Auto merge of #140154 - Berrysoft:cygwin-host, r=jieyouxu

Cygwin support in rustc

This PR builds host rustc targeting cygwin.

- [x] https://github.com/rust-lang/stacker/pull/122
- [x] https://github.com/nagisa/rust_libloading/pull/173
- [x] https://github.com/Detegr/rust-ctrlc/pull/131
- [x] https://github.com/rust-random/getrandom/pull/654
- [x] https://github.com/msys2/MSYS2-packages/issues/5350
- [x] https://github.com/rust-lang/rust/pull/140886
- [x] https://github.com/rust-lang/rust/pull/140921
- [x] https://github.com/rust-lang/rust/pull/140973

Currently supported:
* rustc
* rustdoc
* rustfmt
* clippy

Blocking:
* cargo: blocked by https://github.com/rust-lang/socket2/pull/568
* rust-analyzer: needs `cargo update`, fixed upstream

```
$ rustc --version --verbose
rustc 1.88.0-dev
binary: rustc
commit-hash: unknown
commit-date: unknown
host: x86_64-pc-cygwin
release: 1.88.0-dev
LLVM version: 20.1.4
```

3 files changed, 3 insertions, 3 deletions

diff --git a/src/bootstrap/src/core/builder/mod.rs b/src/bootstrap/src/core/builder/mod.rs
index cc4fa953ddc..af3e3cc37b9 100644
--- a/src/bootstrap/src/core/builder/mod.rs
+++ b/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;
         }
 
diff --git a/src/bootstrap/src/utils/helpers.rs b/src/bootstrap/src/utils/helpers.rs
index b29c1fb3889..f2c3e8c0df4 100644
--- a/src/bootstrap/src/utils/helpers.rs
+++ b/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.
diff --git a/src/bootstrap/src/utils/shared_helpers.rs b/src/bootstrap/src/utils/shared_helpers.rs
index 1297a53d488..08e1c21e58e 100644
--- a/src/bootstrap/src/utils/shared_helpers.rs
+++ b/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"