fix(renderer): silence release-build warnings from debug-only validation

crates/renderer/src/instance.rs

@@ -2,7 +2,11 @@
 
 use crate::error::RendererError;
 use ash::{Entry, Instance, ext, vk};
-use std::ffi::{CStr, c_char};
+use std::ffi::c_char;
+// `CStr` and the tracing macros are used only by the debug-build validation callback.
+#[cfg(debug_assertions)]
+use std::ffi::CStr;
+#[cfg(debug_assertions)]
 use tracing::{debug, error, info, warn};
 
 /// Creates a Vulkan instance and optionally a debug messenger.
@@ -21,7 +25,22 @@ pub fn create_instance(
     ),
     RendererError,
 > {
+    // The validation extension and layer are pushed only in debug builds, so in release builds these vectors are never mutated after initialization.
+    #[cfg_attr(
+        not(debug_assertions),
+        expect(
+            unused_mut,
+            reason = "the debug-only block below mutates these vectors"
+        )
+    )]
     let mut extensions = required_extensions.to_vec();
+    #[cfg_attr(
+        not(debug_assertions),
+        expect(
+            unused_mut,
+            reason = "the debug-only block below mutates these vectors"
+        )
+    )]
     let mut layers = Vec::new();
 
     #[cfg(debug_assertions)]
@@ -70,6 +89,7 @@ pub fn create_instance(
 /// # Safety
 ///
 /// Invoked by the Vulkan loader, which must pass a valid `p_callback_data` pointer whose `p_message` is either null or a valid NUL-terminated C string. Not to be called directly.
+#[cfg(debug_assertions)]
 unsafe extern "system" fn vulkan_debug_callback(
     message_severity: vk::DebugUtilsMessageSeverityFlagsEXT,
     _message_type: vk::DebugUtilsMessageTypeFlagsEXT,