Struct field "system_program" is unsafe, but is not documented.

[bluestar0505]

Anchor:
anchor-cli 0.24.2
Rust:
rustc 1.60.0 (7737e0b5c 2022-04-04)

`yokoiakiomini2@yokoiakiomini2s-Mac-mini solana-twitter % anchor build                                               
BPF SDK: /Users/yokoiakiomini2/.local/share/solana/install/releases/1.10.8/solana-release/bin/sdk/bpf
cargo-build-bpf child: rustup toolchain list -v
cargo-build-bpf child: cargo +bpf build --target bpfel-unknown-unknown --release
   Compiling solana-twitter v0.1.0 (/Volumes/Data/04_OutSourcing/Github/solana-reference/Anchor/solana-twitter/programs/solana-twitter)
    Finished release [optimized] target(s) in 6.48s
cargo-build-bpf child: /Users/yokoiakiomini2/.local/share/solana/install/releases/1.10.8/solana-release/bin/sdk/bpf/scripts/strip.sh /Volumes/Data/04_OutSourcing/Github/solana-reference/Anchor/solana-twitter/target/bpfel-unknown-unknown/release/solana_twitter.so /Volumes/Data/04_OutSourcing/Github/solana-reference/Anchor/solana-twitter/target/deploy/solana_twitter.so
cargo-build-bpf child: /Users/yokoiakiomini2/.local/share/solana/install/releases/1.10.8/solana-release/bin/sdk/bpf/dependencies/bpf-tools/llvm/bin/llvm-readelf --dyn-symbols /Volumes/Data/04_OutSourcing/Github/solana-reference/Anchor/solana-twitter/target/deploy/solana_twitter.so

To deploy this program:
  $ solana program deploy /Volumes/Data/04_OutSourcing/Github/solana-reference/Anchor/solana-twitter/target/deploy/solana_twitter.so
The program address will default to this keypair (override with --program-id):
  /Volumes/Data/04_OutSourcing/Github/solana-reference/Anchor/solana-twitter/target/deploy/solana_twitter-keypair.json
Error: 
        /Volumes/Data/04_OutSourcing/Github/solana-reference/Anchor/solana-twitter/programs/solana-twitter/src/lib.rs:28:8
        Struct field "system_program" is unsafe, but is not documented.
        Please add a `/// CHECK:` doc comment explaining why no checks through types are necessary.
        See https://book.anchor-lang.com/chapter_3/the_accounts_struct.html#safety-checks for more information.

[bluestar0505]

EDIT 2022-03-22: In newer versions of Anchor, we can achieve the same result by using yet another type of account called Program and passing it the System type to ensure it is the official System program.

#[derive(Accounts)]
pub struct SendTweet<'info> {
    // ...
    pub system_program: Program<'info, System>,
}