Confusing diagnostics where the error mentions Sized but not that a trait bound is not implemented (where it would be for a Sized type).

[najamelan]

This comes from the user forum, playground, moved from #27964:

use std::error::Error;

fn library_function<F1, E1>(f1: F1) -> Result<(), E1>
where
    F1: FnOnce() -> Result<(), E1>,
    E1: Error, // <-remove this line -> compile
{
    f1()
}

fn main() -> Result<(), Box<dyn Error>> 
{
    library_function( || Ok(()) )
}

Error:

error[E0277]: the size for values of type `dyn std::error::Error` cannot be known at compilation time
  --> src/main.rs:13:5
   |
3  | fn library_function<F1, E1>(f1: F1) -> Result<(), E1>
   |    ----------------
...
6  |     E1: Error, // <-remove this line -> compile
   |         ----- required by this bound in `library_function`
...
13 |     library_function( || Ok(()) )
   |     ^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
   |
   = help: the trait `std::marker::Sized` is not implemented for `dyn std::error::Error`
   = 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 because of the requirements on the impl of `std::error::Error` for `std::boxed::Box<dyn std::error::Error>`

Go figure why "library_function doesn't have a size known at compile time".

Analysis:
There is a blanket impl for Error on Box<T>, so quickly looking it up in the docs makes you think that it should implement Error, but it's not on Box<T: ?Sized>. credit to @yandros

I feel the main issue here is that we have a trait bound on Error that isn't satisfied, so probably it would be nice to replace library_function not being sized by: Box<dyn Error> does not implement Error. The following Impls where found: impl<T: Error> Error for Box<T>, but that requires T: Sized.

[brain0]

The issue can also be reproduced with the following code snippet:

trait Tr1 {}

trait Tr2: Tr1 {}

impl<T: Tr2 + Sized> Tr1 for T {}

//impl Tr1 for [u8] {}

impl Tr2 for () {}

impl Tr2 for [u8] {}

Uncommenting the commented line makes the snippet compile. Without it, the error message is as follows:

error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
  --> src/lib.rs:11:6
   |
11 | impl Tr2 for [u8] {}
   |      ^^^ doesn't have a size known at compile-time
   |
   = help: the trait `std::marker::Sized` is not implemented for `[u8]`
   = 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 because of the requirements on the impl of `Tr1` for `[u8]`

[estebank]

Current output:

error[[E0277]](https://doc.rust-lang.org/nightly/error_codes/E0277.html): the size for values of type `dyn std::error::Error` cannot be known at compilation time
  --> src/main.rs:17:5
   |
17 |     library_function( || Ok(()) )
   |     ^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
   |
   = help: the trait `Sized` is not implemented for `dyn std::error::Error`
   = help: the trait `std::error::Error` is implemented for `Box<T>`
   = note: required for `Box<dyn std::error::Error>` to implement `std::error::Error`
note: required by a bound in `library_function`
  --> src/main.rs:8:9
   |
5  | fn library_function<F1, E1>(f1: F1) -> Result<(), E1>
   |    ---------------- required by a bound in this function
...
8  |     E1: Error,
   |         ^^^^^ required by this bound in `library_function`

error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
  --> src/lib.rs:11:14
   |
11 | impl Tr2 for [u8] {}
   |              ^^^^ doesn't have a size known at compile-time
   |
   = help: the trait `Sized` is not implemented for `[u8]`
note: required for `[u8]` to implement `Tr1`
  --> src/lib.rs:5:22
   |
5  | impl<T: Tr2 + Sized> Tr1 for T {}
   |      -               ^^^     ^
   |      |
   |      unsatisfied trait bound introduced here
note: required by a bound in `Tr2`
  --> src/lib.rs:3:12
   |
3  | trait Tr2: Tr1 {}
   |            ^^^ required by this bound in `Tr2`