Confusing error when using unsized types without indirection

[anmej]

fn foo(arg: Foo) {} //Foo is a trait
fn bar (arg: [i32]) {}
fn quz (arg: str) {}

All those functions return the following error: the trait core::marker::Sized is not implemented for the type...
This is confusing for a beginner, because it makes you think you need to implement a trait to make it work. The message could suggest using &Foo/&[i32]/&str instead.

[arielb1]

Should we add an on_unimplemented_help?

[steveklabnik]

Triage: no change. Here's the actual diagnostic today

rustc 1.15.1 (021bd294c 2017-02-08)
error[E0277]: the trait bound `Foo + 'static: std::marker::Sized` is not satisfied
 --> <anon>:3:8
  |
3 | fn foo(arg: Foo) {} //Foo is a trait
  |        ^^^ the trait `std::marker::Sized` is not implemented for `Foo + 'static`
  |
  = note: `Foo + 'static` does not have a constant size known at compile-time
  = note: all local variables must have a statically known size

error[E0277]: the trait bound `[i32]: std::marker::Sized` is not satisfied
 --> <anon>:4:9
  |
4 | fn bar (arg: [i32]) {}
  |         ^^^ the trait `std::marker::Sized` is not implemented for `[i32]`
  |
  = note: `[i32]` does not have a constant size known at compile-time
  = note: all local variables must have a statically known size

error[E0277]: the trait bound `str: std::marker::Sized` is not satisfied
 --> <anon>:5:9
  |
5 | fn quz (arg: str) {}
  |         ^^^ the trait `std::marker::Sized` is not implemented for `str`
  |
  = note: `str` does not have a constant size known at compile-time
  = note: all local variables must have a statically known size

[cyplo]

Same message on rustc 1.23.0-nightly (8b22e70 2017-10-31)

[estebank]

CC #23286

[estebank]

Current output:

error[E0277]: the size for values of type `(dyn Foo + 'static)` cannot be known at compilation time
 --> src/lib.rs:2:8
  |
2 | fn foo(arg: Foo) {} //Foo is a trait
  |        ^^^ doesn't have a size known at compile-time
  |
  = help: the trait `std::marker::Sized` is not implemented for `(dyn Foo + 'static)`
  = note: to learn more, visit <https://doc.rust-lang.org/book/second-edition/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
  = 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 `[i32]` cannot be known at compilation time
 --> src/lib.rs:3:9
  |
3 | fn bar (arg: [i32]) {}
  |         ^^^ doesn't have a size known at compile-time
  |
  = help: the trait `std::marker::Sized` is not implemented for `[i32]`
  = note: to learn more, visit <https://doc.rust-lang.org/book/second-edition/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
  = 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 `str` cannot be known at compilation time
 --> src/lib.rs:4:9
  |
4 | fn quz (arg: str) {}
  |         ^^^ doesn't have a size known at compile-time
  |
  = help: the trait `std::marker::Sized` is not implemented for `str`
  = note: to learn more, visit <https://doc.rust-lang.org/book/second-edition/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
  = note: all local variables must have a statically known size
  = help: unsized locals are gated as an unstable feature