From 806a05bb6bcbf5d2643d4d9fb691e20fe0af4d5b Mon Sep 17 00:00:00 2001
From: Benjamin Naecker <ben@oxide.computer>
Date: Wed, 11 Jan 2023 20:04:05 +0000
Subject: [PATCH] Better detection of features necessary for USDT probes

---
 dropshot/build.rs   | 16 +++++++++-------
 dropshot/src/lib.rs | 21 +++++++++------------
 2 files changed, 18 insertions(+), 19 deletions(-)

diff --git a/dropshot/build.rs b/dropshot/build.rs
index db67d2159..4c95a1548 100644
--- a/dropshot/build.rs
+++ b/dropshot/build.rs
@@ -1,4 +1,4 @@
-// Copyright 2022 Oxide Computer Company
+// Copyright 2023 Oxide Computer Company
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -16,12 +16,14 @@
 fn main() {
     println!("cargo:rerun-if-changed=build.rs");
 
-    if version_check::is_min_version("1.59").unwrap_or(false) {
-        println!("cargo:rustc-cfg=usdt_stable_asm");
+    if !version_check::is_min_version("1.59").unwrap_or(false) {
+        println!("cargo:rustc-cfg=usdt_need_asm");
     }
 
-    // Once asm_sym is stablilized, add an additional check so that those
-    // building on macos can use the stable toolchain with any hassle.
-    //
-    // A matching rust-cfg option named `usdt_stable_asm_sym` seems appropriate.
+    #[cfg(target_os = "macos")]
+    if version_check::supports_feature("asm_sym").unwrap_or(false)
+        && !version_check::is_min_version("1.67").unwrap_or(false)
+    {
+        println!("cargo:rustc-cfg=usdt_need_asm_sym");
+    }
 }
diff --git a/dropshot/src/lib.rs b/dropshot/src/lib.rs
index 2a3803c16..984f39383 100644
--- a/dropshot/src/lib.rs
+++ b/dropshot/src/lib.rs
@@ -505,11 +505,12 @@
 //! See the [`RequestInfo`] and [`ResponseInfo`] types for a complete listing
 //! of what's available.
 //!
-//! These probes are implemented via the [`usdt`] crate. They require a nightly
-//! toolchain if built on MacOS (which requires the unstable `asm_sym` feature).
-//! Otherwise a stable compiler >= v1.59 is required in order to present the
-//! necessary features.  Given these constraints, usdt functionality is behind
-//! the feature flag `"usdt-probes"`.
+//! These probes are implemented via the [`usdt`] crate. They may require a
+//! nightly toolchain if built on macOS prior to Rust version 1.66. Otherwise a
+//! stable compiler >= v1.59 is required in order to present the necessary
+//! features. Given these constraints, USDT functionality is behind the feature
+//! flag `"usdt-probes"`, which may become a default feature of this crate in
+//! future releases.
 //!
 //! > *Important:* The probes are internally registered with the DTrace kernel
 //! module, making them visible via `dtrace(1M)`. This is done when an `HttpServer`
@@ -540,15 +541,11 @@
 // Clippy's style advice is definitely valuable, but not worth the trouble for
 // automated enforcement.
 #![allow(clippy::style)]
-// The `usdt` crate requires nightly, enabled if our consumer is enabling
+// The `usdt` crate may requires nightly, enabled if our consumer is enabling
 // DTrace probes.
-#![cfg_attr(all(feature = "usdt-probes", not(usdt_stable_asm)), feature(asm))]
+#![cfg_attr(all(feature = "usdt-probes", usdt_need_asm), feature(asm))]
 #![cfg_attr(
-    all(
-        feature = "usdt-probes",
-        target_os = "macos",
-        not(usdt_stable_asm_sym)
-    ),
+    all(feature = "usdt-probes", target_os = "macos", usdt_need_asm_sym),
     feature(asm_sym)
 )]