Account for object safety when suggesting Box<dyn Trait>

src/test/ui/impl-trait/dyn-trait-return-should-be-impl-trait.stderr

@@ -86,7 +86,7 @@ LL | fn bal() -> dyn Trait {
    = note: if all the returned values were of the same type you could use `impl Trait` as the return type
    = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
    = note: you can create a new `enum` with a variant for each returned type
-help: return a trait object instead
+help: return a boxed trait object instead
    |
 LL | fn bal() -> Box<dyn Trait> {
 LL |     if true {

src/test/ui/impl-trait/equality.stderr

@@ -12,7 +12,7 @@ LL |     0_u32
    |
    = note: to return `impl Trait`, all returned values must be of the same type
    = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
-   = help: you can instead return a trait object using `Box<dyn Foo>`
+   = help: if the trait `Foo` were object safe, you could return a boxed trait object
    = note: for information on trait objects, see <https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types>
    = help: alternatively, create a new `enum` with a variant for each returned type
 

src/test/ui/impl-trait/object-unsafe-trait-in-return-position-dyn-trait.rs

@@ -0,0 +1,35 @@
+#![allow(bare_trait_objects)]
+trait NotObjectSafe {
+    fn foo() -> Self;
+}
+
+struct A;
+struct B;
+
+impl NotObjectSafe for A {
+    fn foo() -> Self {
+        A
+    }
+}
+
+impl NotObjectSafe for B {
+    fn foo() -> Self {
+        B
+    }
+}
+
+fn car() -> dyn NotObjectSafe { //~ ERROR the trait `NotObjectSafe` cannot be made into an object
+    if true {
+        return A;
+    }
+    B
+}
+
+fn cat() -> Box<dyn NotObjectSafe> { //~ ERROR the trait `NotObjectSafe` cannot be made into an
+    if true {
+        return Box::new(A);
+    }
+    Box::new(B)
+}
+
+fn main() {}

src/test/ui/impl-trait/object-unsafe-trait-in-return-position-dyn-trait.stderr

@@ -0,0 +1,21 @@
+error[E0038]: the trait `NotObjectSafe` cannot be made into an object
+  --> $DIR/object-unsafe-trait-in-return-position-dyn-trait.rs:21:1
+   |
+LL |     fn foo() -> Self;
+   |        --- associated function `foo` has no `self` parameter
+...
+LL | fn car() -> dyn NotObjectSafe {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `NotObjectSafe` cannot be made into an object
+
+error[E0038]: the trait `NotObjectSafe` cannot be made into an object
+  --> $DIR/object-unsafe-trait-in-return-position-dyn-trait.rs:28:1
+   |
+LL |     fn foo() -> Self;
+   |        --- associated function `foo` has no `self` parameter
+...
+LL | fn cat() -> Box<dyn NotObjectSafe> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `NotObjectSafe` cannot be made into an object
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0038`.

src/test/ui/impl-trait/object-unsafe-trait-in-return-position-impl-trait.rs

@@ -0,0 +1,46 @@
+trait NotObjectSafe {
+    fn foo() -> Self;
+}
+
+trait ObjectSafe {
+    fn bar(&self);
+}
+
+struct A;
+struct B;
+
+impl NotObjectSafe for A {
+    fn foo() -> Self {
+        A
+    }
+}
+
+impl NotObjectSafe for B {
+    fn foo() -> Self {
+        B
+    }
+}
+
+impl ObjectSafe for A {
+    fn bar(&self) {}
+}
+
+impl ObjectSafe for B {
+    fn bar(&self) {}
+}
+
+fn can() -> impl NotObjectSafe {
+    if true {
+        return A;
+    }
+    B //~ ERROR mismatched types
+}
+
+fn cat() -> impl ObjectSafe {
+    if true {
+        return A;
+    }
+    B //~ ERROR mismatched types
+}
+
+fn main() {}

src/test/ui/impl-trait/object-unsafe-trait-in-return-position-impl-trait.stderr

@@ -0,0 +1,39 @@
+error[E0308]: mismatched types
+  --> $DIR/object-unsafe-trait-in-return-position-impl-trait.rs:36:5
+   |
+LL | fn can() -> impl NotObjectSafe {
+   |             ------------------ expected because this return type...
+LL |     if true {
+LL |         return A;
+   |                - ...is found to be `A` here
+LL |     }
+LL |     B
+   |     ^ expected struct `A`, found struct `B`
+   |
+   = note: to return `impl Trait`, all returned values must be of the same type
+   = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
+   = help: if the trait `NotObjectSafe` were object safe, you could return a boxed trait object
+   = note: for information on trait objects, see <https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types>
+   = help: alternatively, create a new `enum` with a variant for each returned type
+
+error[E0308]: mismatched types
+  --> $DIR/object-unsafe-trait-in-return-position-impl-trait.rs:43:5
+   |
+LL | fn cat() -> impl ObjectSafe {
+   |             --------------- expected because this return type...
+LL |     if true {
+LL |         return A;
+   |                - ...is found to be `A` here
+LL |     }
+LL |     B
+   |     ^ expected struct `A`, found struct `B`
+   |
+   = note: to return `impl Trait`, all returned values must be of the same type
+   = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
+   = help: you can instead return a boxed trait object using `Box<dyn ObjectSafe>`
+   = note: for information on trait objects, see <https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types>
+   = help: alternatively, create a new `enum` with a variant for each returned type
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.

src/test/ui/point-to-type-err-cause-on-impl-trait-return.stderr

@@ -12,7 +12,7 @@ LL |     1u32
    |
    = note: to return `impl Trait`, all returned values must be of the same type
    = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
-   = help: you can instead return a trait object using `Box<dyn std::fmt::Display>`
+   = help: you can instead return a boxed trait object using `Box<dyn std::fmt::Display>`
    = note: for information on trait objects, see <https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types>
    = help: alternatively, create a new `enum` with a variant for each returned type
 
@@ -30,7 +30,7 @@ LL |         return 1u32;
    |
    = note: to return `impl Trait`, all returned values must be of the same type
    = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
-   = help: you can instead return a trait object using `Box<dyn std::fmt::Display>`
+   = help: you can instead return a boxed trait object using `Box<dyn std::fmt::Display>`
    = note: for information on trait objects, see <https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types>
    = help: alternatively, create a new `enum` with a variant for each returned type
 
@@ -48,7 +48,7 @@ LL |         1u32
    |
    = note: to return `impl Trait`, all returned values must be of the same type
    = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
-   = help: you can instead return a trait object using `Box<dyn std::fmt::Display>`
+   = help: you can instead return a boxed trait object using `Box<dyn std::fmt::Display>`
    = note: for information on trait objects, see <https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types>
    = help: alternatively, create a new `enum` with a variant for each returned type
 
@@ -78,7 +78,7 @@ LL |         _ => 1u32,
    |
    = note: to return `impl Trait`, all returned values must be of the same type
    = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
-   = help: you can instead return a trait object using `Box<dyn std::fmt::Display>`
+   = help: you can instead return a boxed trait object using `Box<dyn std::fmt::Display>`
    = note: for information on trait objects, see <https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types>
    = help: alternatively, create a new `enum` with a variant for each returned type
 
@@ -98,7 +98,7 @@ LL | |     }
    |
    = note: to return `impl Trait`, all returned values must be of the same type
    = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
-   = help: you can instead return a trait object using `Box<dyn std::fmt::Display>`
+   = help: you can instead return a boxed trait object using `Box<dyn std::fmt::Display>`
    = note: for information on trait objects, see <https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types>
    = help: alternatively, create a new `enum` with a variant for each returned type
 
@@ -116,7 +116,7 @@ LL |             1u32
    |
    = note: to return `impl Trait`, all returned values must be of the same type
    = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
-   = help: you can instead return a trait object using `Box<dyn std::fmt::Display>`
+   = help: you can instead return a boxed trait object using `Box<dyn std::fmt::Display>`
    = note: for information on trait objects, see <https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types>
    = help: alternatively, create a new `enum` with a variant for each returned type