Specific error for unsized dyn Trait return type
Suggest `impl Trait` when possible, and `Box<dyn Trait>` otherwise.
This commit is contained in:
Esteban Küber
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9fe05e9456
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6fd564112f
7 changed files with 512 additions and 2 deletions
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#![allow(bare_trait_objects)]
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struct Struct;
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trait Trait {}
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impl Trait for Struct {}
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impl Trait for u32 {}
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fn fuz() -> (usize, Trait) { (42, Struct) }
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//~^ ERROR E0277
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//~| ERROR E0308
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fn bar() -> (usize, dyn Trait) { (42, Struct) }
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//~^ ERROR E0277
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//~| ERROR E0308
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fn bap() -> Trait { Struct }
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//~^ ERROR E0746
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//~| ERROR E0308
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fn ban() -> dyn Trait { Struct }
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//~^ ERROR E0746
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//~| ERROR E0308
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fn bak() -> dyn Trait { unimplemented!() } //~ ERROR E0277
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// Suggest using `Box<dyn Trait>`
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fn bal() -> dyn Trait { //~ ERROR E0746
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if true {
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return Struct; //~ ERROR E0308
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}
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42 //~ ERROR E0308
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}
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// Suggest using `impl Trait`
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fn bat() -> dyn Trait { //~ ERROR E0746
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if true {
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return 0; //~ ERROR E0308
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}
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42 //~ ERROR E0308
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}
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fn main() {}
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error[E0308]: mismatched types
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--> $DIR/dyn-trait-return-should-be-impl-trait.rs:7:35
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LL | fn fuz() -> (usize, Trait) { (42, Struct) }
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| ^^^^^^ expected trait `Trait`, found struct `Struct`
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= note: expected trait object `(dyn Trait + 'static)`
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found struct `Struct`
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error[E0277]: the size for values of type `(dyn Trait + 'static)` cannot be known at compilation time
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--> $DIR/dyn-trait-return-should-be-impl-trait.rs:7:13
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LL | fn fuz() -> (usize, Trait) { (42, Struct) }
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| ^^^^^^^^^^^^^^ doesn't have a size known at compile-time
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= help: within `(usize, (dyn Trait + 'static))`, the trait `std::marker::Sized` is not implemented for `(dyn Trait + 'static)`
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= note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
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= note: required because it appears within the type `(usize, (dyn Trait + 'static))`
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= note: the return type of a function must have a statically known size
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error[E0308]: mismatched types
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--> $DIR/dyn-trait-return-should-be-impl-trait.rs:10:39
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LL | fn bar() -> (usize, dyn Trait) { (42, Struct) }
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| ^^^^^^ expected trait `Trait`, found struct `Struct`
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= note: expected trait object `(dyn Trait + 'static)`
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found struct `Struct`
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error[E0277]: the size for values of type `(dyn Trait + 'static)` cannot be known at compilation time
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--> $DIR/dyn-trait-return-should-be-impl-trait.rs:10:13
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LL | fn bar() -> (usize, dyn Trait) { (42, Struct) }
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| ^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
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= help: within `(usize, (dyn Trait + 'static))`, the trait `std::marker::Sized` is not implemented for `(dyn Trait + 'static)`
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= note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
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= note: required because it appears within the type `(usize, (dyn Trait + 'static))`
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= note: the return type of a function must have a statically known size
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error[E0308]: mismatched types
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--> $DIR/dyn-trait-return-should-be-impl-trait.rs:13:21
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LL | fn bap() -> Trait { Struct }
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| ----- ^^^^^^ expected trait `Trait`, found struct `Struct`
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| |
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| expected `(dyn Trait + 'static)` because of return type
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= note: expected trait object `(dyn Trait + 'static)`
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found struct `Struct`
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error[E0746]: return type cannot have a bare trait because it must be `Sized`
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--> $DIR/dyn-trait-return-should-be-impl-trait.rs:13:13
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LL | fn bap() -> Trait { Struct }
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| ^^^^^ doesn't have a size known at compile-time
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= note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
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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`
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LL | fn bap() -> impl Trait { Struct }
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| ^^^^^^^^^^
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error[E0308]: mismatched types
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--> $DIR/dyn-trait-return-should-be-impl-trait.rs:16:25
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LL | fn ban() -> dyn Trait { Struct }
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| --------- ^^^^^^ expected trait `Trait`, found struct `Struct`
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| |
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| expected `(dyn Trait + 'static)` because of return type
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= note: expected trait object `(dyn Trait + 'static)`
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found struct `Struct`
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error[E0746]: return type cannot have a bare trait because it must be `Sized`
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--> $DIR/dyn-trait-return-should-be-impl-trait.rs:16:13
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LL | fn ban() -> dyn Trait { Struct }
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| ^^^^^^^^^ doesn't have a size known at compile-time
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= note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
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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`
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LL | fn ban() -> impl Trait { Struct }
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| ^^^^^^^^^^
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error[E0277]: the size for values of type `(dyn Trait + 'static)` cannot be known at compilation time
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--> $DIR/dyn-trait-return-should-be-impl-trait.rs:19:13
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LL | fn bak() -> dyn Trait { unimplemented!() }
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| ^^^^^^^^^ doesn't have a size known at compile-time
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= help: the trait `std::marker::Sized` is not implemented for `(dyn Trait + 'static)`
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= note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
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= note: the return type of a function must have a statically known size
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error[E0308]: mismatched types
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--> $DIR/dyn-trait-return-should-be-impl-trait.rs:23:16
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LL | fn bal() -> dyn Trait {
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| --------- expected `(dyn Trait + 'static)` because of return type
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LL | if true {
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LL | return Struct;
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| ^^^^^^ expected trait `Trait`, found struct `Struct`
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= note: expected trait object `(dyn Trait + 'static)`
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found struct `Struct`
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error[E0746]: return type cannot have a bare trait because it must be `Sized`
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--> $DIR/dyn-trait-return-should-be-impl-trait.rs:21:13
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LL | fn bal() -> dyn Trait {
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| ^^^^^^^^^ doesn't have a size known at compile-time
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help: if all the returned values were of the same type you could use `impl Trait` as the return type
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--> $DIR/dyn-trait-return-should-be-impl-trait.rs:25:5
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LL | return Struct;
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| ^^^^^^
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LL | }
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LL | 42
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| ^^
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= help: alternatively, you can always create a new `enum` with a variant for each returned type
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= note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
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= 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>
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help: if the performance implications are acceptable, you can return a trait object
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LL | fn bal() -> Box<dyn Trait> {
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LL | if true {
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LL | return Box::new(Struct);
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LL | }
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LL | Box::new(42)
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error[E0308]: mismatched types
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--> $DIR/dyn-trait-return-should-be-impl-trait.rs:25:5
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LL | fn bal() -> dyn Trait {
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| --------- expected `(dyn Trait + 'static)` because of return type
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...
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LL | 42
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| ^^ expected trait `Trait`, found integer
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= note: expected trait object `(dyn Trait + 'static)`
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found type `{integer}`
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error[E0746]: return type cannot have a bare trait because it must be `Sized`
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--> $DIR/dyn-trait-return-should-be-impl-trait.rs:29:13
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LL | fn bat() -> dyn Trait {
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| ^^^^^^^^^ doesn't have a size known at compile-time
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= note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
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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`
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LL | fn bat() -> impl Trait {
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| ^^^^^^^^^^
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error[E0308]: mismatched types
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--> $DIR/dyn-trait-return-should-be-impl-trait.rs:31:16
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LL | fn bat() -> dyn Trait {
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| --------- expected `(dyn Trait + 'static)` because of return type
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LL | if true {
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LL | return 0;
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| ^ expected trait `Trait`, found integer
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= note: expected trait object `(dyn Trait + 'static)`
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found type `{integer}`
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error[E0308]: mismatched types
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--> $DIR/dyn-trait-return-should-be-impl-trait.rs:33:5
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LL | fn bat() -> dyn Trait {
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| --------- expected `(dyn Trait + 'static)` because of return type
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...
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LL | 42
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| ^^ expected trait `Trait`, found integer
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= note: expected trait object `(dyn Trait + 'static)`
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found type `{integer}`
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error: aborting due to 15 previous errors
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Some errors have detailed explanations: E0277, E0308.
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For more information about an error, try `rustc --explain E0277`.
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