Fix discrepancy between host vs target "std" feature

[dtolnay]

derive(Decode) was incorrectly looking at whether the "std" feature is enabled in the host platform to decide on how to generate decoding code. This is not correct. When using the Cargo v2 feature resolver (edition 2021+), or when using the v1 feature resolver and cross-compiling, it can happen that "std" is enabled in the host platform and not enabled in the target platform, leading to generated code that does not compile.

The std- and alloc-related generated code needs to depend on whether "std" is enabled in the target platform.

Repro:

# Cargo.toml

[package]
name = "repro"
version = "0.0.0"
edition = "2024"
publish = false

[dependencies]
minicbor = { version = "2.1.2", default-features = false, features = ["alloc", "derive"] }

[build-dependencies]
minicbor = { version = "2.1.2", features = ["derive", "std"] }

// build.rs

use std::collections::HashSet;

#[derive(Default, minicbor::Encode)]
#[cbor(transparent)]
struct Thing {
    // Encode something that requires cfg(feature = "std") on the host platform
    set: HashSet<String>,
}

fn main() {
    let thing = Thing::default();
    let mut buffer = [0u8; 4];
    minicbor::encode(&thing, buffer.as_mut()).unwrap();
    println!("cargo:warning={:?}", buffer);
}

// src/lib.rs

#![no_std]

extern crate alloc;

use alloc::borrow::Cow;

#[derive(minicbor::Decode)]
#[cbor(transparent)]
pub struct Thing<'a>(#[cbor(borrow)] pub Cow<'a, str>);

cargo check --target x86_64-unknown-none previously failed like this, because the std feature on the host platform was being applied improperly against generated code built for the target platform.

error[E0433]: failed to resolve: use of unresolved module or unlinked crate `std`
 --> src/lib.rs:9:10
  |
9 | #[derive(minicbor::Decode)]
  |          ^^^^^^^^^^^^^^^^ use of unresolved module or unlinked crate `std`
  |
  = help: if you wanted to use a crate named `std`, use `cargo add std` to add it to your `Cargo.toml`
  = note: this error originates in the derive macro `minicbor::Decode` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider importing this enum
  |
7 + use alloc::borrow::Cow;
  |

[twittner]

Thanks again @dtolnay!

[twittner]

Published as 2.1.3 to crates.io.