Rustc help tips create a loop with borrowing

[NotWearingPants]

I tried compiling this code:

fn main() {
    let x: [u8] = vec!(1, 2, 3)[..];
}

which resulted in the following output of rustc:

error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
 --> src/main.rs:2:9
  |
2 |     let x: [u8] = vec!(1, 2, 3)[..];
  |         ^         ----------------- help: consider borrowing here: `&vec!(1, 2, 3)[..]`
  |         |
  |         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: all local variables must have a statically known size
  = help: unsized locals are gated as an unstable feature

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.

So the compiler says help: consider borrowing here: &vec!(1, 2, 3)[..]
So I tried adding a borrow and recompiling the code:

fn main() {
    let x: [u8] = &vec!(1, 2, 3)[..];
}

which resulted in:

error[E0308]: mismatched types
 --> src/main.rs:2:19
  |
2 |     let x: [u8] = &vec!(1, 2, 3)[..];
  |            ----   ^^^^^^^^^^^^^^^^^^
  |            |      |
  |            |      expected slice `[u8]`, found `&[{integer}]`
  |            |      help: consider removing the borrow: `vec!(1, 2, 3)[..]`
  |            expected due to this

error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
 --> src/main.rs:2:9
  |
2 |     let x: [u8] = &vec!(1, 2, 3)[..];
  |         ^ 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: all local variables must have a statically known size
  = help: unsized locals are gated as an unstable feature

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0277, E0308.
For more information about an error, try `rustc --explain E0277`.

so the compiler says help: consider removing the borrow: vec!(1, 2, 3)[..]

and so I'm stuck in a loop. The compiler can't decide whether I should borrow or not.

As a newbie to Rust, this is very frustrating and the opposite of helpful, it just confuses me further.
Please fix this indecisiveness :)

Meta

rustc --version --verbose:

rustc 1.43.1 (8d69840ab 2020-05-04)
binary: rustc
commit-hash: 8d69840ab92ea7f4d323420088dd8c9775f180cd
commit-date: 2020-05-04
host: x86_64-pc-windows-msvc
release: 1.43.1
LLVM version: 9.0

[estebank]

For reference, what you likely want to do is one of the following

use std::convert::TryInto;

fn main() {
    let x: [u8; 3] = vec!(1, 2, 3)[..].try_into().unwrap(); // sized array
    let y: &[u8] = &vec!(1, 2, 3)[..]; // `u8` slice
}

Bare slices ([_]) are !Sized (unsized) which means they cannot be the type of a local binding. You can give a binding a slice type (&[_]), because that's morally equivalent to a pointer (with sizing info), which has a compile time determined size, as all borrows are Sized.

[estebank]

Current output:

error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
 --> src/test/ui/unsized-locals/suggest-borrow.rs:2:9
  |
2 |     let x: [u8] = vec!(1, 2, 3)[..]; //~ ERROR E0277
  |         ^ doesn't have a size known at compile-time
  |
  = help: the trait `Sized` is not implemented for `[u8]`
  = note: all local variables must have a statically known size
  = help: unsized locals are gated as an unstable feature
help: consider borrowing here
  |
2 |     let x: [u8] = &vec!(1, 2, 3)[..]; //~ ERROR E0277
  |                   +

error[E0308]: mismatched types
 --> src/test/ui/unsized-locals/suggest-borrow.rs:3:20
  |
3 |     let x: &[u8] = vec!(1, 2, 3)[..]; //~ ERROR E0308
  |            -----   ^^^^^^^^^^^^^^^^^
  |            |       |
  |            |       expected `&[u8]`, found slice `[{integer}]`
  |            |       help: consider borrowing here: `&vec!(1, 2, 3)[..]`
  |            expected due to this

error[E0308]: mismatched types
 --> src/test/ui/unsized-locals/suggest-borrow.rs:4:19
  |
4 |     let x: [u8] = &vec!(1, 2, 3)[..]; //~ ERROR E0308
  |            ----   ^^^^^^^^^^^^^^^^^^ expected slice `[u8]`, found `&[{integer}]`
  |            |
  |            expected due to this
  |
help: consider removing the borrow
  |
4 -     let x: [u8] = &vec!(1, 2, 3)[..]; //~ ERROR E0308
4 +     let x: [u8] = vec!(1, 2, 3)[..]; //~ ERROR E0308
  |
help: alternatively, consider changing the type annotation
  |
4 |     let x: &[u8] = &vec!(1, 2, 3)[..]; //~ ERROR E0308
  |            +

error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
 --> src/test/ui/unsized-locals/suggest-borrow.rs:4:9
  |
4 |     let x: [u8] = &vec!(1, 2, 3)[..]; //~ ERROR E0308
  |         ^ doesn't have a size known at compile-time
  |
  = help: the trait `Sized` is not implemented for `[u8]`
  = note: all local variables must have a statically known size
  = help: unsized locals are gated as an unstable feature

After #106223:

error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
 --> src/test/ui/unsized-locals/suggest-borrow.rs:2:9
  |
2 |     let x: [u8] = vec!(1, 2, 3)[..]; //~ ERROR E0277
  |         ^ doesn't have a size known at compile-time
  |
  = help: the trait `Sized` is not implemented for `[u8]`
  = note: all local variables must have a statically known size
  = help: unsized locals are gated as an unstable feature
help: consider borrowing here
  |
2 |     let x: &[u8] = vec!(1, 2, 3)[..]; //~ ERROR E0277
  |            +

error[E0308]: mismatched types
 --> src/test/ui/unsized-locals/suggest-borrow.rs:3:20
  |
3 |     let x: &[u8] = vec!(1, 2, 3)[..]; //~ ERROR E0308
  |            -----   ^^^^^^^^^^^^^^^^^
  |            |       |
  |            |       expected `&[u8]`, found slice `[{integer}]`
  |            |       help: consider borrowing here: `&vec!(1, 2, 3)[..]`
  |            expected due to this

error[E0308]: mismatched types
 --> src/test/ui/unsized-locals/suggest-borrow.rs:4:19
  |
4 |     let x: [u8] = &vec!(1, 2, 3)[..]; //~ ERROR E0308
  |            ----   ^^^^^^^^^^^^^^^^^^ expected slice `[u8]`, found `&[{integer}]`
  |            |
  |            expected due to this
  |
help: consider removing the borrow
  |
4 -     let x: [u8] = &vec!(1, 2, 3)[..]; //~ ERROR E0308
4 +     let x: [u8] = vec!(1, 2, 3)[..]; //~ ERROR E0308
  |
help: alternatively, consider changing the type annotation
  |
4 |     let x: &[u8] = &vec!(1, 2, 3)[..]; //~ ERROR E0308
  |            +

error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
 --> src/test/ui/unsized-locals/suggest-borrow.rs:4:9
  |
4 |     let x: [u8] = &vec!(1, 2, 3)[..]; //~ ERROR E0308
  |         ^ doesn't have a size known at compile-time
  |
  = help: the trait `Sized` is not implemented for `[u8]`
  = note: all local variables must have a statically known size
  = help: unsized locals are gated as an unstable feature
help: consider borrowing here
  |
4 |     let x: &[u8] = &vec!(1, 2, 3)[..]; //~ ERROR E0308
  |            +