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()
        }
    }
}