Rust error E0605: non-primitive cast

What rustc prints

error[E0605]: non-primitive cast: `u8` as `Vec<u8>`
 --> main.rs:4:1
  |
4 | x as Vec<u8>; // error: non-primitive cast: `u8` as `std::vec::Vec<u8>`
  | ^^^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object

Real output of rustc 1.99.0 compiling the official example below (2021 edition). Your names and line numbers will differ; the first line is the part to match.

Why it happens

• x as String or v as Vec<u8>.
• Casting between your own structs or enums with data.

How to fix it

let s: String = 42.to_string();
let bytes: Vec<u8> = "hi".into();

let small = u8::try_from(300_u32).unwrap_or(u8::MAX);

Other wordings of this error

non-primitive cast, an `as` expression can only be used to convert between primitive types

let x = 0u8;
x as Vec<u8>; // error: non-primitive cast: `u8` as `std::vec::Vec<u8>`

// Another example

let v = core::ptr::null::<u8>(); // So here, `v` is a `*const u8`.
v as &u8; // error: non-primitive cast: `*const u8` as `&u8`

let x = 0u8;
x as u32; // ok!

let v = core::ptr::null::<u8>();
v as *const i8; // ok!

Related errors

• E0061this function takes N arguments but M were supplied
• E0412cannot find type in this scope
• E0515cannot return reference to local variable
• E0728await is only allowed inside async functions and blocks
• E0590`break` or `continue` with no label in the condition of a `while` loop
• E0591Per [RFC 401][rfc401], if you have a function declaration `foo`:
• E0606casting `&u8` as `u32` is invalid
• E0618expected function, found

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