fix docstring

tools/src/main.rs

@@ -343,9 +343,13 @@ fn extract_docstring(attrs: &[Attribute]) -> Option<String> {
             // #[doc = "…"]
             if let Ok(meta) = attr.parse_meta() {
                 if let syn::Meta::NameValue(nv) = meta {
-                    // The field is `value`, not `lit`
+                    // The value is an `Expr`.  Pull out a string literal.
-                    if let syn::Lit::Str(lit) = nv.value {
+                    if let syn::Expr::Lit(syn::ExprLit {
-                        docs.push(lit.value());
+                        lit: syn::Lit::Str(lit_str),
+                        ..
+                    }) = nv.value
+                    {
+                        docs.push(lit_str.value());
                     }
                 }
             }