Rust requires size for struct parameterized by PhantomData

[iwahbe]

Rust requires size for structs parameterized with PhantomData.

I'm honestly not sure if this is a bug or type system limitation.

I tried this code: playground

trait Trait{
    fn func() -> Struct<Self>;    
}

struct Struct<T>{
    _t: std::marker::PhantomData<*const T>,
}

I expected to see this happen: Compilation complete. std::marker::PhantomData<*const T> is zero-size. And *const T is known size.

Instead, this happened: The build failed.

Meta

rustc --version --verbose:

rustc 1.44.0-nightly (94d346360 2020-04-09)
binary: rustc
commit-hash: 94d346360da50f159e0dc777dc9bc3c5b6b51a00
commit-date: 2020-04-09
host: x86_64-unknown-linux-gnu
release: 1.44.0-nightly
LLVM version: 9.0

Backtrace

  Compiling playground v0.0.1 (/playground)
error[E0277]: the size for values of type `Self` cannot be known at compilation time
 --> src/main.rs:6:18
  |
6 |     fn func() -> Struct<Self>;
  |                  ^^^^^^^^^^^^- help: consider further restricting `Self`: `where Self: std::marker::Sized`
  |                  |
  |                  doesn't have a size known at compile-time
...
9 | struct Struct<T> {
  |               - required by this bound in `Struct`
  |
  = help: the trait `std::marker::Sized` is not implemented for `Self`
  = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground`.

To learn more, run the command again with --verbose.

[lcnr]

Unlike structs, traits do not implicitly add a Sized bound.
This means that your snippet can be thought of as:

trait Trait: ?Sized {
    fn func() -> Struct<Self>;    
}

struct Struct<T: Sized>{
    _t: std::marker::PhantomData<*const T>,
}

Which must not compile.

You can fix this by either adding an explicit ?Sized bound to Struct or a Sized bound to Trait.
i.e.

trait Trait {
    fn func() -> Struct<Self>;    
}

struct Struct<T: ?Sized>{
    _t: std::marker::PhantomData<*const T>,
}

[jonas-schievink]

Closing as expected behavior