Too many dyn Trait is not Sized errors #105753
Description
When writing
trait T {} fn foo() -> dyn T { todo!() } fn main() { let x = foo(); }We emit three errors for dyn T: Sized not being fulfilled. We should likely only emit 1, in foo's return type, but we certainly shouldn't be emitting both one for x and one for foo's call.
error[E0746]: return type cannot have an unboxed trait object --> src/main.rs:3:13 | 3 | fn foo() -> dyn T { | ^^^^^ doesn't have a size known at compile-time | help: use `impl T` as the return type if all return paths have the same type but you want to expose only the trait in the signature | 3 | fn foo() -> impl T { | ~~~~~~ help: use a boxed trait object if all return paths implement trait `T` | 3 | fn foo() -> Box<dyn T> { | ++++ + error[E0277]: the size for values of type `dyn T` cannot be known at compilation time --> src/main.rs:8:9 | 8 | let x = foo(); | ^ doesn't have a size known at compile-time | = help: the trait `Sized` is not implemented for `dyn T` = note: all local variables must have a statically known size = help: unsized locals are gated as an unstable feature error[E0277]: the size for values of type `(dyn T + 'static)` cannot be known at compilation time --> src/main.rs:8:13 | 8 | let x = foo(); | ^^^ doesn't have a size known at compile-time | = help: the trait `Sized` is not implemented for `(dyn T + 'static)` = note: the return type of a function must have a statically known size