Phased Initialization

At this stage, some compound value expressions remain, namely struct, enum and union constructors. In this step we desugar those into individual assignments, using Phased Initialization.

#![allow(unused)]
fn main() {
x = Struct { a: $expr_a, b: $expr_b };

// becomes:
x.a = $expr_a;
x.b = $expr_b;
}

#![allow(unused)]
fn main() {
x = Enum::Variant { a: $expr_a, b: $expr_b };

// becomes:
x.Variant.a = $expr_a;
x.Variant.b = $expr_b;
x.enum#discriminant = discriminant_of!(Enum, Variant));
}

Note that we don't desugar tuple struct/enum constructors since these are semantically function calls.

The one aggregate we keep is array repeat expressions [$expr; $const].