set default path separator to '/' in MSYS

MSYS and MSYS2 environments (such as Git Bash) have a UNIX like
filesystem which uses '/' as the path separator rather than '\', but
Rust doesn't know about this by default.

On Windows, check the MSYSTEM environment variable and set the default
value of the --path-separator option to '/' for convenience.

There is no similar detection of Cygwin because there seems to be no way
for Rust (and any native Win32) programs to detect that they're being
called from a Cygwin environment. Cygwin users can use a shell
alias/function/script to wrap fd.

Fixes: https://github.com/sharkdp/fd/issues/537

src/filesystem.rs

@@ -1,5 +1,5 @@
 use std::borrow::Cow;
-use std::env::current_dir;
+use std::env;
 use std::ffi::OsStr;
 use std::fs;
 use std::io;
@@ -15,7 +15,7 @@ pub fn path_absolute_form(path: &Path) -> io::Result<PathBuf> {
     }
 
     let path = path.strip_prefix(".").unwrap_or(path);
-    current_dir().map(|path_buf| path_buf.join(path))
+    env::current_dir().map(|path_buf| path_buf.join(path))
 }
 
 pub fn absolute_path(path: &Path) -> io::Result<PathBuf> {
@@ -108,6 +108,23 @@ pub fn strip_current_dir(path: &Path) -> &Path {
     path.strip_prefix(".").unwrap_or(path)
 }
 
+/// Default value for the path_separator, mainly for MSYS/MSYS2, which set the MSYSTEM
+/// environment variable, and we set fd's path separator to '/' rather than Rust's default of '\'.
+///
+/// Returns Some to use a nonstandard path separator, or None to use rust's default on the target
+/// platform.
+pub fn default_path_separator() -> Option<String> {
+    if cfg!(windows) {
+        let msystem = env::var("MSYSTEM").ok()?;
+        match msystem.as_str() {
+            "MINGW64" | "MINGW32" | "MSYS" => Some("/".to_owned()),
+            _ => None,
+        }
+    } else {
+        None
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::strip_current_dir;

src/main.rs

@@ -173,7 +173,9 @@ fn run() -> Result<ExitCode> {
         _ => ansi_colors_support && env::var_os("NO_COLOR").is_none() && interactive_terminal,
     };
 
-    let path_separator = matches.value_of("path-separator").map(|str| str.to_owned());
+    let path_separator = matches
+        .value_of("path-separator")
+        .map_or_else(filesystem::default_path_separator, |s| Some(s.to_owned()));
 
     let ls_colors = if colored_output {
         Some(LsColors::from_env().unwrap_or_else(|| LsColors::from_string(DEFAULT_LS_COLORS)))