Bogus extra error about unsized variable (the trait bound str: std::marker::Sized is not satisfied)

[bluss]

Given code like this (not complete):

extern crate serde_json;
let g1 = vec![0; 16];
let json1 = serde_json::to_pretty_string(&g1).unwrap();
let foo: Foo = serde_json::from_str(&json1).unwrap();

The first error is correct but the second is nonsense:

error[E0425]: cannot find function `to_pretty_string` in module `serde_json`
   --> serialization-tests/tests/serialization.rs:498:35
    |
498 |         let json1 = serde_json::to_pretty_string(&g1).unwrap();
    |                                 ^^^^^^^^^^^^^^^^ not found in `serde_json`

error[E0277]: the trait bound `str: std::marker::Sized` is not satisfied
   --> serialization-tests/tests/serialization.rs:498:13
    |
498 |         let json1 = serde_json::to_pretty_string(&g1).unwrap();
    |             ^^^^^ `str` does not have a constant size known at compile-time
    |
    = help: the trait `std::marker::Sized` is not implemented for `str`
    = note: all local variables must have a statically known size

There's an extra error here. json1 is used in a way (with the from_str function) that it should be a str or something that derefs to str. The second error message supposes it is a str, which is nonsense that the user doesn't intend at all. (If the user would only find the right name of the function, they would get a String in the json variable and the next line compiles fine.)

[jkarns275]

Ran into, what I believe to be the same issue, earlier today. My code boiled down to:

trait Hello {
    fn howdy(&self) -> i32;
}

struct Woah { pub field: i32 }

impl Hello for Woah {
    fn howdy(&self) -> i32 { self.field }
}

struct Container {
    pub boxed_trait: Box<Hello>
}

impl Container {
    pub fn new() -> Self {
        Container { boxed_trait: Box::new(Woah) }
    }
}

Which gave me the same bogus error when compiled.

error[E0423]: expected value, found struct `Woah`
  --> src/main.rs:21:43
   |
21 |         Container { boxed_trait: Box::new(Woah) }
   |                                           ^^^^ did you mean `Woah { /* fields */ }`?

error[E0277]: the trait bound `Hello: std::marker::Sized` is not satisfied
  --> src/main.rs:21:34
   |
21 |         Container { boxed_trait: Box::new(Woah) }
   |                                  ^^^^^^^^ `Hello` does not have a constant size known at compile-time
   |
   = help: the trait `std::marker::Sized` is not implemented for `Hello`
   = note: required by `<std::boxed::Box<T>>::new`