Specific error for unsized `dyn Trait` return type

Suggest `impl Trait` when possible, and `Box<dyn Trait>` otherwise.

4 files changed, 301 insertions, 0 deletions

diff --git a/src/test/ui/error-codes/E0746.rs b/src/test/ui/error-codes/E0746.rs
new file mode 100644
index 00000000000..ad257b01e1b
--- /dev/null
+++ b/src/test/ui/error-codes/E0746.rs
@@ -0,0 +1,17 @@
+struct Struct;
+trait Trait {}
+impl Trait for Struct {}
+impl Trait for u32 {}
+
+fn foo() -> dyn Trait { Struct }
+//~^ ERROR E0746
+//~| ERROR E0308
+
+fn bar() -> dyn Trait { //~ ERROR E0746
+    if true {
+        return 0; //~ ERROR E0308
+    }
+    42 //~ ERROR E0308
+}
+
+fn main() {}
diff --git a/src/test/ui/error-codes/E0746.stderr b/src/test/ui/error-codes/E0746.stderr
new file mode 100644
index 00000000000..baafcd27c29
--- /dev/null
+++ b/src/test/ui/error-codes/E0746.stderr
@@ -0,0 +1,62 @@
+error[E0308]: mismatched types
+  --> $DIR/E0746.rs:6:25
+   |
+LL | fn foo() -> dyn Trait { Struct }
+   |             ---------   ^^^^^^ expected trait `Trait`, found struct `Struct`
+   |             |
+   |             expected `(dyn Trait + 'static)` because of return type
+   |
+   = note: expected trait object `(dyn Trait + 'static)`
+                    found struct `Struct`
+
+error[E0746]: return type cannot have a bare trait because it must be `Sized`
+  --> $DIR/E0746.rs:6:13
+   |
+LL | fn foo() -> dyn Trait { Struct }
+   |             ^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = 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 use the `impl Trait` feature in the return type because all the return paths are of type `Struct`, which implements `dyn Trait`
+   |
+LL | fn foo() -> impl Trait { Struct }
+   |             ^^^^^^^^^^
+
+error[E0746]: return type cannot have a bare trait because it must be `Sized`
+  --> $DIR/E0746.rs:10:13
+   |
+LL | fn bar() -> dyn Trait {
+   |             ^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = 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 use the `impl Trait` feature in the return type because all the return paths are of type `{integer}`, which implements `dyn Trait`
+   |
+LL | fn bar() -> impl Trait {
+   |             ^^^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/E0746.rs:12:16
+   |
+LL | fn bar() -> dyn Trait {
+   |             --------- expected `(dyn Trait + 'static)` because of return type
+LL |     if true {
+LL |         return 0;
+   |                ^ expected trait `Trait`, found integer
+   |
+   = note: expected trait object `(dyn Trait + 'static)`
+                      found type `{integer}`
+
+error[E0308]: mismatched types
+  --> $DIR/E0746.rs:14:5
+   |
+LL | fn bar() -> dyn Trait {
+   |             --------- expected `(dyn Trait + 'static)` because of return type
+...
+LL |     42
+   |     ^^ expected trait `Trait`, found integer
+   |
+   = note: expected trait object `(dyn Trait + 'static)`
+                      found type `{integer}`
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/impl-trait/dyn-trait-return-should-be-impl-trait.rs b/src/test/ui/impl-trait/dyn-trait-return-should-be-impl-trait.rs
new file mode 100644
index 00000000000..80168ca8257
--- /dev/null
+++ b/src/test/ui/impl-trait/dyn-trait-return-should-be-impl-trait.rs
@@ -0,0 +1,36 @@
+#![allow(bare_trait_objects)]
+struct Struct;
+trait Trait {}
+impl Trait for Struct {}
+impl Trait for u32 {}
+
+fn fuz() -> (usize, Trait) { (42, Struct) }
+//~^ ERROR E0277
+//~| ERROR E0308
+fn bar() -> (usize, dyn Trait) { (42, Struct) }
+//~^ ERROR E0277
+//~| ERROR E0308
+fn bap() -> Trait { Struct }
+//~^ ERROR E0746
+//~| ERROR E0308
+fn ban() -> dyn Trait { Struct }
+//~^ ERROR E0746
+//~| ERROR E0308
+fn bak() -> dyn Trait { unimplemented!() } //~ ERROR E0277
+// Suggest using `Box<dyn Trait>`
+fn bal() -> dyn Trait { //~ ERROR E0746
+    if true {
+        return Struct; //~ ERROR E0308
+    }
+    42 //~ ERROR E0308
+}
+
+// Suggest using `impl Trait`
+fn bat() -> dyn Trait { //~ ERROR E0746
+    if true {
+        return 0; //~ ERROR E0308
+    }
+    42 //~ ERROR E0308
+}
+
+fn main() {}
diff --git a/src/test/ui/impl-trait/dyn-trait-return-should-be-impl-trait.stderr b/src/test/ui/impl-trait/dyn-trait-return-should-be-impl-trait.stderr
new file mode 100644
index 00000000000..ce4c141a0af
--- /dev/null
+++ b/src/test/ui/impl-trait/dyn-trait-return-should-be-impl-trait.stderr
@@ -0,0 +1,186 @@
+error[E0308]: mismatched types
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:7:35
+   |
+LL | fn fuz() -> (usize, Trait) { (42, Struct) }
+   |                                   ^^^^^^ expected trait `Trait`, found struct `Struct`
+   |
+   = note: expected trait object `(dyn Trait + 'static)`
+                    found struct `Struct`
+
+error[E0277]: the size for values of type `(dyn Trait + 'static)` cannot be known at compilation time
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:7:13
+   |
+LL | fn fuz() -> (usize, Trait) { (42, Struct) }
+   |             ^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: within `(usize, (dyn Trait + 'static))`, the trait `std::marker::Sized` is not implemented for `(dyn Trait + 'static)`
+   = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
+   = note: required because it appears within the type `(usize, (dyn Trait + 'static))`
+   = note: the return type of a function must have a statically known size
+
+error[E0308]: mismatched types
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:10:39
+   |
+LL | fn bar() -> (usize, dyn Trait) { (42, Struct) }
+   |                                       ^^^^^^ expected trait `Trait`, found struct `Struct`
+   |
+   = note: expected trait object `(dyn Trait + 'static)`
+                    found struct `Struct`
+
+error[E0277]: the size for values of type `(dyn Trait + 'static)` cannot be known at compilation time
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:10:13
+   |
+LL | fn bar() -> (usize, dyn Trait) { (42, Struct) }
+   |             ^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: within `(usize, (dyn Trait + 'static))`, the trait `std::marker::Sized` is not implemented for `(dyn Trait + 'static)`
+   = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
+   = note: required because it appears within the type `(usize, (dyn Trait + 'static))`
+   = note: the return type of a function must have a statically known size
+
+error[E0308]: mismatched types
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:13:21
+   |
+LL | fn bap() -> Trait { Struct }
+   |             -----   ^^^^^^ expected trait `Trait`, found struct `Struct`
+   |             |
+   |             expected `(dyn Trait + 'static)` because of return type
+   |
+   = note: expected trait object `(dyn Trait + 'static)`
+                    found struct `Struct`
+
+error[E0746]: return type cannot have a bare trait because it must be `Sized`
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:13:13
+   |
+LL | fn bap() -> Trait { Struct }
+   |             ^^^^^ doesn't have a size known at compile-time
+   |
+   = 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 use the `impl Trait` feature in the return type because all the return paths are of type `Struct`, which implements `dyn Trait`
+   |
+LL | fn bap() -> impl Trait { Struct }
+   |             ^^^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:16:25
+   |
+LL | fn ban() -> dyn Trait { Struct }
+   |             ---------   ^^^^^^ expected trait `Trait`, found struct `Struct`
+   |             |
+   |             expected `(dyn Trait + 'static)` because of return type
+   |
+   = note: expected trait object `(dyn Trait + 'static)`
+                    found struct `Struct`
+
+error[E0746]: return type cannot have a bare trait because it must be `Sized`
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:16:13
+   |
+LL | fn ban() -> dyn Trait { Struct }
+   |             ^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = 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 use the `impl Trait` feature in the return type because all the return paths are of type `Struct`, which implements `dyn Trait`
+   |
+LL | fn ban() -> impl Trait { Struct }
+   |             ^^^^^^^^^^
+
+error[E0277]: the size for values of type `(dyn Trait + 'static)` cannot be known at compilation time
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:19:13
+   |
+LL | fn bak() -> dyn Trait { unimplemented!() }
+   |             ^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `std::marker::Sized` is not implemented for `(dyn Trait + 'static)`
+   = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
+   = note: the return type of a function must have a statically known size
+
+error[E0308]: mismatched types
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:23:16
+   |
+LL | fn bal() -> dyn Trait {
+   |             --------- expected `(dyn Trait + 'static)` because of return type
+LL |     if true {
+LL |         return Struct;
+   |                ^^^^^^ expected trait `Trait`, found struct `Struct`
+   |
+   = note: expected trait object `(dyn Trait + 'static)`
+                    found struct `Struct`
+
+error[E0746]: return type cannot have a bare trait because it must be `Sized`
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:21:13
+   |
+LL | fn bal() -> dyn Trait {
+   |             ^^^^^^^^^ doesn't have a size known at compile-time
+   |
+help: if all the returned values were of the same type you could use `impl Trait` as the return type
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:25:5
+   |
+LL |         return Struct;
+   |                ^^^^^^
+LL |     }
+LL |     42
+   |     ^^
+   = help: alternatively, you can always create a new `enum` with a variant for each returned type
+   = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
+   = 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: if the performance implications are acceptable, you can return a trait object
+   |
+LL | fn bal() -> Box<dyn Trait> {
+LL |     if true {
+LL |         return Box::new(Struct);
+LL |     }
+LL |     Box::new(42)
+   |
+
+error[E0308]: mismatched types
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:25:5
+   |
+LL | fn bal() -> dyn Trait {
+   |             --------- expected `(dyn Trait + 'static)` because of return type
+...
+LL |     42
+   |     ^^ expected trait `Trait`, found integer
+   |
+   = note: expected trait object `(dyn Trait + 'static)`
+                      found type `{integer}`
+
+error[E0746]: return type cannot have a bare trait because it must be `Sized`
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:29:13
+   |
+LL | fn bat() -> dyn Trait {
+   |             ^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = 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 use the `impl Trait` feature in the return type because all the return paths are of type `{integer}`, which implements `dyn Trait`
+   |
+LL | fn bat() -> impl Trait {
+   |             ^^^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:31:16
+   |
+LL | fn bat() -> dyn Trait {
+   |             --------- expected `(dyn Trait + 'static)` because of return type
+LL |     if true {
+LL |         return 0;
+   |                ^ expected trait `Trait`, found integer
+   |
+   = note: expected trait object `(dyn Trait + 'static)`
+                      found type `{integer}`
+
+error[E0308]: mismatched types
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:33:5
+   |
+LL | fn bat() -> dyn Trait {
+   |             --------- expected `(dyn Trait + 'static)` because of return type
+...
+LL |     42
+   |     ^^ expected trait `Trait`, found integer
+   |
+   = note: expected trait object `(dyn Trait + 'static)`
+                      found type `{integer}`
+
+error: aborting due to 15 previous errors
+
+Some errors have detailed explanations: E0277, E0308.
+For more information about an error, try `rustc --explain E0277`.