`<Default>::default()` makes the compiler very mad at me

[scottmcm]

Suppose I learned

• That [f32; 2]::deserialize(deserializer) doesn't work, and I need to do <[f32; 2]>::deserialize(deserializer) instead (thank you syntax: recovery for incorrect associated item paths like [T; N]::clone #46788), and
• That I can do Default::default(), instead of ConcreteType::default()

If I combine those two things together and try

let x: u32 = <Default>::default();

Then I get this spew of errors:

error[E0277]: the trait bound `std::default::Default: std::default::Default` is not satisfied
 --> src/main.rs:2:18
  |
2 |     let x: u32 = <Default>::default();
  |                  ^^^^^^^^^^^^^^^^^^ the trait `std::default::Default` is not implemented for `std::default::Default`
  |
  = note: required by `std::default::Default::default`

error[E0038]: the trait `std::default::Default` cannot be made into an object
 --> src/main.rs:2:19
  |
2 |     let x: u32 = <Default>::default();
  |                   ^^^^^^^ the trait `std::default::Default` cannot be made into an object
  |
  = note: the trait cannot require that `Self : Sized`

error[E0308]: mismatched types
 --> src/main.rs:2:18
  |
2 |     let x: u32 = <Default>::default();
  |                  ^^^^^^^^^^^^^^^^^^^^ expected u32, found trait std::default::Default
  |
  = note: expected type `u32`
             found type `std::default::Default`

error[E0038]: the trait `std::default::Default` cannot be made into an object
 --> src/main.rs:2:18
  |
2 |     let x: u32 = <Default>::default();
  |                  ^^^^^^^^^^^^^^^^^^^^ the trait `std::default::Default` cannot be made into an object
  |
  = note: the trait cannot require that `Self : Sized`

error: aborting due to 4 previous errors

Some errors occurred: E0038, E0277, E0308.
For more information about an error, try `rustc --explain E0038`.

I don't know what I'm concretely asking for here (please retitle better), so here are some thoughts:

• The second error is the valuable one here
• "found trait std::default::Default" should perhaps be "found trait object std::default::Default"?
• Maybe this all will just work in Rust 2021 when <Default> can mean Default, not dyn Default>...

[eddyb]

This can't work, as <T>::name syntax requires T to be a type, so at best in the future it will complain about Default not having dyn before it. Also, we should print the type as dyn Default.

That is, Foo::bar means two different things depending on whether Foo is a type or a trait:

• if Foo is a type, it means <Foo>::bar which may further mean <Foo as Trait>::bar
• if Foo is a trait, it means <_ as Foo>::Bar, which is different from </*dyn*/ Foo>::bar

[Centril]

@eddyb I think what Scott is saying is that you could also have a new rule in 2021 that given <Foo>::name, if Foo is a trait, it means <_ as Foo>::name.

That said, the first error message above is pretty darn confusing and we should do something about it now.

[eddyb]

@Centril I disagree on the grounds of <T>::name being "type-qualified path syntax" and putting a trait path in the slot for T muddling those waters, but I can see how it would work grammar-wise.

[Centril]

@eddyb sure; whether that's desirable is entirely separate from the technical feasibility :) I'm not sure where I stand on desirability at this point.

[scottmcm]

This can't work, as ::name syntax requires T to be a type

Yes, I agree. I did figure out that was what was happening, but it's not incredibly obvious, so I wanted to file something about it. (For example, it's not obvious to me why it must be a type specifically, rather than something in the type namespace. My mental model had been that it just flipped the parser into type-namespace-mode, so no turbofish needed, etc, but apparently that's insufficient.)

[eddyb]

@scottmcm It doesn't do anything to paths, instead it fully contains some arbitrary type.
The primary intention was to use it types that are not paths, e.g. <*const _>::is_null.
That is, the extra <...> permit containing the entire type syntax directly with no other special-cases.

[steveklabnik]

Trying this today gives a slightly better, but still not great error:

   Compiling playground v0.0.1 (/playground)
warning: trait objects without an explicit `dyn` are deprecated
 --> src/main.rs:4:15
  |
4 | let x: u32 = <Default>::default();
  |               ^^^^^^^ help: use `dyn`: `dyn Default`
  |
  = note: #[warn(bare_trait_objects)] on by default

error[E0277]: the size for values of type `dyn std::default::Default` cannot be known at compilation time
 --> src/main.rs:4:14
  |
4 | let x: u32 = <Default>::default();
  |              ^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
  |
  = help: the trait `std::marker::Sized` is not implemented for `dyn std::default::Default`
  = 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 by `std::default::Default::default`

error[E0038]: the trait `std::default::Default` cannot be made into an object
 --> src/main.rs:4:15
  |
4 | let x: u32 = <Default>::default();
  |               ^^^^^^^ the trait `std::default::Default` cannot be made into an object
  |
  = note: the trait cannot require that `Self : Sized`

error[E0308]: mismatched types
 --> src/main.rs:4:14
  |
4 | let x: u32 = <Default>::default();
  |              ^^^^^^^^^^^^^^^^^^^^ expected u32, found trait std::default::Default
  |
  = note: expected type `u32`
             found type `dyn std::default::Default`

error[E0038]: the trait `std::default::Default` cannot be made into an object
 --> src/main.rs:4:14
  |
4 | let x: u32 = <Default>::default();
  |              ^^^^^^^^^^^^^^^^^^^^ the trait `std::default::Default` cannot be made into an object
  |
  = note: the trait cannot require that `Self : Sized`

error: aborting due to 4 previous errors

[eddyb]

At the very least, this part is still misleading: expected u32, found trait std::default::Default.
I think the only part that improved is:

  = note: expected type `u32`
             found type `dyn std::default::Default`