Create Static Slice from another Static Slice? - rust

Question

I have two static slices that look something like this:
static ITEMS_1 : &'static [&str] = &[
"abc",
"def",
];
static ITEMS_2 : &'static [&str] = &[
"abc",
"def",
"ghi"
];
The first two entries of ITEMS_2 should match those of ITEMS_1. So, I want to define ITEMS_2 as ITEMS_1 appended with another item (or more) to avoid duplication. Is there any sensible way of doing this?

Answer 1

If you really only have 3 items, I wouldn't unpack the macro hammer.
While you can't do
static ITEMS_2 : &'static [&str] = &[
"abc",
"def",
"ghi"
];
static ITEMS_1 : &'static [&str] = &ITEMS_2[0..1];
you can do
static ITEMS_2: &'static [&str] = &["abc", "def", "ghi"];
static ITEMS_1: &'static [&str] = match ITEMS_2.split_last() {
None => &[],
Some((_, i)) => i,
};
Alternatively, you could also reference the elements one by one
static ITEMS_1 : &'static [&str] = &[
"abc",
"def",
];
static ITEMS_2 : &'static [&str] = &[
ITEMS_1[0],
ITEMS_1[1],
"ghi"
];

Answer 2

This can be done by encapsulating the common part in a macro:
macro_rules! with_common_strings {
($id:ident [ $($item:expr),* ]) => {
static $id: &'static [&str] = &[
"abc",
"def",
$($item),*
];
}
}
with_common_strings!(ITEMS_1 []);
with_common_strings!(ITEMS_2 [ "ghi" ]);
fn main() {
println!("Items 1: {:?}", ITEMS_1);
println!("Items 2: {:?}", ITEMS_2);
}
Playground
Or with a more complex but more generic macro that doesn't hardcode the common part of the arrays:
macro_rules! make_cumul_arrays {
(#rec ($($accum:expr),*)) => {};
(#rec ($($accum:expr),*) $id:ident [ $($item:expr),* ] $($tail:tt)*) => {
static $id: &'static[&str] = &[
$($accum),*,
$($item),*
];
make_cumul_arrays!{ #rec ($($accum),*, $($item),*) $($tail)* }
};
($id:ident [ $($item:expr),* ] $($tail:tt)*) => {
static $id: &'static[&str] = &[ $($item),* ];
make_cumul_arrays!{ #rec ($($item),*) $($tail)* }
};
}
make_cumul_arrays!{
ITEMS_1 [ "abc", "def" ]
ITEMS_2 [ "ghi" ]
}
Playground