RFC 2027: "first draft" of implementation

These are a squashed series of commits.

6 files changed, 169 insertions, 0 deletions

diff --git a/src/test/ui/wf/wf-convert-unsafe-trait-obj-box.rs b/src/test/ui/wf/wf-convert-unsafe-trait-obj-box.rs
new file mode 100644
index 00000000000..ffdb49a3be5
--- /dev/null
+++ b/src/test/ui/wf/wf-convert-unsafe-trait-obj-box.rs
@@ -0,0 +1,18 @@
+// Check that we do not allow casts or coercions
+// to object unsafe trait objects inside a Box
+
+#![feature(object_safe_for_dispatch)]
+
+trait Trait: Sized {}
+
+struct S;
+
+impl Trait for S {}
+
+fn takes_box(t: Box<dyn Trait>) {}
+
+fn main() {
+    Box::new(S) as Box<dyn Trait>; //~ ERROR E0038
+    let t_box: Box<dyn Trait> = Box::new(S); //~ ERROR E0038
+    takes_box(Box::new(S)); //~ ERROR E0038
+}
diff --git a/src/test/ui/wf/wf-convert-unsafe-trait-obj-box.stderr b/src/test/ui/wf/wf-convert-unsafe-trait-obj-box.stderr
new file mode 100644
index 00000000000..0b63aef2bce
--- /dev/null
+++ b/src/test/ui/wf/wf-convert-unsafe-trait-obj-box.stderr
@@ -0,0 +1,33 @@
+error[E0038]: the trait `Trait` cannot be made into an object
+  --> $DIR/wf-convert-unsafe-trait-obj-box.rs:16:33
+   |
+LL |     let t_box: Box<dyn Trait> = Box::new(S);
+   |                                 ^^^^^^^^^^^ the trait `Trait` 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<std::boxed::Box<dyn Trait>>` for `std::boxed::Box<S>`
+   = note: required by cast to type `std::boxed::Box<dyn Trait>`
+
+error[E0038]: the trait `Trait` cannot be made into an object
+  --> $DIR/wf-convert-unsafe-trait-obj-box.rs:17:15
+   |
+LL |     takes_box(Box::new(S));
+   |               ^^^^^^^^^^^ the trait `Trait` 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<std::boxed::Box<dyn Trait>>` for `std::boxed::Box<S>`
+   = note: required by cast to type `std::boxed::Box<(dyn Trait + 'static)>`
+
+error[E0038]: the trait `Trait` cannot be made into an object
+  --> $DIR/wf-convert-unsafe-trait-obj-box.rs:15:5
+   |
+LL |     Box::new(S) as Box<dyn Trait>;
+   |     ^^^^^^^^^^^ the trait `Trait` 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<std::boxed::Box<dyn Trait>>` for `std::boxed::Box<S>`
+   = note: required by cast to type `std::boxed::Box<dyn Trait>`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0038`.
diff --git a/src/test/ui/wf/wf-convert-unsafe-trait-obj.rs b/src/test/ui/wf/wf-convert-unsafe-trait-obj.rs
new file mode 100644
index 00000000000..143b854ed6b
--- /dev/null
+++ b/src/test/ui/wf/wf-convert-unsafe-trait-obj.rs
@@ -0,0 +1,18 @@
+// Check that we do not allow casts or coercions
+// to object unsafe trait objects by ref
+
+#![feature(object_safe_for_dispatch)]
+
+trait Trait: Sized {}
+
+struct S;
+
+impl Trait for S {}
+
+fn takes_trait(t: &dyn Trait) {}
+
+fn main() {
+    &S as &dyn Trait; //~ ERROR E0038
+    let t: &dyn Trait = &S; //~ ERROR E0038
+    takes_trait(&S); //~ ERROR E0038
+}
diff --git a/src/test/ui/wf/wf-convert-unsafe-trait-obj.stderr b/src/test/ui/wf/wf-convert-unsafe-trait-obj.stderr
new file mode 100644
index 00000000000..7aeefd731fb
--- /dev/null
+++ b/src/test/ui/wf/wf-convert-unsafe-trait-obj.stderr
@@ -0,0 +1,33 @@
+error[E0038]: the trait `Trait` cannot be made into an object
+  --> $DIR/wf-convert-unsafe-trait-obj.rs:16:25
+   |
+LL |     let t: &dyn Trait = &S;
+   |                         ^^ the trait `Trait` 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 Trait>` for `&S`
+   = note: required by cast to type `&dyn Trait`
+
+error[E0038]: the trait `Trait` cannot be made into an object
+  --> $DIR/wf-convert-unsafe-trait-obj.rs:17:17
+   |
+LL |     takes_trait(&S);
+   |                 ^^ the trait `Trait` 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 Trait>` for `&S`
+   = note: required by cast to type `&dyn Trait`
+
+error[E0038]: the trait `Trait` cannot be made into an object
+  --> $DIR/wf-convert-unsafe-trait-obj.rs:15:5
+   |
+LL |     &S as &dyn Trait;
+   |     ^^ the trait `Trait` 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 Trait>` for `&S`
+   = note: required by cast to type `&dyn Trait`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0038`.
diff --git a/src/test/ui/wf/wf-unsafe-trait-obj-match.rs b/src/test/ui/wf/wf-unsafe-trait-obj-match.rs
new file mode 100644
index 00000000000..c8731a8ecaf
--- /dev/null
+++ b/src/test/ui/wf/wf-unsafe-trait-obj-match.rs
@@ -0,0 +1,29 @@
+// Check that we do not allow coercions to object
+// unsafe trait objects in match arms
+
+#![feature(object_safe_for_dispatch)]
+
+trait Trait: Sized {}
+
+struct S;
+
+impl Trait for S {}
+
+struct R;
+
+impl Trait for R {}
+
+fn opt() -> Option<()> {
+    Some(())
+}
+
+fn main() {
+    match opt() {
+        Some(()) => &S,
+        None => &R,  //~ ERROR E0308
+    }
+    let t: &dyn Trait = match opt() { //~ ERROR E0038
+        Some(()) => &S, //~ ERROR E0038
+        None => &R,
+    };
+}
diff --git a/src/test/ui/wf/wf-unsafe-trait-obj-match.stderr b/src/test/ui/wf/wf-unsafe-trait-obj-match.stderr
new file mode 100644
index 00000000000..185b1e6c36b
--- /dev/null
+++ b/src/test/ui/wf/wf-unsafe-trait-obj-match.stderr
@@ -0,0 +1,38 @@
+error[E0308]: match arms have incompatible types
+  --> $DIR/wf-unsafe-trait-obj-match.rs:23:17
+   |
+LL | /     match opt() {
+LL | |         Some(()) => &S,
+   | |                     -- this is found to be of type `&S`
+LL | |         None => &R,
+   | |                 ^^ expected struct `S`, found struct `R`
+LL | |     }
+   | |_____- `match` arms have incompatible types
+   |
+   = note: expected type `&S`
+              found type `&R`
+
+error[E0038]: the trait `Trait` cannot be made into an object
+  --> $DIR/wf-unsafe-trait-obj-match.rs:26:21
+   |
+LL |         Some(()) => &S,
+   |                     ^^ the trait `Trait` 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 Trait>` for `&S`
+   = note: required by cast to type `&dyn Trait`
+
+error[E0038]: the trait `Trait` cannot be made into an object
+  --> $DIR/wf-unsafe-trait-obj-match.rs:25:25
+   |
+LL |     let t: &dyn Trait = match opt() {
+   |                         ^^^^^^^^^^^ the trait `Trait` 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 Trait>` for `&R`
+   = note: required by cast to type `&dyn Trait`
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0038, E0308.
+For more information about an error, try `rustc --explain E0038`.