Add wrapper for -Z gcc-ld=lld to invoke rust-lld with the correct flavor

The wrapper is installed as `ld` and `ld64` in the `lib\rustlib\<host_target>\bin\gcc-ld`
directory and its sole purpose is to invoke `rust-lld` in the parent directory with
the correct flavor.

src/tools/lld-wrapper/Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+name = "lld-wrapper"
+version = "0.1.0"
+edition = "2021"
+license = "MIT OR Apache-2.0"
+
+[dependencies]
+
+[features]
+ld = []
+ld64 = []

src/tools/lld-wrapper/src/main.rs

@@ -0,0 +1,125 @@
+//! Script to invoke the bundled rust-lld with the correct flavor. The flavor is selected by
+//! feature.
+//!
+//! lld supports multiple command line interfaces. If `-flavor <flavor>` are passed as the first
+//! two arguments the `<flavor>` command line interface is used to process the remaining arguments.
+//! If no `-flavor` argument is present the flavor is determined by the executable name.
+//!
+//! In Rust with `-Z gcc-ld=lld` we have gcc or clang invoke rust-lld. Since there is no way to
+//! make gcc/clang pass `-flavor <flavor>` as the first two arguments in the linker invocation
+//! and since Windows does not support symbolic links for files this wrapper is used in place of a
+//! symblic link. It execs `../rust-lld -flavor ld` if the feature `ld` is enabled and
+//! `../rust-lld -flavor ld64` if `ld64` is enabled. On Windows it spawns a `..\rust-lld.exe`
+//! child process.
+
+#[cfg(not(any(feature = "ld", feature = "ld64")))]
+compile_error!("One of the features ld and ld64 must be enabled.");
+
+#[cfg(all(feature = "ld", feature = "ld64"))]
+compile_error!("Only one of the feature ld or ld64 can be enabled.");
+
+#[cfg(feature = "ld")]
+const FLAVOR: &str = "ld";
+
+#[cfg(feature = "ld64")]
+const FLAVOR: &str = "ld64";
+
+use std::env;
+use std::fmt::Display;
+use std::path::{Path, PathBuf};
+use std::process;
+
+trait ResultExt<T, E> {
+    fn unwrap_or_exit_with(self, context: &str) -> T;
+}
+
+impl<T, E> ResultExt<T, E> for Result<T, E>
+where
+    E: Display,
+{
+    fn unwrap_or_exit_with(self, context: &str) -> T {
+        match self {
+            Ok(t) => t,
+            Err(e) => {
+                eprintln!("lld-wrapper: {}: {}", context, e);
+                process::exit(1);
+            }
+        }
+    }
+}
+
+trait OptionExt<T> {
+    fn unwrap_or_exit_with(self, context: &str) -> T;
+}
+
+impl<T> OptionExt<T> for Option<T> {
+    fn unwrap_or_exit_with(self, context: &str) -> T {
+        match self {
+            Some(t) => t,
+            None => {
+                eprintln!("lld-wrapper: {}", context);
+                process::exit(1);
+            }
+        }
+    }
+}
+
+/// Returns the path to rust-lld in the parent directory.
+///
+/// Exits if the parent directory cannot be determined.
+fn get_rust_lld_path(current_exe_path: &Path) -> PathBuf {
+    let mut rust_lld_exe_name = "rust-lld".to_owned();
+    rust_lld_exe_name.push_str(env::consts::EXE_SUFFIX);
+    let mut rust_lld_path = current_exe_path
+        .parent()
+        .unwrap_or_exit_with("directory containing current executable could not be determined")
+        .parent()
+        .unwrap_or_exit_with("parent directory could not be determined")
+        .to_owned();
+    rust_lld_path.push(rust_lld_exe_name);
+    rust_lld_path
+}
+
+/// Returns the command for invoking rust-lld with the correct flavor.
+///
+/// Exits on error.
+fn get_rust_lld_command(current_exe_path: &Path) -> process::Command {
+    let rust_lld_path = get_rust_lld_path(current_exe_path);
+    let mut command = process::Command::new(rust_lld_path);
+    command.arg("-flavor");
+    command.arg(FLAVOR);
+    command.args(env::args_os().skip(1));
+    command
+}
+
+#[cfg(unix)]
+fn exec_lld(mut command: process::Command) {
+    use std::os::unix::prelude::CommandExt;
+    Result::<(), _>::Err(command.exec()).unwrap_or_exit_with("could not exec rust-lld");
+    unreachable!("lld-wrapper: after exec without error");
+}
+
+#[cfg(not(unix))]
+fn exec_lld(mut command: process::Command) {
+    // Windows has no exec(), spawn a child process and wait for it
+    let exit_status = command.status().unwrap_or_exit_with("error running rust-lld child process");
+    if !exit_status.success() {
+        match exit_status.code() {
+            Some(code) => {
+                // return the original lld exit code
+                process::exit(code)
+            }
+            None => {
+                eprintln!("lld-wrapper: rust-lld child process exited with error: {}", exit_status,);
+                process::exit(1);
+            }
+        }
+    }
+}
+
+fn main() {
+    let current_exe_path =
+        env::current_exe().unwrap_or_exit_with("could not get the path of the current executable");
+
+    exec_lld(get_rust_lld_command(current_exe_path.as_ref()));
+}