Silence unexpected_cfgs error about unknown cygwin target

Extract our own cfg value, to avoid noisy warnings like:

	warning: unexpected `cfg` condition value: `cygwin`
	  --> src/fallback.rs:78:23
	   |
	78 |             #[cfg(not(target_os = "cygwin"))]
	   |                       ^^^^^^^^^^^^^^^^^^^^
	   |

The cygwin target will be added to Rust 1.86, so we can get rid of this
after some time.

build.rs

@@ -90,6 +90,7 @@ fn detect_cfgs(target: &mut Target) {
         ),
         ("apple", &detect_apple),
         ("bsd", &detect_bsd),
+        ("cygwin", &detect_cygwin),
         ("gettext", &have_gettext),
         ("small_main_stack", &has_small_stack),
         // See if libc supports the thread-safe localeconv_l(3) alternative to localeconv(3).
@@ -128,6 +129,11 @@ fn detect_apple(_: &Target) -> Result<bool, Box<dyn Error>> {
     Ok(cfg!(any(target_os = "ios", target_os = "macos")))
 }
 
+#[allow(unexpected_cfgs)]
+fn detect_cygwin(_: &Target) -> Result<bool, Box<dyn Error>> {
+    Ok(cfg!(target_os = "cygwin"))
+}
+
 /// Detect if we're being compiled for a BSD-derived OS, allowing targeting code conditionally with
 /// `#[cfg(bsd)]`.
 ///

src/common.rs

@@ -1072,7 +1072,7 @@ pub fn get_obfuscation_read_char() -> char {
 /// the multiline prompt usable. See [#2859](https://github.com/fish-shell/fish-shell/issues/2859)
 /// and <https://github.com/Microsoft/BashOnWindows/issues/545>
 pub fn has_working_tty_timestamps() -> bool {
-    if cfg!(any(target_os = "windows", target_os = "cygwin")) {
+    if cfg!(any(target_os = "windows", cygwin)) {
         false
     } else if cfg!(target_os = "linux") {
         !is_windows_subsystem_for_linux(WSL::V1)

src/env_dispatch.rs

@@ -199,9 +199,9 @@ fn guess_emoji_width(vars: &EnvStack) {
     } else {
         // Default to whatever the system's wcwidth gives for U+1F603, but only if it's at least
         // 1 and at most 2.
-        #[cfg(not(target_os = "cygwin"))]
+        #[cfg(not(cygwin))]
         let width = crate::fallback::wcwidth('😃').clamp(1, 2);
-        #[cfg(target_os = "cygwin")]
+        #[cfg(cygwin)]
         let width = 2_isize;
         FISH_EMOJI_WIDTH.store(width, Ordering::Relaxed);
         FLOG!(term_support, "default emoji width:", width);

src/fallback.rs

@@ -32,7 +32,7 @@
 static WC_LOOKUP_TABLE: Lazy<WcLookupTable> = Lazy::new(WcLookupTable::new);
 
 /// A safe wrapper around the system `wcwidth()` function
-#[cfg(not(target_os = "cygwin"))]
+#[cfg(not(cygwin))]
 pub fn wcwidth(c: char) -> isize {
     extern "C" {
         pub fn wcwidth(c: libc::wchar_t) -> libc::c_int;
@@ -49,7 +49,7 @@ pub fn fish_wcwidth(c: char) -> isize {
     // The system version of wcwidth should accurately reflect the ability to represent characters
     // in the console session, but knows nothing about the capabilities of other terminal emulators
     // or ttys. Use it from the start only if we are logged in to the physical console.
-    #[cfg(not(target_os = "cygwin"))]
+    #[cfg(not(cygwin))]
     if crate::common::is_console_session() {
         return wcwidth(c);
     }
@@ -75,12 +75,12 @@ pub fn fish_wcwidth(c: char) -> isize {
     let width = WC_LOOKUP_TABLE.classify(c);
     match width {
         WcWidth::NonCharacter | WcWidth::NonPrint | WcWidth::Combining | WcWidth::Unassigned => {
-            #[cfg(not(target_os = "cygwin"))]
+            #[cfg(not(cygwin))]
             {
                 // Fall back to system wcwidth in this case.
                 wcwidth(c)
             }
-            #[cfg(target_os = "cygwin")]
+            #[cfg(cygwin)]
             {
                 // No system wcwidth for UTF-32 on cygwin.
                 0

src/tests/topic_monitor.rs

@@ -38,7 +38,7 @@ fn test_topic_monitor() {
 #[test]
 // FIXME: Does not compile on NetBSD & Cygwin
 // "`*mut sem` cannot be sent between threads safely"
-#[cfg(not(any(target_os = "netbsd", target_os = "cygwin")))]
+#[cfg(not(any(target_os = "netbsd", cygwin)))]
 #[serial]
 fn test_topic_monitor_torture() {
     let _cleanup = test_init();

src/wutil/encoding.rs

@@ -1,7 +1,7 @@
 extern "C" {
-    #[cfg_attr(target_os = "cygwin", link_name = "c32rtomb")]
+    #[cfg_attr(cygwin, link_name = "c32rtomb")]
     pub fn wcrtomb(s: *mut libc::c_char, wc: u32, ps: *mut mbstate_t) -> usize;
-    #[cfg_attr(target_os = "cygwin", link_name = "mbrtoc32")]
+    #[cfg_attr(cygwin, link_name = "mbrtoc32")]
     pub fn mbrtowc(pwc: *mut u32, s: *const libc::c_char, n: usize, p: *mut mbstate_t) -> usize;
 }