RFC 2027: "first draft" of implementation

These are a squashed series of commits.

18 files changed, 183 insertions, 25 deletions

diff --git a/src/test/ui/object-safety/object-safety-associated-consts.stderr b/src/test/ui/object-safety/object-safety-associated-consts.curr.stderr
index 7d5aa00356e..67ef7a62f10 100644
--- a/src/test/ui/object-safety/object-safety-associated-consts.stderr
+++ b/src/test/ui/object-safety/object-safety-associated-consts.curr.stderr
@@ -1,5 +1,5 @@
 error[E0038]: the trait `Bar` cannot be made into an object
-  --> $DIR/object-safety-associated-consts.rs:9:1
+  --> $DIR/object-safety-associated-consts.rs:12:1
    |
 LL |     const X: usize;
    |           - the trait cannot contain associated consts like `X`
diff --git a/src/test/ui/object-safety/object-safety-associated-consts.object_safe_for_dispatch.stderr b/src/test/ui/object-safety/object-safety-associated-consts.object_safe_for_dispatch.stderr
new file mode 100644
index 00000000000..20993a680ba
--- /dev/null
+++ b/src/test/ui/object-safety/object-safety-associated-consts.object_safe_for_dispatch.stderr
@@ -0,0 +1,15 @@
+error[E0038]: the trait `Bar` cannot be made into an object
+  --> $DIR/object-safety-associated-consts.rs:14:5
+   |
+LL |     const X: usize;
+   |           - the trait cannot contain associated consts like `X`
+...
+LL |     t
+   |     ^ the trait `Bar` cannot be made into an object
+   |
+   = note: required because of the requirements on the impl of `std::ops::CoerceUnsized<&dyn Bar>` for `&T`
+   = note: required by cast to type `&dyn Bar`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0038`.
diff --git a/src/test/ui/object-safety/object-safety-associated-consts.rs b/src/test/ui/object-safety/object-safety-associated-consts.rs
index 5900019ea91..e1a772e5ab2 100644
--- a/src/test/ui/object-safety/object-safety-associated-consts.rs
+++ b/src/test/ui/object-safety/object-safety-associated-consts.rs
@@ -1,14 +1,18 @@
 // Check that we correctly prevent users from making trait objects
 // from traits with associated consts.
+//
+// revisions: curr object_safe_for_dispatch
 
+#![cfg_attr(object_safe_for_dispatch, feature(object_safe_for_dispatch))]
 
 trait Bar {
     const X: usize;
 }
 
 fn make_bar<T:Bar>(t: &T) -> &dyn Bar {
-    //~^ ERROR E0038
+    //[curr]~^ ERROR E0038
     t
+    //[object_safe_for_dispatch]~^ ERROR E0038
 }
 
 fn main() {
diff --git a/src/test/ui/object-safety/object-safety-generics.stderr b/src/test/ui/object-safety/object-safety-generics.curr.stderr
index b25e0052e41..8ae9236a5c3 100644
--- a/src/test/ui/object-safety/object-safety-generics.stderr
+++ b/src/test/ui/object-safety/object-safety-generics.curr.stderr
@@ -1,5 +1,5 @@
 error[E0038]: the trait `Bar` cannot be made into an object
-  --> $DIR/object-safety-generics.rs:14:1
+  --> $DIR/object-safety-generics.rs:18:1
    |
 LL |     fn bar<T>(&self, t: T);
    |        --- method `bar` has generic type parameters
@@ -8,7 +8,7 @@ LL | fn make_bar<T:Bar>(t: &T) -> &dyn Bar {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Bar` cannot be made into an object
 
 error[E0038]: the trait `Bar` cannot be made into an object
-  --> $DIR/object-safety-generics.rs:19:1
+  --> $DIR/object-safety-generics.rs:24:1
    |
 LL |     fn bar<T>(&self, t: T);
    |        --- method `bar` has generic type parameters
diff --git a/src/test/ui/object-safety/object-safety-generics.object_safe_for_dispatch.stderr b/src/test/ui/object-safety/object-safety-generics.object_safe_for_dispatch.stderr
new file mode 100644
index 00000000000..d3d8d368888
--- /dev/null
+++ b/src/test/ui/object-safety/object-safety-generics.object_safe_for_dispatch.stderr
@@ -0,0 +1,27 @@
+error[E0038]: the trait `Bar` cannot be made into an object
+  --> $DIR/object-safety-generics.rs:20:5
+   |
+LL |     fn bar<T>(&self, t: T);
+   |        --- method `bar` has generic type parameters
+...
+LL |     t
+   |     ^ the trait `Bar` cannot be made into an object
+   |
+   = note: required because of the requirements on the impl of `std::ops::CoerceUnsized<&dyn Bar>` for `&T`
+   = note: required by cast to type `&dyn Bar`
+
+error[E0038]: the trait `Bar` cannot be made into an object
+  --> $DIR/object-safety-generics.rs:26:5
+   |
+LL |     fn bar<T>(&self, t: T);
+   |        --- method `bar` has generic type parameters
+...
+LL |     t as &dyn Bar
+   |     ^ the trait `Bar` cannot be made into an object
+   |
+   = note: required because of the requirements on the impl of `std::ops::CoerceUnsized<&dyn Bar>` for `&T`
+   = note: required by cast to type `&dyn Bar`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0038`.
diff --git a/src/test/ui/object-safety/object-safety-generics.rs b/src/test/ui/object-safety/object-safety-generics.rs
index d63ea28c8f2..63dcd169925 100644
--- a/src/test/ui/object-safety/object-safety-generics.rs
+++ b/src/test/ui/object-safety/object-safety-generics.rs
@@ -1,6 +1,10 @@
 // Check that we correctly prevent users from making trait objects
 // from traits with generic methods, unless `where Self : Sized` is
 // present.
+// revisions: curr object_safe_for_dispatch
+
+#![cfg_attr(object_safe_for_dispatch, feature(object_safe_for_dispatch))]
+
 
 trait Bar {
     fn bar<T>(&self, t: T);
@@ -12,13 +16,15 @@ trait Quux {
 }
 
 fn make_bar<T:Bar>(t: &T) -> &dyn Bar {
-        //~^ ERROR E0038
+    //[curr]~^ ERROR E0038
     t
+    //[object_safe_for_dispatch]~^ ERROR E0038
 }
 
 fn make_bar_explicit<T:Bar>(t: &T) -> &dyn Bar {
-    //~^ ERROR E0038
+    //[curr]~^ ERROR E0038
     t as &dyn Bar
+    //[object_safe_for_dispatch]~^ ERROR E0038
 }
 
 fn make_quux<T:Quux>(t: &T) -> &dyn Quux {
diff --git a/src/test/ui/object-safety/object-safety-mentions-Self.stderr b/src/test/ui/object-safety/object-safety-mentions-Self.curr.stderr
index 971e79cb021..297cd876187 100644
--- a/src/test/ui/object-safety/object-safety-mentions-Self.stderr
+++ b/src/test/ui/object-safety/object-safety-mentions-Self.curr.stderr
@@ -1,5 +1,5 @@
 error[E0038]: the trait `Bar` cannot be made into an object
-  --> $DIR/object-safety-mentions-Self.rs:17:1
+  --> $DIR/object-safety-mentions-Self.rs:22:1
    |
 LL |     fn bar(&self, x: &Self);
    |        --- method `bar` references the `Self` type in its parameters or return type
@@ -8,10 +8,10 @@ LL | fn make_bar<T:Bar>(t: &T) -> &dyn Bar {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Bar` cannot be made into an object
 
 error[E0038]: the trait `Baz` cannot be made into an object
-  --> $DIR/object-safety-mentions-Self.rs:22:1
+  --> $DIR/object-safety-mentions-Self.rs:28:1
    |
-LL |     fn bar(&self) -> Self;
-   |        --- method `bar` references the `Self` type in its parameters or return type
+LL |     fn baz(&self) -> Self;
+   |        --- method `baz` references the `Self` type in its parameters or return type
 ...
 LL | fn make_baz<T:Baz>(t: &T) -> &dyn Baz {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Baz` cannot be made into an object
diff --git a/src/test/ui/object-safety/object-safety-mentions-Self.object_safe_for_dispatch.stderr b/src/test/ui/object-safety/object-safety-mentions-Self.object_safe_for_dispatch.stderr
new file mode 100644
index 00000000000..03b2b8da075
--- /dev/null
+++ b/src/test/ui/object-safety/object-safety-mentions-Self.object_safe_for_dispatch.stderr
@@ -0,0 +1,27 @@
+error[E0038]: the trait `Bar` cannot be made into an object
+  --> $DIR/object-safety-mentions-Self.rs:24:5
+   |
+LL |     fn bar(&self, x: &Self);
+   |        --- method `bar` references the `Self` type in its parameters or return type
+...
+LL |     t
+   |     ^ the trait `Bar` cannot be made into an object
+   |
+   = note: required because of the requirements on the impl of `std::ops::CoerceUnsized<&dyn Bar>` for `&T`
+   = note: required by cast to type `&dyn Bar`
+
+error[E0038]: the trait `Baz` cannot be made into an object
+  --> $DIR/object-safety-mentions-Self.rs:30:5
+   |
+LL |     fn baz(&self) -> Self;
+   |        --- method `baz` references the `Self` type in its parameters or return type
+...
+LL |     t
+   |     ^ the trait `Baz` cannot be made into an object
+   |
+   = note: required because of the requirements on the impl of `std::ops::CoerceUnsized<&dyn Baz>` for `&T`
+   = note: required by cast to type `&dyn Baz`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0038`.
diff --git a/src/test/ui/object-safety/object-safety-mentions-Self.rs b/src/test/ui/object-safety/object-safety-mentions-Self.rs
index f13ffe53626..412d16ff3c7 100644
--- a/src/test/ui/object-safety/object-safety-mentions-Self.rs
+++ b/src/test/ui/object-safety/object-safety-mentions-Self.rs
@@ -1,27 +1,34 @@
 // Check that we correctly prevent users from making trait objects
 // form traits that make use of `Self` in an argument or return
 // position, unless `where Self : Sized` is present..
+//
+// revisions: curr object_safe_for_dispatch
+
+#![cfg_attr(object_safe_for_dispatch, feature(object_safe_for_dispatch))]
+
 
 trait Bar {
     fn bar(&self, x: &Self);
 }
 
 trait Baz {
-    fn bar(&self) -> Self;
+    fn baz(&self) -> Self;
 }
 
 trait Quux {
-    fn get(&self, s: &Self) -> Self where Self : Sized;
+    fn quux(&self, s: &Self) -> Self where Self : Sized;
 }
 
 fn make_bar<T:Bar>(t: &T) -> &dyn Bar {
-        //~^ ERROR E0038
-    loop { }
+    //[curr]~^ ERROR E0038
+    t
+    //[object_safe_for_dispatch]~^ ERROR E0038
 }
 
 fn make_baz<T:Baz>(t: &T) -> &dyn Baz {
-        //~^ ERROR E0038
+    //[curr]~^ ERROR E0038
     t
+    //[object_safe_for_dispatch]~^ ERROR E0038
 }
 
 fn make_quux<T:Quux>(t: &T) -> &dyn Quux {
@@ -32,5 +39,4 @@ fn make_quux_explicit<T:Quux>(t: &T) -> &dyn Quux {
     t as &dyn Quux
 }
 
-fn main() {
-}
+fn main() {}
diff --git a/src/test/ui/object-safety/object-safety-no-static.curr.stderr b/src/test/ui/object-safety/object-safety-no-static.curr.stderr
new file mode 100644
index 00000000000..1641ce57771
--- /dev/null
+++ b/src/test/ui/object-safety/object-safety-no-static.curr.stderr
@@ -0,0 +1,12 @@
+error[E0038]: the trait `Foo` cannot be made into an object
+  --> $DIR/object-safety-no-static.rs:12:1
+   |
+LL |     fn foo() {}
+   |        --- associated function `foo` has no `self` parameter
+...
+LL | fn diverges() -> Box<dyn Foo> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Foo` cannot be made into an object
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0038`.
diff --git a/src/test/ui/object-safety/object-safety-no-static.object_safe_for_dispatch.stderr b/src/test/ui/object-safety/object-safety-no-static.object_safe_for_dispatch.stderr
new file mode 100644
index 00000000000..91a9285b63c
--- /dev/null
+++ b/src/test/ui/object-safety/object-safety-no-static.object_safe_for_dispatch.stderr
@@ -0,0 +1,15 @@
+error[E0038]: the trait `Foo` cannot be made into an object
+  --> $DIR/object-safety-no-static.rs:22:27
+   |
+LL |     fn foo() {}
+   |        --- associated function `foo` has no `self` parameter
+...
+LL |     let b: Box<dyn Foo> = Box::new(Bar);
+   |                           ^^^^^^^^^^^^^ the trait `Foo` cannot be made into an object
+   |
+   = note: required because of the requirements on the impl of `std::ops::CoerceUnsized<std::boxed::Box<dyn Foo>>` for `std::boxed::Box<Bar>`
+   = note: required by cast to type `std::boxed::Box<dyn Foo>`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0038`.
diff --git a/src/test/ui/object-safety/object-safety-no-static.rs b/src/test/ui/object-safety/object-safety-no-static.rs
index 55d31ce8087..03b62217483 100644
--- a/src/test/ui/object-safety/object-safety-no-static.rs
+++ b/src/test/ui/object-safety/object-safety-no-static.rs
@@ -1,14 +1,24 @@
 // Check that we correctly prevent users from making trait objects
 // from traits with static methods.
+//
+// revisions: curr object_safe_for_dispatch
+
+#![cfg_attr(object_safe_for_dispatch, feature(object_safe_for_dispatch))]
 
 trait Foo {
-    fn foo();
+    fn foo() {}
 }
 
-fn foo_implicit<T:Foo+'static>(b: Box<T>) -> Box<dyn Foo + 'static> {
-    //~^ ERROR E0038
+fn diverges() -> Box<dyn Foo> {
+    //[curr]~^ ERROR E0038
     loop { }
 }
 
+struct Bar;
+
+impl Foo for Bar {}
+
 fn main() {
+    let b: Box<dyn Foo> = Box::new(Bar);
+    //[object_safe_for_dispatch]~^ ERROR E0038
 }
diff --git a/src/test/ui/object-safety/object-safety-sized-2.stderr b/src/test/ui/object-safety/object-safety-sized-2.curr.stderr
index dcaf2ff0bc2..1e1d2bf64c4 100644
--- a/src/test/ui/object-safety/object-safety-sized-2.stderr
+++ b/src/test/ui/object-safety/object-safety-sized-2.curr.stderr
@@ -1,5 +1,5 @@
 error[E0038]: the trait `Bar` cannot be made into an object
-  --> $DIR/object-safety-sized-2.rs:10:1
+  --> $DIR/object-safety-sized-2.rs:14:1
    |
 LL | fn make_bar<T:Bar>(t: &T) -> &dyn Bar {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Bar` cannot be made into an object
diff --git a/src/test/ui/object-safety/object-safety-sized-2.object_safe_for_dispatch.stderr b/src/test/ui/object-safety/object-safety-sized-2.object_safe_for_dispatch.stderr
new file mode 100644
index 00000000000..06ecfd019c8
--- /dev/null
+++ b/src/test/ui/object-safety/object-safety-sized-2.object_safe_for_dispatch.stderr
@@ -0,0 +1,13 @@
+error[E0038]: the trait `Bar` cannot be made into an object
+  --> $DIR/object-safety-sized-2.rs:16:5
+   |
+LL |     t
+   |     ^ the trait `Bar` cannot be made into an object
+   |
+   = note: the trait cannot require that `Self : Sized`
+   = note: required because of the requirements on the impl of `std::ops::CoerceUnsized<&dyn Bar>` for `&T`
+   = note: required by cast to type `&dyn Bar`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0038`.
diff --git a/src/test/ui/object-safety/object-safety-sized-2.rs b/src/test/ui/object-safety/object-safety-sized-2.rs
index 7235b22404e..1e79b8cd917 100644
--- a/src/test/ui/object-safety/object-safety-sized-2.rs
+++ b/src/test/ui/object-safety/object-safety-sized-2.rs
@@ -1,5 +1,9 @@
 // Check that we correctly prevent users from making trait objects
 // from traits where `Self : Sized`.
+//
+// revisions: curr object_safe_for_dispatch
+
+#![cfg_attr(object_safe_for_dispatch, feature(object_safe_for_dispatch))]
 
 trait Bar
     where Self : Sized
@@ -8,8 +12,9 @@ trait Bar
 }
 
 fn make_bar<T:Bar>(t: &T) -> &dyn Bar {
-        //~^ ERROR E0038
-    loop { }
+    //[curr]~^ ERROR E0038
+    t
+    //[object_safe_for_dispatch]~^ ERROR E0038
 }
 
 fn main() {
diff --git a/src/test/ui/object-safety/object-safety-sized.stderr b/src/test/ui/object-safety/object-safety-sized.curr.stderr
index 98bc73e38d4..1a67e79e83d 100644
--- a/src/test/ui/object-safety/object-safety-sized.stderr
+++ b/src/test/ui/object-safety/object-safety-sized.curr.stderr
@@ -1,5 +1,5 @@
 error[E0038]: the trait `Bar` cannot be made into an object
-  --> $DIR/object-safety-sized.rs:8:1
+  --> $DIR/object-safety-sized.rs:12:1
    |
 LL | fn make_bar<T:Bar>(t: &T) -> &dyn Bar {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Bar` cannot be made into an object
diff --git a/src/test/ui/object-safety/object-safety-sized.object_safe_for_dispatch.stderr b/src/test/ui/object-safety/object-safety-sized.object_safe_for_dispatch.stderr
new file mode 100644
index 00000000000..3d88dfc40ed
--- /dev/null
+++ b/src/test/ui/object-safety/object-safety-sized.object_safe_for_dispatch.stderr
@@ -0,0 +1,13 @@
+error[E0038]: the trait `Bar` cannot be made into an object
+  --> $DIR/object-safety-sized.rs:14:5
+   |
+LL |     t
+   |     ^ the trait `Bar` cannot be made into an object
+   |
+   = note: the trait cannot require that `Self : Sized`
+   = note: required because of the requirements on the impl of `std::ops::CoerceUnsized<&dyn Bar>` for `&T`
+   = note: required by cast to type `&dyn Bar`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0038`.
diff --git a/src/test/ui/object-safety/object-safety-sized.rs b/src/test/ui/object-safety/object-safety-sized.rs
index 1312bb34717..b424b892d3b 100644
--- a/src/test/ui/object-safety/object-safety-sized.rs
+++ b/src/test/ui/object-safety/object-safety-sized.rs
@@ -1,13 +1,18 @@
 // Check that we correctly prevent users from making trait objects
 // from traits where `Self : Sized`.
+//
+// revisions: curr object_safe_for_dispatch
+
+#![cfg_attr(object_safe_for_dispatch, feature(object_safe_for_dispatch))]
 
 trait Bar : Sized {
     fn bar<T>(&self, t: T);
 }
 
 fn make_bar<T:Bar>(t: &T) -> &dyn Bar {
-        //~^ ERROR E0038
+    //[curr]~^ ERROR E0038
     t
+    //[object_safe_for_dispatch]~^ ERROR E0038
 }
 
 fn main() {