chore: use proc-macro-error2

The `proc-macro-error` crate is deprecated, hence use `proc-macro-error2`
instead.

Closes #370.

derive-impl/Cargo.toml

@@ -13,7 +13,7 @@ proc-macro = true
 [dependencies]
 proc-macro2 = { version = "1.0.24", features = ["span-locations"] }
 proc-macro-crate = "3.1.0"
-proc-macro-error = "1.0.4"
+proc-macro-error2 = "2.0.1"
 quote = "1.0.7"
 syn = "2.0.66"
 synstructure = "0.13.1"

derive-impl/src/lib.rs

@@ -4,13 +4,11 @@
 //! For example, the actual trait that we are deriving `MultihashDigest`, as well as the `Hasher` trait and
 //! the `UnsupportedCode` error type.
 
-extern crate proc_macro;
-
 mod multihash;
 mod utils;
 
 use proc_macro::TokenStream;
-use proc_macro_error::proc_macro_error;
+use proc_macro_error2::proc_macro_error;
 use synstructure::macros::{parse, DeriveInput};
 use synstructure::{MacroResult, Structure};
 

derive-impl/src/multihash.rs

@@ -112,14 +112,14 @@ impl<'a> From<&'a VariantInfo<'a>> for Hash {
             #[cfg(test)]
             panic!("{}", msg);
             #[cfg(not(test))]
-            proc_macro_error::abort!(ident, msg);
+            proc_macro_error2::abort!(ident, msg);
         });
         let hasher = hasher.unwrap_or_else(|| {
             let msg = "Missing hasher attribute: e.g. #[mh(hasher = multihash::Sha2_256)]";
             #[cfg(test)]
             panic!("{}", msg);
             #[cfg(not(test))]
-            proc_macro_error::abort!(ident, msg);
+            proc_macro_error2::abort!(ident, msg);
         });
         Self {
             ident,
@@ -156,7 +156,7 @@ fn parse_code_enum_attrs(ast: &syn::DeriveInput) -> syn::LitInt {
             #[cfg(test)]
             panic!("{}", msg);
             #[cfg(not(test))]
-            proc_macro_error::abort!(&ast.ident, msg);
+            proc_macro_error2::abort!(&ast.ident, msg);
         }
     }
 }
@@ -184,7 +184,7 @@ fn error_code_duplicates(hashes: &[Hash]) {
             {
                 let already_defined = uniq.get(code).unwrap();
                 let line = already_defined.to_token_stream().span().start().line;
-                proc_macro_error::emit_error!(
+                proc_macro_error2::emit_error!(
                     &hash.code, msg;
                     note = "previous definition of `{}` at line {}", quote!(#code), line;
                 );