1 files changed, 39 insertions, 0 deletions
diff --git a/src/emit.rs b/src/emit.rs
new file mode 100644
index 0000000..d1ddda8
--- /dev/null
+++ b/src/emit.rs
@@ -0,0 +1,39 @@
+use proc_macro::TokenStream;
+use proc_macro2::Span;
+use quote::quote;
+use syn::Error;
+
+#[derive(Copy, Clone)]
+pub enum Kind {
+    Enum,
+    Match,
+    Struct,
+    Let,
+}
+
+pub fn emit(err: Error, kind: Kind, original: TokenStream) -> TokenStream {
+    let mut err = err;
+    if !probably_has_spans(kind) {
+        // Otherwise the error is printed without any line number.
+        err = Error::new(Span::call_site(), &err.to_string());
+    }
+
+    let err = err.to_compile_error();
+    let original = proc_macro2::TokenStream::from(original);
+
+    let expanded = match kind {
+        Kind::Enum | Kind::Let | Kind::Struct => quote!(#err #original),
+        Kind::Match => quote!({ #err #original }),
+    };
+
+    TokenStream::from(expanded)
+}
+
+// Rustc is so bad at spans.
+// https://github.com/rust-lang/rust/issues/43081
+fn probably_has_spans(kind: Kind) -> bool {
+    match kind {
+        Kind::Enum | Kind::Struct => true,
+        Kind::Match | Kind::Let => false,
+    }
+}