Impl Bake for BTreeMap

[adamchalmers]

Part of #4266

[sffc]

Is there a more efficient way to construct this than From<&[(K, V)]>?

[adamchalmers]

The implementation could use this instead:

    fn bake(&self, ctx: &CrateEnv) -> TokenStream {
        ctx.insert("alloc");
        let data = self.iter().map(|(k, v)| {
            let k = k.bake(ctx);
            let v = v.bake(ctx);
            quote!((#k, #v))
        });
        quote! {
            {
                let mut map = alloc::collections::BTreeMap::new();
                for (k, v) in [#(#data),*] {
                    map.insert(k, v);
                }
                map
            }
        }
    }

but I suspect that's actually less efficient. I don't see any benchmarks for databake, maybe I could add one.

[sffc]

Yeah, in theory there should be a way to get an O(N) construction of BTreeMap if we know that the elements are sorted and how many elements there are, but From is likely the most efficient construction mechanism that is currently stable, and we can improve it later if Rust gives us such an API in the future.

[Manishearth]

Ideally this PR would also contain a changelog entry