can't use BTreeMap with custom key struct for offchain Local Storage

Question

I'm trying to use sp_std::collections::btree_map::BTreeMap for offchain worker Local Storage.

While

let foo = StorageValueRef::persistent(b"bar::foo");
foo.get::<BTreeMap<u32, u32>>();

compiles,

when a custom struct is used as a key as follows:

use sp_core::{Decode, Encode};

#[derive(Clone, PartialEq, Eq, Encode, Decode, Debug)]
struct Foo(u32);

let foo = StorageValueRef::persistent(b"bar::foo");
foo.get::<BTreeMap<Foo, u32>>();

compilation fails due to:

  18 |     foo.get::<BTreeMap<Foo, u32>>();
     |               ^^^^^^^^^^^^^^^^^^ the trait `sp_core::Decode` is not implemented for `BTreeMap<Foo, u32>`
     |
     = help: the trait `sp_core::Decode` is implemented for `BTreeMap<K, V>`

Curiously, I can use a sp_std::vec::Vec like so

    let foo = StorageValueRef::persistent(b"bar::foo");
    foo.get::<Vec<(Foo, u32)>>();

which compiles just fine.

Answer 1

https://docs.rs/parity-scale-codec/3.4.0/src/parity_scale_codec/codec.rs.html#1084-1093

impl_codec_through_iterator! {
    BTreeMap { K: Ord, V } { LikeK, LikeV}
        { K: EncodeLike<LikeK>, LikeK: Encode, V: EncodeLike<LikeV>, LikeV: Encode }
    BTreeSet { T: Ord } { LikeT }
        { T: EncodeLike<LikeT>, LikeT: Encode }
    LinkedList { T } { LikeT }
        { T: EncodeLike<LikeT>, LikeT: Encode }
    BinaryHeap { T: Ord } { LikeT }
        { T: EncodeLike<LikeT>, LikeT: Encode }
}

It only implements for BTreeMap<K, V: Decode>.

But in your case, your K is a Decodeable thing, which does not satisfy this implementation.

BTreeMap<u32, Foo> will work.