From 2305b76f44bc73573cc8b8b44d988d3ef94a717d Mon Sep 17 00:00:00 2001
From: Rain <rain@oxide.computer>
Date: Tue, 1 Sep 2026 19:37:56 -0700
Subject: [PATCH] better message for stray tokens after a ParseWrapper value

When a `ParseWrapper<P>` value is followed by tokens that `P` did not consume (for example `bool_expr = true false`), `syn::parse2` rejected them with a bare "unexpected token" (though the span was fine). Produce a better error message in this case, similar to the message you would get from a non-`ParseWrapper` type.

Doing this in `ParseWrapper` rather than in individual `Parse` impls keeps those impls composable.
---
 src/ibidem.rs                                  | 18 +++++++++++++++++-
 ui-tests/tests/ui/bad_parse_wrapper_stray.rs   | 17 +++++++++++++++++
 .../tests/ui/bad_parse_wrapper_stray.stderr    |  5 +++++
 3 files changed, 39 insertions(+), 1 deletion(-)
 create mode 100644 ui-tests/tests/ui/bad_parse_wrapper_stray.rs
 create mode 100644 ui-tests/tests/ui/bad_parse_wrapper_stray.stderr

diff --git a/src/ibidem.rs b/src/ibidem.rs
index ab403a1..d65623d 100644
--- a/src/ibidem.rs
+++ b/src/ibidem.rs
@@ -5,6 +5,8 @@ use std::cell::RefCell;
 use proc_macro2::{TokenStream, TokenTree};
 use serde::{Deserialize, de::Error, de::Visitor};
 
+use crate::serde_tokenstream::spanned_error;
+
 /// A wrapper around [`TokenStream`] that implements [`Deserialize`] in the
 /// context of [`from_tokenstream`].
 ///
@@ -74,7 +76,21 @@ impl<'de, P: syn::parse::Parse> Deserialize<'de> for ParseWrapper<P> {
     {
         let token_stream = deserializer.deserialize_bytes(WrapperVisitor)?;
 
-        match syn::parse2::<P>(token_stream) {
+        let parser = |input: syn::parse::ParseStream<'_>| -> syn::Result<P> {
+            let parsed = P::parse(input)?;
+            // The deserializer hands over every token up to the next `,`, `=`,
+            // or EOF, so anything left after `P` is a stray token. Report such
+            // tokens as errors.
+            if let Some((tt, _)) = input.cursor().token_tree() {
+                return Err(spanned_error(
+                    &tt,
+                    format!("expected `,` or nothing, but found `{tt}`"),
+                ));
+            }
+            Ok(parsed)
+        };
+
+        match syn::parse::Parser::parse2(parser, token_stream) {
             Ok(parsed) => Ok(Self(parsed)),
             Err(err) => {
                 let msg = err.to_string();
diff --git a/ui-tests/tests/ui/bad_parse_wrapper_stray.rs b/ui-tests/tests/ui/bad_parse_wrapper_stray.rs
new file mode 100644
index 0000000..0c4d9e3
--- /dev/null
+++ b/ui-tests/tests/ui/bad_parse_wrapper_stray.rs
@@ -0,0 +1,17 @@
+// Copyright 2026 Oxide Computer Company
+
+// Ensure that tokens left over after a ParseWrapper value are reported at the
+// first stray token.
+
+use testlib::annotation;
+
+#[annotation {
+    string = "test",
+    options = OptionA,
+    unit = (),
+    tup = (1, 2.0),
+    bool_expr = true false
+}]
+fn test() {}
+
+fn main() {}
diff --git a/ui-tests/tests/ui/bad_parse_wrapper_stray.stderr b/ui-tests/tests/ui/bad_parse_wrapper_stray.stderr
new file mode 100644
index 0000000..cdd315d
--- /dev/null
+++ b/ui-tests/tests/ui/bad_parse_wrapper_stray.stderr
@@ -0,0 +1,5 @@
+error: expected `,` or nothing, but found `false`
+  --> tests/ui/bad_parse_wrapper_stray.rs:13:22
+   |
+13 |     bool_expr = true false
+   |                      ^^^^^