rust: qemu-api-macros: extend error reporting facility to parse errors

Generalize the CompileError tuple to an enum, that can be either an error
message or a parse error from syn.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/qemu-api-macros/src/utils.rs

@@ -0,0 +1,26 @@
+// Procedural macro utilities.
+// Author(s): Paolo Bonzini <pbonzini@redhat.com>
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+use proc_macro2::Span;
+use quote::quote_spanned;
+
+pub enum MacroError {
+    Message(String, Span),
+    ParseError(syn::Error),
+}
+
+impl From<syn::Error> for MacroError {
+    fn from(err: syn::Error) -> Self {
+        MacroError::ParseError(err)
+    }
+}
+
+impl From<MacroError> for proc_macro2::TokenStream {
+    fn from(err: MacroError) -> Self {
+        match err {
+            MacroError::Message(msg, span) => quote_spanned! { span => compile_error!(#msg); },
+            MacroError::ParseError(err) => err.into_compile_error(),
+        }
+    }
+}