Const and unsafe blocks

help
1

Why does this not work?

const _:() = unsafe {
    const VAL: usize = std::mem::transmute(5_isize);
};

Here's the error:

warning: unnecessary `unsafe` block
 --> src/lib.rs:1:14
  |
1 | const _:() = unsafe {
  |              ^^^^^^ unnecessary `unsafe` block
  |
  = note: `#[warn(unused_unsafe)]` on by default

error[E0133]: call to unsafe function is unsafe and requires unsafe function or block
 --> src/lib.rs:2:24
  |
2 |     const VAL: usize = std::mem::transmute(5_isize);
  |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ call to unsafe function
  |
  = note: consult the function's documentation for information on how to avoid undefined behavior

error: aborting due to previous error; 1 warning emitted

Playground link.

2

This is because the scope of the unsafe block does not extend to the definition of the constant, even if the constant is defined in the unsafe block.

3

Is there a reason for that?

4

The body of the definition of the constant is a different function in some sense.

5

Ah I see. It's frustrating in the context of a macro. I can workaround it though. It just makes things more complex.