fail/specialization-default-projection.rs
diff --git a/src/test/ui/specialization/specialization-projection.stderr b/src/test/ui/specialization/specialization-projection.stderr
new file mode 100644
index 00000000000..ef676d2f240
--- /dev/null
+++ b/src/test/ui/specialization/specialization-projection.stderr
@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/specialization-projection.rs:4:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/specialization/specialization-super-traits.rs b/src/test/ui/specialization/specialization-super-traits.rs
index a0f71d87693..fb85d801921 100644
--- a/src/test/ui/specialization/specialization-super-traits.rs
+++ b/src/test/ui/specialization/specialization-super-traits.rs
@@ -1,6 +1,6 @@
 // run-pass
 
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 // Test that you can specialize via an explicit trait hierarchy
 
diff --git a/src/test/ui/specialization/specialization-super-traits.stderr b/src/test/ui/specialization/specialization-super-traits.stderr
new file mode 100644
index 00000000000..05bdfd40136
--- /dev/null
+++ b/src/test/ui/specialization/specialization-super-traits.stderr
@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/specialization-super-traits.rs:3:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/specialization/specialization-translate-projections-with-lifetimes.rs b/src/test/ui/specialization/specialization-translate-projections-with-lifetimes.rs
index 2e32e3ff02d..5c2781a9c63 100644
--- a/src/test/ui/specialization/specialization-translate-projections-with-lifetimes.rs
+++ b/src/test/ui/specialization/specialization-translate-projections-with-lifetimes.rs
@@ -1,6 +1,6 @@
 // run-pass
 
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 trait Iterator {
     fn next(&self);
diff --git a/src/test/ui/specialization/specialization-translate-projections-with-lifetimes.stderr b/src/test/ui/specialization/specialization-translate-projections-with-lifetimes.stderr
new file mode 100644
index 00000000000..3fc6f350ba2
--- /dev/null
+++ b/src/test/ui/specialization/specialization-translate-projections-with-lifetimes.stderr
@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/specialization-translate-projections-with-lifetimes.rs:3:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/specialization/specialization-translate-projections-with-params.rs b/src/test/ui/specialization/specialization-translate-projections-with-params.rs
index bdc6501df44..62d63590a66 100644
--- a/src/test/ui/specialization/specialization-translate-projections-with-params.rs
+++ b/src/test/ui/specialization/specialization-translate-projections-with-params.rs
@@ -4,7 +4,7 @@
 // type parameters *and* rely on projections, and the type parameters are input
 // types on the trait.
 
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 trait Trait<T> {
     fn convert(&self) -> T;
diff --git a/src/test/ui/specialization/specialization-translate-projections-with-params.stderr b/src/test/ui/specialization/specialization-translate-projections-with-params.stderr
new file mode 100644
index 00000000000..58ac70a309c
--- /dev/null
+++ b/src/test/ui/specialization/specialization-translate-projections-with-params.stderr
@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/specialization-translate-projections-with-params.rs:7:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/specialization/specialization-translate-projections.rs b/src/test/ui/specialization/specialization-translate-projections.rs
index fcccb67902e..92ea9e2b85d 100644
--- a/src/test/ui/specialization/specialization-translate-projections.rs
+++ b/src/test/ui/specialization/specialization-translate-projections.rs
@@ -3,7 +3,7 @@
 // Ensure that provided items are inherited properly even when impls vary in
 // type parameters *and* rely on projections.
 
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 use std::convert::Into;
 
diff --git a/src/test/ui/specialization/specialization-translate-projections.stderr b/src/test/ui/specialization/specialization-translate-projections.stderr
new file mode 100644
index 00000000000..8d5156d5a99
--- /dev/null
+++ b/src/test/ui/specialization/specialization-translate-projections.stderr
@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/specialization-translate-projections.rs:6:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/traits/negative-impls/negative-default-impls.rs b/src/test/ui/traits/negative-impls/negative-default-impls.rs
index 2d50bc83ec3..c68bca432fa 100644
--- a/src/test/ui/traits/negative-impls/negative-default-impls.rs
+++ b/src/test/ui/traits/negative-impls/negative-default-impls.rs
@@ -1,5 +1,6 @@
 #![feature(negative_impls)]
 #![feature(specialization)]
+//~^ WARN the feature `specialization` is incomplete
 
 trait MyTrait {
     type Foo;
diff --git a/src/test/ui/traits/negative-impls/negative-default-impls.stderr b/src/test/ui/traits/negative-impls/negative-default-impls.stderr
index a70bbe6b948..50e74373b53 100644
--- a/src/test/ui/traits/negative-impls/negative-default-impls.stderr
+++ b/src/test/ui/traits/negative-impls/negative-default-impls.stderr
@@ -1,9 +1,18 @@
+warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/negative-default-impls.rs:2:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
 error[E0750]: negative impls cannot be default impls
-  --> $DIR/negative-default-impls.rs:8:1
+  --> $DIR/negative-default-impls.rs:9:1
    |
 LL | default impl !MyTrait for u32 {}
    | ^^^^^^^      ^
 
-error: aborting due to previous error
+error: aborting due to previous error; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0750`.
diff --git a/src/test/ui/traits/negative-impls/negative-specializes-negative.rs b/src/test/ui/traits/negative-impls/negative-specializes-negative.rs
index 877c3e8af4f..35297ab124e 100644
--- a/src/test/ui/traits/negative-impls/negative-specializes-negative.rs
+++ b/src/test/ui/traits/negative-impls/negative-specializes-negative.rs
@@ -1,4 +1,4 @@
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 #![feature(negative_impls)]
 
 // Test a negative impl that "specializes" another negative impl.
diff --git a/src/test/ui/traits/negative-impls/negative-specializes-negative.stderr b/src/test/ui/traits/negative-impls/negative-specializes-negative.stderr
new file mode 100644
index 00000000000..8b536de3786
--- /dev/null
+++ b/src/test/ui/traits/negative-impls/negative-specializes-negative.stderr
@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/negative-specializes-negative.rs:1:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/traits/negative-impls/negative-specializes-positive-item.rs b/src/test/ui/traits/negative-impls/negative-specializes-positive-item.rs
index da22e43377f..4281eedaf63 100644
--- a/src/test/ui/traits/negative-impls/negative-specializes-positive-item.rs
+++ b/src/test/ui/traits/negative-impls/negative-specializes-positive-item.rs
@@ -1,4 +1,4 @@
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 #![feature(negative_impls)]
 
 // Negative impl for u32 cannot "specialize" the base impl.
diff --git a/src/test/ui/traits/negative-impls/negative-specializes-positive-item.stderr b/src/test/ui/traits/negative-impls/negative-specializes-positive-item.stderr
index 079546a7df4..89ef15e89ac 100644
--- a/src/test/ui/traits/negative-impls/negative-specializes-positive-item.stderr
+++ b/src/test/ui/traits/negative-impls/negative-specializes-positive-item.stderr
@@ -1,3 +1,12 @@
+warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/negative-specializes-positive-item.rs:1:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
 error[E0751]: found both positive and negative implementation of trait `MyTrait` for type `u32`:
   --> $DIR/negative-specializes-positive-item.rs:11:1
    |
@@ -7,6 +16,6 @@ LL | impl<T> MyTrait for T {
 LL | impl !MyTrait for u32 {}
    | ^^^^^^^^^^^^^^^^^^^^^ negative implementation here
 
-error: aborting due to previous error
+error: aborting due to previous error; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0751`.
diff --git a/src/test/ui/traits/negative-impls/negative-specializes-positive.rs b/src/test/ui/traits/negative-impls/negative-specializes-positive.rs
index 1939a098b50..0e227691e04 100644
--- a/src/test/ui/traits/negative-impls/negative-specializes-positive.rs
+++ b/src/test/ui/traits/negative-impls/negative-specializes-positive.rs
@@ -1,4 +1,4 @@
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 #![feature(negative_impls)]
 
 // Negative impl for u32 cannot "specialize" the base impl.
diff --git a/src/test/ui/traits/negative-impls/negative-specializes-positive.stderr b/src/test/ui/traits/negative-impls/negative-specializes-positive.stderr
index ea005c1cbe0..e45d5a251ab 100644
--- a/src/test/ui/traits/negative-impls/negative-specializes-positive.stderr
+++ b/src/test/ui/traits/negative-impls/negative-specializes-positive.stderr
@@ -1,3 +1,12 @@
+warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/negative-specializes-positive.rs:1:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
 error[E0751]: found both positive and negative implementation of trait `MyTrait` for type `u32`:
   --> $DIR/negative-specializes-positive.rs:7:1
    |
@@ -6,6 +15,6 @@ LL | impl<T> MyTrait for T {}
 LL | impl !MyTrait for u32 {}
    | ^^^^^^^^^^^^^^^^^^^^^ negative implementation here
 
-error: aborting due to previous error
+error: aborting due to previous error; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0751`.
diff --git a/src/test/ui/traits/negative-impls/positive-specializes-negative.rs b/src/test/ui/traits/negative-impls/positive-specializes-negative.rs
index f2c5f507a4e..a06b3576540 100644
--- a/src/test/ui/traits/negative-impls/positive-specializes-negative.rs
+++ b/src/test/ui/traits/negative-impls/positive-specializes-negative.rs
@@ -1,4 +1,4 @@
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 #![feature(negative_impls)]
 
 trait MyTrait {}
diff --git a/src/test/ui/traits/negative-impls/positive-specializes-negative.stderr b/src/test/ui/traits/negative-impls/positive-specializes-negative.stderr
index a24d7aa442f..49c16d47404 100644
--- a/src/test/ui/traits/negative-impls/positive-specializes-negative.stderr
+++ b/src/test/ui/traits/negative-impls/positive-specializes-negative.stderr
@@ -1,3 +1,12 @@
+warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/positive-specializes-negative.rs:1:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
 error[E0751]: found both positive and negative implementation of trait `MyTrait` for type `u32`:
   --> $DIR/positive-specializes-negative.rs:7:1
    |
@@ -6,6 +15,6 @@ LL | impl<T> !MyTrait for T {}
 LL | impl MyTrait for u32 {}
    | ^^^^^^^^^^^^^^^^^^^^ positive implementation here
 
-error: aborting due to previous error
+error: aborting due to previous error; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0751`.
diff --git a/src/test/ui/transmute-specialization.rs b/src/test/ui/transmute-specialization.rs
index 002fba9ce81..499334d983b 100644
--- a/src/test/ui/transmute-specialization.rs
+++ b/src/test/ui/transmute-specialization.rs
@@ -1,6 +1,6 @@
 // run-pass
 
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 trait Specializable { type Output; }
 
diff --git a/src/test/ui/transmute-specialization.stderr b/src/test/ui/transmute-specialization.stderr
new file mode 100644
index 00000000000..9158e3142d8
--- /dev/null
+++ b/src/test/ui/transmute-specialization.stderr
@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may be unsafe to use and/or cause compiler crashes
+  --> $DIR/transmute-specialization.rs:3:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+
+warning: 1 warning emitted
+