Handle ASM features more gracefully

usdt-impl/build.rs

@@ -0,0 +1,90 @@
+// Copyright 2022 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.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+use std::env;
+
+use version_check;
+
+#[derive(Copy, Clone)]
+enum Backend {
+    // Standard (read: illumos) probe registration
+    Standard,
+    // MacOS linker-aware probe registration
+    Linker,
+    // Provide probe macros, but probes are no-ops (dtrace-less OSes)
+    NoOp,
+}
+
+fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
+
+    // `asm` feature was stabilized in 1.59
+    let have_stable_asm = version_check::is_min_version("1.59").unwrap_or(false);
+    // XXX: `asm_sym` feature is not yet stable
+    let have_stable_asm_sym = false;
+
+    // Are we being built with a compiler which allows feature flags (nightly)
+    let is_nightly = version_check::is_feature_flaggable().unwrap_or(false);
+
+    let feat_asm = env::var_os("CARGO_FEATURE_ASM").is_some();
+    let feat_strict_asm = env::var_os("CARGO_FEATURE_STRICT_ASM").is_some();
+
+    let backend = match env::var("CARGO_CFG_TARGET_OS").ok().as_deref() {
+        Some("macos") if feat_asm => {
+            if have_stable_asm && have_stable_asm_sym {
+                Backend::Linker
+            } else if feat_strict_asm || is_nightly {
+                if !have_stable_asm {
+                    println!("cargo:rustc-cfg=usdt_need_feat_asm");
+                }
+                if !have_stable_asm_sym {
+                    println!("cargo:rustc-cfg=usdt_need_feat_asm_sym");
+                }
+                Backend::Linker
+            } else {
+                Backend::NoOp
+            }
+        }
+        Some("illumos") | Some("solaris") if feat_asm => {
+            if have_stable_asm {
+                Backend::Standard
+            } else if feat_strict_asm || is_nightly {
+                println!("cargo:rustc-cfg=usdt_need_feat_asm");
+                Backend::Standard
+            } else {
+                Backend::NoOp
+            }
+        }
+        _ => Backend::NoOp,
+    };
+
+    // Since visibility of the `asm!()` macro differs between the nightly feature and the
+    // stabilized version, the consumer requires information about its availability
+    if have_stable_asm {
+        println!("cargo:rustc-cfg=usdt_stable_asm");
+    }
+
+    match backend {
+        Backend::NoOp => {
+            println!("cargo:rustc-cfg=usdt_backend_noop");
+        }
+        Backend::Linker => {
+            println!("cargo:rustc-cfg=usdt_backend_linker");
+        }
+        Backend::Standard => {
+            println!("cargo:rustc-cfg=usdt_backend_standard");
+        }
+    }
+}

usdt-impl/src/lib.rs

@@ -14,36 +14,27 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#![cfg_attr(all(not(usdt_stable_asm), feature = "asm"), feature(asm))]
+#![cfg_attr(usdt_need_feat_asm, feature(asm))]
+#![cfg_attr(usdt_need_feat_asm_sym, feature(asm_sym))]
 
 use serde::Deserialize;
 use std::cell::RefCell;
 use thiserror::Error;
 
-#[cfg(all(
-    feature = "asm",
-    any(
-        all(not(target_os = "linux"), not(target_os = "macos")),
-        feature = "des",
-    )
-))]
+// Probe record parsing required for standard backend (and `des` feature used by `dusty util)
+#[cfg(any(usdt_backend_standard, feature = "des"))]
 pub mod record;
 
-#[cfg_attr(any(target_os = "linux", not(feature = "asm")), allow(dead_code))]
-mod common;
-
-#[cfg_attr(
-    feature = "asm",
-    cfg_attr(target_os = "linux", path = "empty.rs"),
-    cfg_attr(target_os = "macos", path = "linker.rs"),
-    cfg_attr(
-        all(not(target_os = "linux"), not(target_os = "macos")),
-        path = "no-linker.rs"
-    )
-)]
-#[cfg_attr(not(feature = "asm"), path = "empty.rs")]
+#[cfg_attr(usdt_backend_noop, path = "empty.rs")]
+#[cfg_attr(usdt_backend_linker, path = "linker.rs")]
+#[cfg_attr(usdt_backend_standard, path = "no-linker.rs")]
 mod internal;
 
+// Since the `empty` is mostly a no-op, parts of the common code will go unused when it is
+// selected for use.
+#[cfg_attr(usdt_backend_noop, allow(dead_code))]
+mod common;
+
 /// Register an application's probe points with DTrace.
 ///
 /// This function collects information about the probe points defined in an application and ensures