More tests

tests/ui/object-safety/erroneous_signature.rs

@@ -0,0 +1,18 @@
+trait Foo {
+    fn err(&self) -> MissingType;
+    //~^ ERROR cannot find type `MissingType` in this scope
+}
+
+impl Foo for i32 {
+    fn err(&self) -> MissingType {
+        //~^ ERROR cannot find type `MissingType` in this scope
+        0
+    }
+}
+
+fn coerce(x: &i32) -> &dyn Foo {
+    //~^ ERROR `Foo` cannot be made into an object
+    x
+}
+
+fn main() {}

tests/ui/object-safety/erroneous_signature.stderr

@@ -0,0 +1,32 @@
+error[E0412]: cannot find type `MissingType` in this scope
+  --> $DIR/erroneous_signature.rs:2:22
+   |
+LL |     fn err(&self) -> MissingType;
+   |                      ^^^^^^^^^^^ not found in this scope
+
+error[E0412]: cannot find type `MissingType` in this scope
+  --> $DIR/erroneous_signature.rs:7:22
+   |
+LL |     fn err(&self) -> MissingType {
+   |                      ^^^^^^^^^^^ not found in this scope
+
+error[E0038]: the trait `Foo` cannot be made into an object
+  --> $DIR/erroneous_signature.rs:13:24
+   |
+LL | fn coerce(x: &i32) -> &dyn Foo {
+   |                        ^^^^^^^ `Foo` cannot be made into an object
+   |
+note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
+  --> $DIR/erroneous_signature.rs:2:8
+   |
+LL | trait Foo {
+   |       --- this trait cannot be made into an object...
+LL |     fn err(&self) -> MissingType;
+   |        ^^^ ...because method `err` has errors in its signature
+   = help: consider moving `err` to another trait
+   = help: only type `i32` implements the trait, consider using it directly instead
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0038, E0412.
+For more information about an error, try `rustc --explain E0038`.