Experimental cygwin support in rustc

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

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")