fix: fixing msrv ci test by making the library use std for components that don't exist on older versions of rust core

Author: ngildenhuys

Cargo.toml

@@ -22,6 +22,7 @@ include = ["Cargo.toml", "CHANGELOG.md", "LICENSE-MIT", "LICENSE-APACHE", "READM
 
 [dependencies]
 derive_arbitrary = { version = "~1.4.0", path = "./derive", optional = true }
+rustversion = "1"
 
 [features]
 default = ["std"]

README.md

@@ -130,6 +130,12 @@ compile with older versions but that may change in any new patch release.
 We reserve the right to increment the MSRV on minor releases, however we will
 strive to only do it deliberately and for good reasons.
 
+### Notes on no_std compatability:
+
+- When compiling for versions older than **1.81.0** the `std` feature must be enabled.
+- When compiling for versions older than **1.64.0** the `impl` for `CString` is not available
+  with the `no_std` feature. And
+
 ## License
 
 Licensed under dual MIT or Apache-2.0 at your choice.

src/error.rs

@@ -1,4 +1,10 @@
-use core::{error, fmt};
+use core::fmt;
+
+#[rustversion::before(1.81)]
+use std::error;
+
+#[rustversion::since(1.81)]
+use core::error;
 
 /// An enumeration of buffer creation errors
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]

src/foreign/alloc/ffi/c_str.rs

@@ -1,6 +1,10 @@
+#[rustversion::since(1.64)]
+use alloc::ffi::CString;
+#[rustversion::before(1.64)]
+use std::ffi::CString;
 use {
     crate::{Arbitrary, Result, Unstructured},
-    alloc::{ffi::CString, vec::Vec},
+    alloc::vec::Vec,
 };
 
 impl<'a> Arbitrary<'a> for CString {

src/lib.rs

@@ -53,6 +53,10 @@ impl core::fmt::Display for MaxRecursionReached {
     }
 }
 
+#[rustversion::before(1.81)]
+impl std::error::Error for MaxRecursionReached {}
+
+#[rustversion::since(1.81)]
 impl core::error::Error for MaxRecursionReached {}
 
 /// Generate arbitrary structured values from raw, unstructured data.