False/Irrelevant Error E0277

[isaacg1]

On type mismatchses when using trait objects, rust emits error E0277, saying that the trait does not have a constant size known at compile-time, and that all local variables must have a statically known type. In fact, the problem lies elsewhere, and the program can be fixed without addressing the E0277 error at all.

I wrote this code:

trait T {}

struct A;
impl T for A {}
impl A {
    fn new() -> Self {
        Self {}
    }
}

fn main() {
    let (a, b, c) = (A::new(), A::new()); // This tuple is 2 elements, should be three
    let ts: Vec<&T> = vec![&a, &b, &c];
    println!("{}", ts.len());
}

The marked tuple has 2 A::new() copies, when it should have 3. If this is fixed, the code compiles. I would expect to see a single error pointing out this fact. Instead, four errors are returned:

   Compiling bug v0.1.0 (file:///home/isaac/prog/rust/bug)
error[E0308]: mismatched types
  --> src/main.rs:12:9
   |
12 |     let (a, b, c) = (A::new(), A::new());
   |         ^^^^^^^^^ expected a tuple with 2 elements, found one with 3 elements
   |
   = note: expected type `(A, A)`
              found type `(_, _, _)`

error[E0277]: the trait bound `T: std::marker::Sized` is not satisfied
  --> src/main.rs:12:10
   |
12 |     let (a, b, c) = (A::new(), A::new());
   |          ^ `T` does not have a constant size known at compile-time
   |
   = help: the trait `std::marker::Sized` is not implemented for `T`
   = note: all local variables must have a statically known size

error[E0277]: the trait bound `T: std::marker::Sized` is not satisfied
  --> src/main.rs:12:13
   |
12 |     let (a, b, c) = (A::new(), A::new());
   |             ^ `T` does not have a constant size known at compile-time
   |
   = help: the trait `std::marker::Sized` is not implemented for `T`
   = note: all local variables must have a statically known size

error[E0277]: the trait bound `T: std::marker::Sized` is not satisfied
  --> src/main.rs:12:16
   |
12 |     let (a, b, c) = (A::new(), A::new());
   |                ^ `T` does not have a constant size known at compile-time
   |
   = help: the trait `std::marker::Sized` is not implemented for `T`
   = note: all local variables must have a statically known size

error: aborting due to 4 previous errors

error: Could not compile `bug`.

Only the first of these errors is related to the bug - the other three are all false and/or irrelevant.

Meta

rustc --version --verbose:

rustc 1.25.0 (84203ca 2018-03-25)
binary: rustc
commit-hash: 84203ca
commit-date: 2018-03-25
host: x86_64-unknown-linux-gnu
release: 1.25.0
LLVM version: 6.0

Also produced on nightly 1.27.0