Resolve std_instead_of_core clippy restriction on std::error

    warning: used import from `std` instead of `core`
       --> src/exception.rs:21:6
        |
    21  | impl std::error::Error for Exception {}
        |      ^^^ help: consider importing the item from `core`: `core`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#std_instead_of_core
    note: the lint level is defined here
       --> src/lib.rs:378:5
        |
    378 |     clippy::std_instead_of_core
        |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^

build.rs

@@ -32,6 +32,7 @@ fn main() {
             println!("cargo:rustc-check-cfg=cfg(compile_error_if_alloc)");
             println!("cargo:rustc-check-cfg=cfg(compile_error_if_std)");
             println!("cargo:rustc-check-cfg=cfg(cxx_experimental_no_alloc)");
+            println!("cargo:rustc-check-cfg=cfg(no_core_error)");
             println!("cargo:rustc-check-cfg=cfg(no_core_ffi_c_char)");
             println!("cargo:rustc-check-cfg=cfg(skip_ui_tests)");
         }
@@ -48,6 +49,11 @@ fn main() {
             // core::ffi::c_char
             println!("cargo:rustc-cfg=no_core_ffi_c_char");
         }
+
+        if rustc.minor < 81 {
+            // core::error::Error
+            println!("cargo:rustc-cfg=no_core_error");
+        }
     }
 }
 

src/exception.rs

@@ -3,6 +3,11 @@
 use alloc::boxed::Box;
 use core::fmt::{self, Display};
 
+#[cfg(all(feature = "std", not(no_core_error)))]
+use core::error::Error as StdError;
+#[cfg(all(feature = "std", no_core_error))]
+use std::error::Error as StdError;
+
 /// Exception thrown from an `extern "C++"` function.
 #[cfg_attr(docsrs, doc(cfg(feature = "alloc")))]
 #[derive(Debug)]
@@ -18,7 +23,7 @@ impl Display for Exception {
 
 #[cfg(feature = "std")]
 #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
-impl std::error::Error for Exception {}
+impl StdError for Exception {}
 
 impl Exception {
     #[allow(missing_docs)]