Unused variables lead to scary diagnostics

fn main() {
    let x;
}

<anon>:2:9: 2:10 error: unable to infer enough type information to locate the impl of the trait `core::kinds::Sized` for the type `<generic #1>`; type annotations required
<anon>:2     let x;
                 ^
<anon>:2:9: 2:10 note: all local variables must have a statically known size
<anon>:2     let x;

cc @nick29581

[code-ape]

Jakub, not sure what exactly you mean by "scary" for the error that the compiler returns. But I do know that it isn't because x is unused. It's because it isn't declared correctly. Because Rust is a systems level language all variables have to be a known type so that the compiler known how much memory space is needed. A safety feature of Rust is also that it tracks type to make sure that you don't try to set an integer variable to something other than an integer. Rust is after all a strongly typed language and so whatever type you declare a variable to be that's the only kind of data it can have. You may want to check out the guide here:
http://doc.rust-lang.org/guide.html#variable-bindings

It actually lists this error as an example.

@code-ape The message in the guide is out of date. The one I provided in the original report is what I considered "scary".

[nrc]

This related to #17567, where we decided we should just give a better error message. Here it seems that type inference could do better - if x is a local variable and we have no other type info, then we should infer that it has the Sized bound.

Aside: this would be a cleaner way to implement the check that local vars are sized, but we would have to require that the Sized bound is a requirement of any type, since I believe unifying Sized and Sized? just gives Sized.