base64ct: downgrade edition to 2021; MSRV 1.81

We had some complaints about the steep MSRV upgrade from 1.60 to 1.85
which occurred in the 1.7.0 release (#1695).

This lowers the MSRV to just what we need for `core::error::Error`
support, downgrading the edition in the process.

We can bump edition in a future release, once 1.85 has been out longer.

Closes #1695

Author: tarcieri

.github/workflows/base64ct.yml

@@ -23,7 +23,7 @@ jobs:
     strategy:
       matrix:
         rust:
-          - 1.85.0 # MSRV
+          - 1.81.0 # MSRV
           - stable
         target:
           - thumbv7em-none-eabi
@@ -47,7 +47,7 @@ jobs:
     strategy:
       matrix:
         rust:
-          - 1.85.0 # MSRV
+          - 1.81.0 # MSRV
           - stable
     steps:
       - uses: actions/checkout@v4

base64ct/Cargo.toml

@@ -14,8 +14,8 @@ repository = "https://github.com/RustCrypto/formats"
 categories = ["cryptography", "encoding", "no-std", "parser-implementations"]
 keywords = ["crypto", "base64", "pem", "phc"]
 readme = "README.md"
-edition = "2024"
-rust-version = "1.85"
+edition = "2021"
+rust-version = "1.81"
 
 [dev-dependencies]
 base64 = "0.22"

base64ct/README.md

@@ -41,6 +41,13 @@ fixed-width line wrapping.
 - bcrypt Base64: `.`, `/`, `[A-Z]`, `[a-z]`, `[0-9]`
 - `crypt(3)` Base64: `.`, `-`, `[0-9]`, `[A-Z]`, `[a-z]`
 
+## Minimum Supported Rust Version
+
+This crate requires **Rust 1.81** at a minimum.
+   
+We may change the MSRV in the future, but it will be accompanied by a minor
+version bump.
+
 ## License
 
 Licensed under either of:
@@ -65,7 +72,7 @@ dual licensed as above, without any additional terms or conditions.
 [build-image]: https://github.com/RustCrypto/formats/actions/workflows/base64ct.yml/badge.svg
 [build-link]: https://github.com/RustCrypto/formats/actions/workflows/base64ct.yml
 [license-image]: https://img.shields.io/badge/license-Apache2.0/MIT-blue.svg
-[rustc-image]: https://img.shields.io/badge/rustc-1.85+-blue.svg
+[rustc-image]: https://img.shields.io/badge/rustc-1.81+-blue.svg
 [chat-image]: https://img.shields.io/badge/zulip-join_chat-blue.svg
 [chat-link]: https://rustcrypto.zulipchat.com/#narrow/stream/300570-formats