Bitcoin CLI in Rust.

The main tool provided by Bitcoin-core is bitcoind, a daemon that implements full node, wallet and mining logic.

Some of the shortcomings of bitcoin-cli:

• Nonstandard single dash long arguments, e.g., -datadir, when most other CLI tools would use --datadir.
• Argument values must be given with an equals sign, e.g., -datadir=foo, when most other tools would accept both --datadir=foo and --datadir foo.
• No colored help output, and help text formatting could be improved.
• By default, arguments are passed as JSON which is verbose and hard to get right, especially when values must be quoted.
• Passing arguments by name is not the default, when it probably should be.
• Arguments are not validated up-front, but instead passed along to the RPC server as-is, which leads to poor error messages.
• No completion script.
• bitcoin-cli is a bit long to type.
• Like the rest of Bitcoin Core, bitcoin-cli is written in C++, which deters potential contributors, increases review burden, and is error prone and unsafe.

Improvements:

• The new binary could be written in Rust, which has excellent libraries for argument parsing and JSON serialization, including Bitcoin-specific libraries, namely rust-bitcoin and rust-bitcoincore-rpc, for parsing bitcoin types and interacting with bitcoind's RPC interface.
• Many people want to learn Rust, write Rust, and use Rust-written CLI tools. Adding a way for people to contribute to Bitcoin Core using Rust would open the door to a lot of new contributors, and a rewite of bitcoin-cli in particular would provide an extremely approachable way for them to do so.
• The new binary could be called bitcoin or even btc, so it's easier to type.
• Eventually, the binary could support a subcommand to invoke bitcoind, e.g., bitcoin daemon, to improve the same issues with bitcoind that bitcoin-cli has, e.g., argument parsing, argument validation, help text formatting, and completion script support.
• Long arguments could be accepted with --arg and short arguments with -a, which is standard and familiar among the vast majority of CLI tools.
• Values could be passed as standard named arguments, e.g., --address foo, instead of as JSON. The ability to pass JSON arguments could be retained with a subcommand for making arbitrary RPC calls which takes a single json argument: bitcoin rpc --name generateblock --json SOME_JSON_BLOB.
• Arguments could be validated up-front, instead of being passed to the RPC server. This would allow the binary to report which argument failed, specifically, instead of receiving a somewhat inscrutable error from the RPC server.