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.

Cargo.lock

@@ -1965,6 +1965,10 @@ dependencies = [
  "walkdir",
 ]
 
+[[package]]
+name = "lld-wrapper"
+version = "0.1.0"
+
 [[package]]
 name = "lock_api"
 version = "0.4.1"

Cargo.toml

@@ -36,6 +36,7 @@ members = [
   "src/tools/jsondocck",
   "src/tools/html-checker",
   "src/tools/bump-stage0",
+  "src/tools/lld-wrapper",
 ]
 
 exclude = [

src/bootstrap/compile.rs

@@ -1136,14 +1136,14 @@ impl Step for Assemble {
             // for `-Z gcc-ld=lld`
             let gcc_ld_dir = libdir_bin.join("gcc-ld");
             t!(fs::create_dir(&gcc_ld_dir));
-            builder.copy(
-                &lld_install.join("bin").join(&src_exe),
-                &gcc_ld_dir.join(exe("ld", target_compiler.host)),
-            );
-            builder.copy(
-                &lld_install.join("bin").join(&src_exe),
-                &gcc_ld_dir.join(exe("ld64", target_compiler.host)),
-            );
+            for flavor in ["ld", "ld64"] {
+                let lld_wrapper_exe = builder.ensure(crate::tool::LldWrapper {
+                    compiler: build_compiler,
+                    target: target_compiler.host,
+                    flavor_feature: flavor,
+                });
+                builder.copy(&lld_wrapper_exe, &gcc_ld_dir.join(exe(flavor, target_compiler.host)));
+            }
         }
 
         // Similarly, copy `llvm-dwp` into libdir for Split DWARF. Only copy it when the LLVM

src/bootstrap/dist.rs

@@ -409,11 +409,14 @@ impl Step for Rustc {
                 let rust_lld = exe("rust-lld", compiler.host);
                 builder.copy(&src_dir.join(&rust_lld), &dst_dir.join(&rust_lld));
                 // for `-Z gcc-ld=lld`
-                let gcc_lld_dir = dst_dir.join("gcc-ld");
-                t!(fs::create_dir(&gcc_lld_dir));
-                builder.copy(&src_dir.join(&rust_lld), &gcc_lld_dir.join(exe("ld", compiler.host)));
-                builder
-                    .copy(&src_dir.join(&rust_lld), &gcc_lld_dir.join(exe("ld64", compiler.host)));
+                let gcc_lld_src_dir = src_dir.join("gcc-ld");
+                let gcc_lld_dst_dir = dst_dir.join("gcc-ld");
+                t!(fs::create_dir(&gcc_lld_dst_dir));
+                for flavor in ["ld", "ld64"] {
+                    let exe_name = exe(flavor, compiler.host);
+                    builder
+                        .copy(&gcc_lld_src_dir.join(&exe_name), &gcc_lld_dst_dir.join(&exe_name));
+                }
             }
 
             // Copy over llvm-dwp if it's there

src/bootstrap/tool.rs

@@ -664,6 +664,38 @@ impl Step for Cargo {
     }
 }
 
+#[derive(Debug, Copy, Clone, Hash, PartialEq, Eq)]
+pub struct LldWrapper {
+    pub compiler: Compiler,
+    pub target: TargetSelection,
+    pub flavor_feature: &'static str,
+}
+
+impl Step for LldWrapper {
+    type Output = PathBuf;
+
+    fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
+        run.never()
+    }
+
+    fn run(self, builder: &Builder<'_>) -> PathBuf {
+        let src_exe = builder
+            .ensure(ToolBuild {
+                compiler: self.compiler,
+                target: self.target,
+                tool: "lld-wrapper",
+                mode: Mode::ToolStd,
+                path: "src/tools/lld-wrapper",
+                is_optional_tool: false,
+                source_type: SourceType::InTree,
+                extra_features: vec![self.flavor_feature.to_owned()],
+            })
+            .expect("expected to build -- essential tool");
+
+        src_exe
+    }
+}
+
 macro_rules! tool_extended {
     (($sel:ident, $builder:ident),
        $($name:ident,

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()));
+}