Experimental cygwin support in rustc

Co-authored-by: Ookiineko <chiisaineko@protonmail.com>

compiler/rustc_llvm/build.rs

@@ -255,6 +255,7 @@ fn main() {
     } else if target.contains("haiku")
         || target.contains("darwin")
         || (is_crossed && (target.contains("dragonfly") || target.contains("solaris")))
+        || target.contains("cygwin")
     {
         println!("cargo:rustc-link-lib=z");
     } else if target.contains("netbsd") {

compiler/rustc_session/src/filesearch.rs

@@ -58,7 +58,7 @@ pub fn make_target_bin_path(sysroot: &Path, target_triple: &str) -> PathBuf {
     sysroot.join(rustlib_path).join("bin")
 }
 
-#[cfg(unix)]
+#[cfg(all(unix, not(target_os = "cygwin")))]
 fn current_dll_path() -> Result<PathBuf, String> {
     use std::sync::OnceLock;
 
@@ -132,6 +132,23 @@ fn current_dll_path() -> Result<PathBuf, String> {
         .clone()
 }
 
+#[cfg(target_os = "cygwin")]
+fn current_dll_path() -> Result<PathBuf, String> {
+    use std::ffi::{CStr, OsStr};
+    use std::os::unix::prelude::*;
+
+    unsafe {
+        let addr = current_dll_path as usize as *mut _;
+        let mut info = std::mem::zeroed();
+        if libc::dladdr(addr, &mut info) == 0 {
+            return Err("dladdr failed".into());
+        }
+        let bytes = CStr::from_ptr(info.dli_fname.as_ptr()).to_bytes();
+        let os = OsStr::from_bytes(bytes);
+        Ok(PathBuf::from(os))
+    }
+}
+
 #[cfg(windows)]
 fn current_dll_path() -> Result<PathBuf, String> {
     use std::ffi::OsString;

src/bootstrap/src/core/builder/mod.rs

@@ -1388,7 +1388,7 @@ impl<'a> Builder<'a> {
         // Windows doesn't need dylib path munging because the dlls for the
         // compiler live next to the compiler and the system will find them
         // automatically.
-        if cfg!(windows) {
+        if cfg!(any(windows, target_os = "cygwin")) {
             return;
         }
 

src/bootstrap/src/utils/helpers.rs

@@ -130,7 +130,7 @@ pub fn is_debug_info(name: &str) -> bool {
 /// Returns the corresponding relative library directory that the compiler's
 /// dylibs will be found in.
 pub fn libdir(target: TargetSelection) -> &'static str {
-    if target.is_windows() { "bin" } else { "lib" }
+    if target.is_windows() || target.contains("cygwin") { "bin" } else { "lib" }
 }
 
 /// Adds a list of lookup paths to `cmd`'s dynamic library lookup path.

src/bootstrap/src/utils/shared_helpers.rs

@@ -20,7 +20,7 @@ use std::str::FromStr;
 /// Returns the environment variable which the dynamic library lookup path
 /// resides in for this platform.
 pub fn dylib_path_var() -> &'static str {
-    if cfg!(target_os = "windows") {
+    if cfg!(any(target_os = "windows", target_os = "cygwin")) {
         "PATH"
     } else if cfg!(target_vendor = "apple") {
         "DYLD_LIBRARY_PATH"
@@ -46,7 +46,7 @@ pub fn dylib_path() -> Vec<std::path::PathBuf> {
 /// Given an executable called `name`, return the filename for the
 /// executable for a particular target.
 pub fn exe(name: &str, target: &str) -> String {
-    if target.contains("windows") {
+    if target.contains("windows") || target.contains("cygwin") {
         format!("{name}.exe")
     } else if target.contains("uefi") {
         format!("{name}.efi")