change: fail early when using Rust 1.57 and earlier, because they don't work (#4435)

* change: set 1.58 as the minimum required Rust version for zebrad and document it

* Update README.md

Co-authored-by: Deirdre Connolly <durumcrustulum@gmail.com>

* Apply suggestions from code review

Co-authored-by: teor <teor@riseup.net>

Co-authored-by: Deirdre Connolly <durumcrustulum@gmail.com>
Co-authored-by: teor <teor@riseup.net>

README.md

@@ -100,6 +100,9 @@ install mechanism. To run `zebrad`, follow the instructions to compile `zebrad`
 for your platform:
 
 1. Install [`cargo` and `rustc`](https://www.rust-lang.org/tools/install).
+     - Zebra is tested with the latest `stable` Rust version.
+        Earlier versions are not supported or tested, but they might work.
+        (Rust 1.57 and earlier are not supported, due to missing features.)
 2. Install Zebra's build dependencies:
      - **libclang:** the `libclang`, `libclang-dev`, `llvm`, or `llvm-dev` packages, depending on your package manager
      - **clang** or another C++ compiler: `g++`, `Xcode`, or `MSVC`

zebrad/Cargo.toml

@@ -4,6 +4,9 @@ authors = ["Zcash Foundation <zebra@zfnd.org>"]
 license = "MIT OR Apache-2.0"
 version = "1.0.0-beta.10"
 edition = "2021"
+# Zebra is only supported on the latest stable Rust version. Some earlier versions might work.
+# Zebra uses features introduced in Rust 1.58.
+rust-version = "1.58"
 repository = "https://github.com/ZcashFoundation/zebra"
 # make `cargo run` use `zebrad` by default
 # when run in the workspace directory