Qualify compile_error!

src/error.rs

@@ -278,8 +278,19 @@ impl ErrorMessage {
             None => (Span::call_site(), Span::call_site()),
         };
 
-        // compile_error!($message)
+        // std::compile_error!($message)
         TokenStream::from_iter(vec![
+            TokenTree::Ident(Ident::new("std", start)),
+            TokenTree::Punct({
+                let mut punct = Punct::new(':', Spacing::Joint);
+                punct.set_span(start);
+                punct
+            }),
+            TokenTree::Punct({
+                let mut punct = Punct::new(':', Spacing::Alone);
+                punct.set_span(start);
+                punct
+            }),
             TokenTree::Ident(Ident::new("compile_error", start)),
             TokenTree::Punct({
                 let mut punct = Punct::new('!', Spacing::Alone);

tests/test_compile_error.rs

@@ -0,0 +1,18 @@
+use quote::quote;
+use syn::Item;
+
+#[test]
+fn parse_crate_root_custom_inner_attribute() {
+    let tokens = quote! {
+        #![feature(custom_inner_attributes)]
+        #[prelude_import]
+        use std::prelude::rust_2021::*;
+        #[macro_use]
+        extern crate std;
+    };
+    let error = syn::parse2::<Item>(tokens).unwrap_err();
+    assert_eq!(
+        error.to_compile_error().to_string(),
+        r#"std :: compile_error ! { "expected square brackets" }"#
+    );
+}