display better message for a missing wrapper value

src/serde_tokenstream.rs

@@ -1083,22 +1083,13 @@ impl<'de> Deserializer<'de> for &mut TokenDe {
         // TODO format a spanned error of some sort
         let mut token = match &next {
             None => {
-                return self.deserialize_error(
-                    next,
-                    "anything but a ',', '=', or EOF",
-                );
+                return self.deserialize_error(next, "a value");
             }
             Some(TokenTree::Punct(punct)) if punct.as_char() == ',' => {
-                return self.deserialize_error(
-                    next,
-                    "anything but a ',', '=', or EOF",
-                );
+                return self.deserialize_error(next, "a value");
             }
             Some(TokenTree::Punct(punct)) if punct.as_char() == '=' => {
-                return self.deserialize_error(
-                    next,
-                    "anything but a ',', '=', or EOF",
-                );
+                return self.deserialize_error(next, "a value");
             }
             Some(token) => token.clone(),
         };

ui-tests/tests/ui/bad_parse_wrapper_missing.rs

@@ -0,0 +1,17 @@
+// Copyright 2026 Oxide Computer Company
+
+// A ParseWrapper value that is missing before a `,` must be reported as a
+// missing value at the `,`.
+
+use testlib::annotation;
+
+#[annotation {
+    string = "test",
+    options = OptionA,
+    unit = (),
+    tup = (1, 2.0),
+    bool_expr = ,
+}]
+fn test() {}
+
+fn main() {}

ui-tests/tests/ui/bad_parse_wrapper_missing.stderr

@@ -0,0 +1,5 @@
+error: expected a value, but found `,`
+  --> tests/ui/bad_parse_wrapper_missing.rs:13:17
+   |
+13 |     bool_expr = ,
+   |                 ^

ui-tests/tests/ui/bad_parse_wrapper_missing_eof.rs

@@ -0,0 +1,17 @@
+// Copyright 2026 Oxide Computer Company
+
+// A ParseWrapper value that is missing at the end of the input must be reported
+// as a missing value at the `=`.
+
+use testlib::annotation;
+
+#[annotation {
+    string = "test",
+    options = OptionA,
+    unit = (),
+    tup = (1, 2.0),
+    bool_expr =
+}]
+fn test() {}
+
+fn main() {}

ui-tests/tests/ui/bad_parse_wrapper_missing_eof.stderr

@@ -0,0 +1,5 @@
+error: expected a value following `=`
+  --> tests/ui/bad_parse_wrapper_missing_eof.rs:13:15
+   |
+13 |     bool_expr =
+   |               ^