core::raw_const - Rust
Description: API documentation for the Rust `raw_const` macro in crate `core`.
Keywords: rust, rustlang, rust-lang, raw_const

# [−][src]Macro core::raw_const

    macro_rules! raw_const {
        ($e:expr) => { ... };
    }

🔬 This is a nightly-only experimental API. (`raw_ref_macros` #73394)

Create a `const` raw pointer to a place, without creating an intermediate reference.

Creating a reference with `&`/`&mut` is only allowed if the pointer is properly aligned and points to initialized data. For cases where those requirements do not hold, raw pointers should be used instead. However, `&expr as *const _` creates a reference before casting it to a raw pointer, and that reference is subject to the same rules as all other references. This macro can create a raw pointer without creating a reference first.

# Example

    #![feature(raw_ref_macros)]
    use std::ptr;

    #[repr(packed)]
    struct Packed {
        f1: u8,
        f2: u16,
    }

    let packed = Packed { f1: 1, f2: 2 };
    // `&packed.f2` would create an unaligned reference, and thus be Undefined Behavior!
    let raw_f2 = ptr::raw_const!(packed.f2);
    assert_eq!(unsafe { raw_f2.read_unaligned() }, 2);

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