Type mismatch error trying to intialize vector with type

I’m trying to create a msg_bytes buffer to store data. But when creating it I get a type mismatch error.

  use libflate::gzip::Decoder;

let mut f = std::fs::File::open("./20181002.bytes").unwrap();
let mut decoder = Decoder::new(&f).unwrap();

  let mut msglen_bytes = [0u8; 4];

    decoder.read_exact(&mut msglen_bytes).unwrap();
    println!("msglen_bytes: {:?}", &msglen_bytes);

    let length = u32::from_le_bytes(msglen_bytes);
    let length_us = usize::try_from(length).unwrap();
    println!("length: {}", length);

    let mut msg_bytes = vec!(0u8, length_us, length_us);
    let mut msg_bytes = [0u8; length_us]; // Option 2
    decoder.read_exact(&mut msg_bytes).unwrap();

error[E0308]: mismatched types
  --> src/main.rs:50:35
   |
50 |     let mut msg_bytes = vec!(0u8, length_us, length_us);
   |                                   ^^^^^^^^^ expected `u8`, found `usize`

If I run with Option 2 I get:

error[E0435]: attempt to use a non-constant value in a constant
  --> src/main.rs:51:31
   |
47 |     let length_us = usize::try_from(length).unwrap();
   |     ------------- help: consider using `const` instead of `let`: `const length_us`
...
51 |     let mut msg_bytes = [0u8; length_us];
   |                               ^^^^^^^^^ non-constant value

>Solution :

You probably want let msg_bytes = vec![0u8; length_us] (note semicolon between 0u8 and the length).

vec!(0u8, length_us, length_us) attempts to create a 3-element vector whose elements have values 0u8, length_us, and length_us respectively. Since all vector elements must be of the same type, that syntax can’t compile.

Option 2 doesn’t compile because arrays must have sizes determined at compile time.