Function core::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()
}
}
}
Run