fix: pre_hook (sync) make request mutable

progenitor-impl/src/method.rs

@@ -1066,7 +1066,7 @@ impl Generator {
         };
         let pre_hook = self.settings.pre_hook.as_ref().map(|hook| {
             quote! {
-                (#hook)(#inner &#request_ident);
+                (#hook)(#inner &mut #request_ident);
             }
         });
         let pre_hook_async = self.settings.pre_hook_async.as_ref().map(|hook| {

progenitor-macro/src/lib.rs

@@ -129,9 +129,10 @@ impl syn::parse::Parse for SpecSource {
 /// The optional `pre_hook` is either a closure (that must be within
 /// parentheses: `(fn |[inner,] request| { .. })`) or a path to a function. The
 /// closure or function must take one or two parameters: the inner type (if one
-/// is specified) and a `&reqwest::Request`. This allows clients to examine
-/// requests before they're sent to the server, for example to log them. The
-/// optional `pre_hook_async` is the `async` variant of the same.
+/// is specified) and a `&mut reqwest::Request`. This allows clients to examine
+/// or modify requests before they're sent to the server, for example to log
+/// them or add a header. The optional `pre_hook_async` is the `async` variant
+/// of the same.
 ///
 /// The optional `post_hook` is either a closure (that must be within
 /// parentheses: `(fn |[inner,] result| { .. })`) or a path to a function. The

progenitor/tests/build_hooks.rs

@@ -0,0 +1,33 @@
+// Copyright 2026 Oxide Computer Company
+
+//! Validate the signatures with which pre and post hooks are invoked.
+
+fn add_header(req: &mut reqwest::Request) {
+    req.headers_mut().insert(
+        "x-pre-hook",
+        reqwest::header::HeaderValue::from_static("present"),
+    );
+}
+
+fn observe(_req: &reqwest::Request) {}
+
+fn observe_result(_result: &Result<reqwest::Response, reqwest::Error>) {}
+
+// The sync pre_hook receives `&mut reqwest::Request` so that it can modify
+// the request before it's sent.
+mod pre_hook_mut {
+    progenitor::generate_api!(
+        spec = "../sample_openapi/keeper.json",
+        pre_hook = crate::add_header,
+        post_hook = crate::observe_result,
+    );
+}
+
+// A pre_hook written against `&reqwest::Request` must continue to compile:
+// the generated call site passes `&mut request`, which coerces to `&_`.
+mod pre_hook_ref {
+    progenitor::generate_api!(
+        spec = "../sample_openapi/keeper.json",
+        pre_hook = crate::observe,
+    );
+}