Rollup merge of #71420 - RalfJung:specialization-incomplete, r=matthewjasper

Specialization is unsound

As discussed in https://github.com/rust-lang/rust/issues/31844#issuecomment-617013949, it might be a good idea to warn users of specialization that the feature they are using is unsound.

I also expanded the "incomplete feature" warning to link the user to the tracking issue.

src/libcore/tests/lib.rs

@@ -19,7 +19,7 @@
 #![feature(raw)]
 #![feature(sort_internals)]
 #![feature(slice_partition_at_index)]
-#![feature(specialization)]
+#![feature(min_specialization)]
 #![feature(step_trait)]
 #![feature(step_trait_ext)]
 #![feature(str_internals)]

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,
 ];

src/test/compile-fail/specialization/issue-50452.rs

@@ -1,6 +1,6 @@
 // compile-fail
-
 #![feature(specialization)]
+//~^ WARN the feature `specialization` is incomplete
 
 pub trait Foo {
     fn foo();

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;

src/test/ui/associated-types/defaults-specialization.stderr

@@ -1,5 +1,14 @@
+warning: the feature `specialization` is incomplete and may not be safe 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`.

src/test/ui/coherence/coherence-inherited-assoc-ty-cycle-err.rs

@@ -4,6 +4,7 @@
 //
 // No we expect to run into a more user-friendly cycle error instead.
 #![feature(specialization)]
+//~^ WARN the feature `specialization` is incomplete
 
 trait Trait<T> { type Assoc; }
 //~^ ERROR E0391

src/test/ui/coherence/coherence-inherited-assoc-ty-cycle-err.stderr

@@ -1,16 +1,25 @@
+warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/coherence-inherited-assoc-ty-cycle-err.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
+
 error[E0391]: cycle detected when building specialization graph of trait `Trait`
-  --> $DIR/coherence-inherited-assoc-ty-cycle-err.rs:8:1
+  --> $DIR/coherence-inherited-assoc-ty-cycle-err.rs:9:1
    |
 LL | trait Trait<T> { type Assoc; }
    | ^^^^^^^^^^^^^^
    |
    = note: ...which again requires building specialization graph of trait `Trait`, completing the cycle
 note: cycle used when coherence checking all impls of trait `Trait`
-  --> $DIR/coherence-inherited-assoc-ty-cycle-err.rs:8:1
+  --> $DIR/coherence-inherited-assoc-ty-cycle-err.rs:9:1
    |
 LL | trait Trait<T> { type Assoc; }
    | ^^^^^^^^^^^^^^
 
-error: aborting due to previous error
+error: aborting due to previous error; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0391`.

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 {}
 

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
+

src/test/ui/error-codes/E0520.rs

@@ -1,4 +1,5 @@
 #![feature(specialization)]
+//~^ WARN the feature `specialization` is incomplete
 
 trait SpaceLlama {
     fn fly(&self);

src/test/ui/error-codes/E0520.stderr

@@ -1,5 +1,14 @@
+warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/E0520.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]: `fly` specializes an item from a parent `impl`, but that item is not marked `default`
-  --> $DIR/E0520.rs:16:5
+  --> $DIR/E0520.rs:17:5
    |
 LL | / impl<T: Clone> SpaceLlama for T {
 LL | |     fn fly(&self) {}
@@ -11,6 +20,6 @@ LL |       default fn fly(&self) {}
    |
    = note: to specialize, `fly` in the parent `impl` must be marked `default`
 
-error: aborting due to previous error
+error: aborting due to previous error; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0520`.

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 {}
 

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
+

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 {}
 

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`.

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 {}
 

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`.

src/test/ui/issues/issue-35376.rs

@@ -1,5 +1,6 @@
 // check-pass
 #![feature(specialization)]
+//~^ WARN the feature `specialization` is incomplete
 
 fn main() {}
 

src/test/ui/issues/issue-35376.stderr

@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/issue-35376.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
+

src/test/ui/issues/issue-38091.rs

@@ -1,5 +1,6 @@
 // run-pass
 #![feature(specialization)]
+//~^ WARN the feature `specialization` is incomplete
 
 trait Iterate<'a> {
     type Ty: Valid;

src/test/ui/issues/issue-38091.stderr

@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/issue-38091.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
+

src/test/ui/issues/issue-55380.rs

@@ -1,6 +1,6 @@
 // run-pass
-
 #![feature(specialization)]
+//~^ WARN the feature `specialization` is incomplete
 
 pub trait Foo {
     fn abc() -> u32;

src/test/ui/issues/issue-55380.stderr

@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/issue-55380.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
+

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 {}

src/test/ui/overlap-doesnt-conflict-with-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/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
+

src/test/ui/parser/assoc-static-semantic-fail.rs

@@ -1,6 +1,7 @@
 // Semantically, we do not allow e.g., `static X: u8 = 0;` as an associated item.
 
 #![feature(specialization)]
+//~^ WARN the feature `specialization` is incomplete
 
 fn main() {}
 

src/test/ui/parser/assoc-static-semantic-fail.stderr

@@ -1,17 +1,17 @@
 error: associated `static` items are not allowed
-  --> $DIR/assoc-static-semantic-fail.rs:9:5
+  --> $DIR/assoc-static-semantic-fail.rs:10:5
    |
 LL |     static IA: u8 = 0;
    |     ^^^^^^^^^^^^^^^^^^
 
 error: associated `static` items are not allowed
-  --> $DIR/assoc-static-semantic-fail.rs:11:5
+  --> $DIR/assoc-static-semantic-fail.rs:12:5
    |
 LL |     static IB: u8;
    |     ^^^^^^^^^^^^^^
 
 error: a static item cannot be `default`
-  --> $DIR/assoc-static-semantic-fail.rs:14:5
+  --> $DIR/assoc-static-semantic-fail.rs:15:5
    |
 LL |     default static IC: u8 = 0;
    |     ^^^^^^^ `default` because of this
@@ -19,13 +19,13 @@ LL |     default static IC: u8 = 0;
    = note: only associated `fn`, `const`, and `type` items can be `default`
 
 error: associated `static` items are not allowed
-  --> $DIR/assoc-static-semantic-fail.rs:14:5
+  --> $DIR/assoc-static-semantic-fail.rs:15:5
    |
 LL |     default static IC: u8 = 0;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: a static item cannot be `default`
-  --> $DIR/assoc-static-semantic-fail.rs:17:16
+  --> $DIR/assoc-static-semantic-fail.rs:18:16
    |
 LL |     pub(crate) default static ID: u8;
    |                ^^^^^^^ `default` because of this
@@ -33,25 +33,25 @@ LL |     pub(crate) default static ID: u8;
    = note: only associated `fn`, `const`, and `type` items can be `default`
 
 error: associated `static` items are not allowed
-  --> $DIR/assoc-static-semantic-fail.rs:17:5
+  --> $DIR/assoc-static-semantic-fail.rs:18:5
    |
 LL |     pub(crate) default static ID: u8;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: associated `static` items are not allowed
-  --> $DIR/assoc-static-semantic-fail.rs:24:5
+  --> $DIR/assoc-static-semantic-fail.rs:25:5
    |
 LL |     static TA: u8 = 0;
    |     ^^^^^^^^^^^^^^^^^^
 
 error: associated `static` items are not allowed
-  --> $DIR/assoc-static-semantic-fail.rs:26:5
+  --> $DIR/assoc-static-semantic-fail.rs:27:5
    |
 LL |     static TB: u8;
    |     ^^^^^^^^^^^^^^
 
 error: a static item cannot be `default`
-  --> $DIR/assoc-static-semantic-fail.rs:28:5
+  --> $DIR/assoc-static-semantic-fail.rs:29:5
    |
 LL |     default static TC: u8 = 0;
    |     ^^^^^^^ `default` because of this
@@ -59,13 +59,13 @@ LL |     default static TC: u8 = 0;
    = note: only associated `fn`, `const`, and `type` items can be `default`
 
 error: associated `static` items are not allowed
-  --> $DIR/assoc-static-semantic-fail.rs:28:5
+  --> $DIR/assoc-static-semantic-fail.rs:29:5
    |
 LL |     default static TC: u8 = 0;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: a static item cannot be `default`
-  --> $DIR/assoc-static-semantic-fail.rs:31:16
+  --> $DIR/assoc-static-semantic-fail.rs:32:16
    |
 LL |     pub(crate) default static TD: u8;
    |                ^^^^^^^ `default` because of this
@@ -73,25 +73,25 @@ LL |     pub(crate) default static TD: u8;
    = note: only associated `fn`, `const`, and `type` items can be `default`
 
 error: associated `static` items are not allowed
-  --> $DIR/assoc-static-semantic-fail.rs:31:5
+  --> $DIR/assoc-static-semantic-fail.rs:32:5
    |
 LL |     pub(crate) default static TD: u8;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: associated `static` items are not allowed
-  --> $DIR/assoc-static-semantic-fail.rs:38:5
+  --> $DIR/assoc-static-semantic-fail.rs:39:5
    |
 LL |     static TA: u8 = 0;
    |     ^^^^^^^^^^^^^^^^^^
 
 error: associated `static` items are not allowed
-  --> $DIR/assoc-static-semantic-fail.rs:40:5
+  --> $DIR/assoc-static-semantic-fail.rs:41:5
    |
 LL |     static TB: u8;
    |     ^^^^^^^^^^^^^^
 
 error: a static item cannot be `default`
-  --> $DIR/assoc-static-semantic-fail.rs:43:5
+  --> $DIR/assoc-static-semantic-fail.rs:44:5
    |
 LL |     default static TC: u8 = 0;
    |     ^^^^^^^ `default` because of this
@@ -99,13 +99,13 @@ LL |     default static TC: u8 = 0;
    = note: only associated `fn`, `const`, and `type` items can be `default`
 
 error: associated `static` items are not allowed
-  --> $DIR/assoc-static-semantic-fail.rs:43:5
+  --> $DIR/assoc-static-semantic-fail.rs:44:5
    |
 LL |     default static TC: u8 = 0;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: a static item cannot be `default`
-  --> $DIR/assoc-static-semantic-fail.rs:46:9
+  --> $DIR/assoc-static-semantic-fail.rs:47:9
    |
 LL |     pub default static TD: u8;
    |         ^^^^^^^ `default` because of this
@@ -113,13 +113,13 @@ LL |     pub default static TD: u8;
    = note: only associated `fn`, `const`, and `type` items can be `default`
 
 error: associated `static` items are not allowed
-  --> $DIR/assoc-static-semantic-fail.rs:46:5
+  --> $DIR/assoc-static-semantic-fail.rs:47:5
    |
 LL |     pub default static TD: u8;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: associated constant in `impl` without body
-  --> $DIR/assoc-static-semantic-fail.rs:11:5
+  --> $DIR/assoc-static-semantic-fail.rs:12:5
    |
 LL |     static IB: u8;
    |     ^^^^^^^^^^^^^-
@@ -127,7 +127,7 @@ LL |     static IB: u8;
    |                  help: provide a definition for the constant: `= <expr>;`
 
 error: associated constant in `impl` without body
-  --> $DIR/assoc-static-semantic-fail.rs:17:5
+  --> $DIR/assoc-static-semantic-fail.rs:18:5
    |
 LL |     pub(crate) default static ID: u8;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^-
@@ -135,13 +135,13 @@ LL |     pub(crate) default static ID: u8;
    |                                     help: provide a definition for the constant: `= <expr>;`
 
 error[E0449]: unnecessary visibility qualifier
-  --> $DIR/assoc-static-semantic-fail.rs:31:5
+  --> $DIR/assoc-static-semantic-fail.rs:32:5
    |
 LL |     pub(crate) default static TD: u8;
    |     ^^^^^^^^^^
 
 error: associated constant in `impl` without body
-  --> $DIR/assoc-static-semantic-fail.rs:40:5
+  --> $DIR/assoc-static-semantic-fail.rs:41:5
    |
 LL |     static TB: u8;
    |     ^^^^^^^^^^^^^-
@@ -149,7 +149,7 @@ LL |     static TB: u8;
    |                  help: provide a definition for the constant: `= <expr>;`
 
 error: associated constant in `impl` without body
-  --> $DIR/assoc-static-semantic-fail.rs:46:5
+  --> $DIR/assoc-static-semantic-fail.rs:47:5
    |
 LL |     pub default static TD: u8;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^-
@@ -157,11 +157,20 @@ LL |     pub default static TD: u8;
    |                              help: provide a definition for the constant: `= <expr>;`
 
 error[E0449]: unnecessary visibility qualifier
-  --> $DIR/assoc-static-semantic-fail.rs:46:5
+  --> $DIR/assoc-static-semantic-fail.rs:47:5
    |
 LL |     pub default static TD: u8;
    |     ^^^ `pub` not permitted here because it's implied
 
-error: aborting due to 24 previous errors
+warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/assoc-static-semantic-fail.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: aborting due to 24 previous errors; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0449`.

src/test/ui/parser/default.rs

@@ -1,6 +1,7 @@
 // Test successful and unsuccessful parsing of the `default` contextual keyword
 
 #![feature(specialization)]
+//~^ WARN the feature `specialization` is incomplete
 
 trait Foo {
     fn foo<T: Default>() -> T;

src/test/ui/parser/default.stderr

@@ -1,5 +1,5 @@
 error: `default` is not followed by an item
-  --> $DIR/default.rs:22:5
+  --> $DIR/default.rs:23:5
    |
 LL |     default pub fn foo<T: Default>() -> T { T::default() }
    |     ^^^^^^^ the `default` qualifier
@@ -7,7 +7,7 @@ LL |     default pub fn foo<T: Default>() -> T { T::default() }
    = note: only `fn`, `const`, `type`, or `impl` items may be prefixed by `default`
 
 error: non-item in item list
-  --> $DIR/default.rs:22:13
+  --> $DIR/default.rs:23:13
    |
 LL | impl Foo for u32 {
    |                  - item list starts here
@@ -18,13 +18,22 @@ LL | }
    | - item list ends here
 
 error[E0449]: unnecessary visibility qualifier
-  --> $DIR/default.rs:16:5
+  --> $DIR/default.rs:17:5
    |
 LL |     pub default fn foo<T: Default>() -> T {
    |     ^^^ `pub` not permitted here because it's implied
 
+warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/default.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`
-  --> $DIR/default.rs:21:1
+  --> $DIR/default.rs:22:1
    |
 LL |     fn foo<T: Default>() -> T;
    |     -------------------------- `foo` from trait
@@ -32,7 +41,7 @@ LL |     fn foo<T: Default>() -> T;
 LL | impl Foo for u32 {
    | ^^^^^^^^^^^^^^^^ missing `foo` in implementation
 
-error: aborting due to 4 previous errors
+error: aborting due to 4 previous errors; 1 warning emitted
 
 Some errors have detailed explanations: E0046, E0449.
 For more information about an error, try `rustc --explain E0046`.

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() {}
 

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
 

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;

src/test/ui/specialization/assoc-ty-graph-cycle.stderr

@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may not be safe 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
+

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;
 

src/test/ui/specialization/cross-crate-defaults.stderr

@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may not be safe 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
+

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;
 

src/test/ui/specialization/defaultimpl/allowed-cross-crate.stderr

@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may not be safe 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
+

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;

src/test/ui/specialization/defaultimpl/out-of-order.stderr

@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may not be safe 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
+

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;

src/test/ui/specialization/defaultimpl/overlap-projection.stderr

@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may not be safe 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
+

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

src/test/ui/specialization/defaultimpl/projection.stderr

@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may not be safe 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
+

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`.

src/test/ui/specialization/defaultimpl/specialization-no-default.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-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`.

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;

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
+

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;

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`.

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;

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`.

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> { }
 

src/test/ui/specialization/defaultimpl/specialization-wfcheck.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-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`.

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;

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 not be safe 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`.

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);
 

src/test/ui/specialization/issue-36804.stderr

@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may not be safe 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
+

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).
 

src/test/ui/specialization/issue-39448.stderr

@@ -1,3 +1,12 @@
+warning: the feature `specialization` is incomplete and may not be safe 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`.

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);

src/test/ui/specialization/issue-39618.stderr

@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may not be safe 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
+

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();

src/test/ui/specialization/issue-50452.stderr

@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may not be safe 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
+

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

src/test/ui/specialization/issue-52050.stderr

@@ -1,3 +1,12 @@
+warning: the feature `specialization` is incomplete and may not be safe 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`.

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() {}
 

src/test/ui/specialization/issue-63716-parse-async.stderr

@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may not be safe 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
+

src/test/ui/specialization/issue-70442.rs

@@ -1,4 +1,4 @@
-#![feature(specialization)]
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 // check-pass
 

src/test/ui/specialization/issue-70442.stderr

@@ -0,0 +1,11 @@
+warning: the feature `specialization` is incomplete and may not be safe 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
+

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.

src/test/ui/specialization/non-defaulted-item-fail.stderr

@@ -1,5 +1,14 @@
+warning: the feature `specialization` is incomplete and may not be safe 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`.

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;
 

src/test/ui/specialization/specialization-allowed-cross-crate.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-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
+

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;

src/test/ui/specialization/specialization-assoc-fns.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-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
+

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.

src/test/ui/specialization/specialization-basics.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-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
+

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;
 

src/test/ui/specialization/specialization-cross-crate.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-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
+

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
 

src/test/ui/specialization/specialization-default-methods.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-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
+

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
 

src/test/ui/specialization/specialization-default-projection.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-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`.

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;

src/test/ui/specialization/specialization-default-types.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-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`.

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`.

src/test/ui/specialization/specialization-no-default.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-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`.

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
 

src/test/ui/specialization/specialization-on-projection.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-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
+

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;

src/test/ui/specialization/specialization-out-of-order.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-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
+

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 {}
 

src/test/ui/specialization/specialization-overlap-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/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`.

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;

src/test/ui/specialization/specialization-overlap-projection.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-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
+

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 {}

src/test/ui/specialization/specialization-overlap.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-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`.

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 {}
 

src/test/ui/specialization/specialization-polarity.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-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`.

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