allow using the system-wide llvm-libunwind as the unwinder

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>

config.toml.example

@@ -527,7 +527,8 @@ changelog-seen = 2
 #test-compare-mode = false
 
 # Use LLVM libunwind as the implementation for Rust's unwinder.
-#llvm-libunwind = false
+# Accepted values are 'in-tree' (formerly true), 'system' or 'no' (formerly false).
+#llvm-libunwind = 'no'
 
 # Enable Windows Control Flow Guard checks in the standard library.
 # This only applies from stage 1 onwards, and only for Windows targets.

library/std/Cargo.toml

@@ -61,6 +61,7 @@ profiler = ["profiler_builtins"]
 compiler-builtins-c = ["alloc/compiler-builtins-c"]
 compiler-builtins-mem = ["alloc/compiler-builtins-mem"]
 llvm-libunwind = ["unwind/llvm-libunwind"]
+system-llvm-libunwind = ["unwind/system-llvm-libunwind"]
 
 # Make panics and failed asserts immediately abort without formatting any message
 panic_immediate_abort = ["core/panic_immediate_abort"]

library/test/Cargo.toml

@@ -27,6 +27,7 @@ backtrace = ["std/backtrace"]
 compiler-builtins-c = ["std/compiler-builtins-c"]
 compiler-builtins-mem = ["std/compiler-builtins-mem"]
 llvm-libunwind = ["std/llvm-libunwind"]
+system-llvm-libunwind = ["std/system-llvm-libunwind"]
 panic-unwind = ["std/panic_unwind"]
 panic_immediate_abort = ["std/panic_immediate_abort"]
 profiler = ["std/profiler"]

library/unwind/Cargo.toml

@@ -23,3 +23,4 @@ cc = { version = "1.0.1" }
 
 [features]
 llvm-libunwind = []
+system-llvm-libunwind = []

library/unwind/src/lib.rs

@@ -45,10 +45,24 @@ extern "C" {}
 // When building with crt-static, we get `gcc_eh` from the `libc` crate, since
 // glibc needs it, and needs it listed later on the linker command line. We
 // don't want to duplicate it here.
-#[cfg(all(target_os = "linux", target_env = "gnu", not(feature = "llvm-libunwind")))]
+#[cfg(all(
+    target_os = "linux",
+    target_env = "gnu",
+    not(feature = "llvm-libunwind"),
+    not(feature = "system-llvm-libunwind")
+))]
 #[link(name = "gcc_s", cfg(not(target_feature = "crt-static")))]
 extern "C" {}
 
+#[cfg(all(
+    target_os = "linux",
+    target_env = "gnu",
+    not(feature = "llvm-libunwind"),
+    feature = "system-llvm-libunwind"
+))]
+#[link(name = "unwind", cfg(not(target_feature = "crt-static")))]
+extern "C" {}
+
 #[cfg(target_os = "redox")]
 #[link(name = "gcc_eh", kind = "static-nobundle", cfg(target_feature = "crt-static"))]
 #[link(name = "gcc_s", cfg(not(target_feature = "crt-static")))]

src/bootstrap/config.rs

@@ -10,6 +10,7 @@ use std::ffi::OsString;
 use std::fmt;
 use std::fs;
 use std::path::{Path, PathBuf};
+use std::str::FromStr;
 
 use crate::cache::{Interned, INTERNER};
 use crate::flags::Flags;
@@ -65,7 +66,7 @@ pub struct Config {
     pub rustc_error_format: Option<String>,
     pub json_output: bool,
     pub test_compare_mode: bool,
-    pub llvm_libunwind: bool,
+    pub llvm_libunwind: Option<LlvmLibunwind>,
 
     pub on_fail: Option<String>,
     pub stage: u32,
@@ -177,6 +178,32 @@ pub struct Config {
     pub out: PathBuf,
 }
 
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub enum LlvmLibunwind {
+    No,
+    InTree,
+    System,
+}
+
+impl Default for LlvmLibunwind {
+    fn default() -> Self {
+        Self::No
+    }
+}
+
+impl FromStr for LlvmLibunwind {
+    type Err = String;
+
+    fn from_str(value: &str) -> Result<Self, Self::Err> {
+        match value {
+            "no" => Ok(Self::No),
+            "in-tree" => Ok(Self::InTree),
+            "system" => Ok(Self::System),
+            invalid => Err(format!("Invalid value '{}' for rust.llvm-libunwind config.", invalid)),
+        }
+    }
+}
+
 #[derive(Debug, Copy, Clone, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub struct TargetSelection {
     pub triple: Interned<String>,
@@ -457,7 +484,7 @@ struct Rust {
     remap_debuginfo: Option<bool>,
     jemalloc: Option<bool>,
     test_compare_mode: Option<bool>,
-    llvm_libunwind: Option<bool>,
+    llvm_libunwind: Option<String>,
     control_flow_guard: Option<bool>,
     new_symbol_mangling: Option<bool>,
 }
@@ -799,7 +826,9 @@ impl Config {
             set(&mut config.rust_rpath, rust.rpath);
             set(&mut config.jemalloc, rust.jemalloc);
             set(&mut config.test_compare_mode, rust.test_compare_mode);
-            set(&mut config.llvm_libunwind, rust.llvm_libunwind);
+            config.llvm_libunwind = rust
+                .llvm_libunwind
+                .map(|v| v.parse().expect("failed to parse rust.llvm-libunwind"));
             set(&mut config.backtrace, rust.backtrace);
             set(&mut config.channel, rust.channel);
             set(&mut config.rust_dist_src, rust.dist_src);

src/bootstrap/configure.py

@@ -65,7 +65,7 @@ def v(*args):
 v("llvm-cxxflags", "llvm.cxxflags", "build LLVM with these extra compiler flags")
 v("llvm-ldflags", "llvm.ldflags", "build LLVM with these extra linker flags")
 
-o("llvm-libunwind", "rust.llvm-libunwind", "use LLVM libunwind")
+v("llvm-libunwind", "rust.llvm-libunwind", "use LLVM libunwind")
 
 # Optimization and debugging options. These may be overridden by the release
 # channel, etc.

src/bootstrap/lib.rs

@@ -121,7 +121,7 @@ use std::os::windows::fs::symlink_file;
 use build_helper::{mtime, output, run, run_suppressed, t, try_run, try_run_suppressed};
 use filetime::FileTime;
 
-use crate::config::TargetSelection;
+use crate::config::{LlvmLibunwind, TargetSelection};
 use crate::util::{exe, libdir, CiEnv};
 
 mod builder;
@@ -537,8 +537,10 @@ impl Build {
     fn std_features(&self) -> String {
         let mut features = "panic-unwind".to_string();
 
-        if self.config.llvm_libunwind {
-            features.push_str(" llvm-libunwind");
+        match self.config.llvm_libunwind.unwrap_or_default() {
+            LlvmLibunwind::InTree => features.push_str(" llvm-libunwind"),
+            LlvmLibunwind::System => features.push_str(" system-llvm-libunwind"),
+            LlvmLibunwind::No => {}
         }
         if self.config.backtrace {
             features.push_str(" backtrace");