# Function std::intrinsics::mir::Field

source ·

    pub fn Field<F>(place: (), field: u32) -> F

🔬This is a nightly-only experimental API. (`custom_mir`)
Expand description

Access the field with the given index of some place.

This only makes sense to use in conjunction with `Variant`. If the type you are looking to access the field of does not have variants, you can use normal field projection syntax.

There is no proper way to do a place projection to a variant in Rust, and so these two functions are a workaround. You can access a field of a variant via `Field(Variant(place, var_idx), field_idx)`, where `var_idx` and `field_idx` are appropriate literals. Some caveats:

- The return type of `Variant` is always `()`. Don’t worry about that, the correct MIR will still be generated.
- In some situations, the return type of `Field` cannot be inferred. You may need to annotate it on the function in these cases.
- Since `Field` is a function call which is not a place expression, using this on the left hand side of an expression is rejected by the compiler. `place!` is a macro provided to work around that issue. Wrap the left hand side of an assignment in the macro to convince the compiler that it’s ok.

## Examples

    #![allow(internal_features)]
    #![feature(custom_mir, core_intrinsics)]

    use core::intrinsics::mir::*;

    #[custom_mir(dialect = "built")]
    fn unwrap_deref(opt: Option<&i32>) -> i32 {
        mir!({
            RET = *Field::<&i32>(Variant(opt, 1), 0);
            Return()
        })
    }

    #[custom_mir(dialect = "built")]
    fn set(opt: &mut Option<i32>) {
        mir!({
            place!(Field(Variant(*opt, 1), 0)) = 5;
            Return()
        })
    }

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