Enum Projections

To desugar enum patterns we need a way to talk about the places inside of an enum variant.

If $place is of an enum type, this features adds a $place.$variant_name place expression that refers to the contents of the $variant_name variant. That place then has one field for each field of the variant.

For example:

#![allow(unused)]
fn main() {
let opt: &mut Option<u32> = ...;
if let Some(ref mut x) = *opt {
    // x: &mut u32 here
}

// desugars to:
if let Some(_) = *opt {
    let x = unsafe { &mut (*opt).Some.0 };
}
}

Using this place is UB if the enum value didn’t have the correct discriminant, and thus this operation requires an unsafe block.

The place $place.$variant_name can’t be used by itself because we don’t have a type for it; it must be directly followed by a field projection.

Backlinks

• Pattern Unnesting