<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`

[estebank]

Current output:

error[E0038]: the trait `Default` cannot be made into an object
 --> src/main.rs:2:19
  |
2 |     let x: u32 = <dyn Default>::default();
  |                   ^^^^^^^^^^^ `Default` cannot be made into an object
  |
  = note: the trait cannot be made into an object because it requires `Self: Sized`
  = note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>

error[E0277]: the size for values of type `dyn Default` cannot be known at compilation time
   --> src/main.rs:2:19
    |
2   |     let x: u32 = <dyn Default>::default();
    |                   ^^^^^^^^^^^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `dyn Default`
note: required by a bound in `default`
   --> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/default.rs:106:20
    |
106 | pub trait Default: Sized {
    |                    ^^^^^ required by this bound in `Default::default`
...
138 |     fn default() -> Self;
    |        ------- required by a bound in this associated function

error[E0308]: mismatched types
 --> src/main.rs:2:18
  |
2 |     let x: u32 = <dyn Default>::default();
  |            ---   ^^^^^^^^^^^^^^^^^^^^^^^^ expected `u32`, found `dyn Default`
  |            |
  |            expected due to this
  |
  = note:      expected type `u32`
          found trait object `dyn Default`
  = help: `u32` implements `Default` so you could change the expected type to `Box<dyn Default>`

error[E0038]: the trait `Default` cannot be made into an object
 --> src/main.rs:2:18
  |
2 |     let x: u32 = <dyn Default>::default();
  |                  ^^^^^^^^^^^^^^^^^^^^^^^^ `Default` cannot be made into an object
  |
  = note: the trait cannot be made into an object because it requires `Self: Sized`
  = note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>

error[E0277]: the size for values of type `dyn Default` cannot be known at compilation time
 --> src/main.rs:2:18
  |
2 |     let x: u32 = <dyn Default>::default();
  |                  ^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
  |
  = help: the trait `Sized` is not implemented for `dyn Default`
  = note: the return type of a function must have a statically known size

The E0038 and E0277 errors both say the same thing (Self: !Sized) so one should silence the other. That would bring the error count from 5 to 3. If we could tie the dyn Default failure to the <dyn Default>::default() one, we could bring the error count down to 2. Finally, we could mark the expression type to be {type error} to avoid the E0308, leaving just 1 error left.

[scottmcm]

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

Note that I wasn't actually trying to use dyn here, so I don't know why the output shows that.

I think it's possible that the root issue might be things turning Default into dyn Default too early -- at least in new editions we don't need to do that, and I think it'd be better not to assume that's what was meant. Giving a "hey, we don't know if you meant impl Default or dyn Default" when seeing a trait in type position might avoid going down this garden path in the first place.

[estebank]

Note that I wasn't actually trying to use dyn here, so I don't know why the output shows that.

I'd added it to the playground to see the other errors without the "you must use dyn here". The "bad trait use" should also check whether we're in an item body or not, and propose using impl Trait besides dyn Trait. And we should "poison" the node's type with {type error} whenever we encounter a bare dyn Trait, in general.