derive: reject unions as they require unsafe code

rinja_derive/src/input.rs

@@ -302,9 +302,17 @@
 
 impl TemplateArgs {
     pub(crate) fn new(ast: &syn::DeriveInput) -> Result<Self, CompileError> {
+        // FIXME: implement once <https://github.com/rust-lang/rfcs/pull/3715> is stable
+        if let syn::Data::Union(data) = &ast.data {
+            return Err(CompileError::new_with_span(
+                "rinja templates are not supported for `union` types, only `struct` and `enum`",
+                None,
+                Some(data.union_token.span),
+            ));
+        }
+
         // Check that an attribute called `template()` exists at least once and that it is
         // the proper type (list).
-
         let mut templates_attrs = ast
             .attrs
             .iter()
@@ -532,6 +540,7 @@
     let kind = match &ast.data {
         syn::Data::Struct(_) => "struct",
         syn::Data::Enum(_) => "enum",
+        // actually unreachable: `union`s are rejected by `TemplateArgs::new()`
         syn::Data::Union(_) => "union",
     };
     CompileError::no_file_info(

testing/tests/ui/rinja-block.stderr

@@ -42,8 +42,8 @@ error: when using `in_doc = true`, the enum's documentation needs a `rinja` code
 60 | #[template(ext = "txt", in_doc = true)]
    |                         ^^^^^^
 
-error: when using `in_doc = true`, the union's documentation needs a `rinja` code block
-  --> tests/ui/rinja-block.rs:67:25
+error: rinja templates are not supported for `union` types, only `struct` and `enum`
+  --> tests/ui/rinja-block.rs:68:1
    |
-67 | #[template(ext = "txt", in_doc = true)]
-   |                         ^^^^^^
+68 | union NoDocForUnion {
+   | ^^^^^

testing/tests/ui/union.rs

@@ -0,0 +1,10 @@
+use rinja::Template;
+
+#[derive(Template)]
+#[template(source = "a={{ a }} b={{ b }}", ext = "html")]
+union Tmpl {
+    a: i32,
+    b: u32,
+}
+
+fn main() {}

testing/tests/ui/union.stderr

@@ -0,0 +1,5 @@
+error: rinja templates are not supported for `union` types, only `struct` and `enum`
+ --> tests/ui/union.rs:5:1
+  |
+5 | union Tmpl {
+  | ^^^^^