Only use cygpath on shebang if it contains /

On Windows, skip conversion if a shebang path does not include `/`. In
this case it is not a Unix path, and does not need to be converted to a
Windows path before running.

README.adoc

@@ -694,6 +694,25 @@ Yo from a shell script!
 Hello from ruby!
 ```
 
+==== Shebang Recipe Execution on Windows
+
+On Windows, shebang interpreter paths containing a `/` are translated from Unix-style
+paths to Windows-style paths using `cygpath`, a utility that ships with http://www.cygwin.com[Cygwin].
+
+For example, to execute this recipe on Windows:
+
+```make
+echo:
+  #!/bin/sh
+
+  echo "Hello!"
+```
+
+The interpreter path `/bin/sh` will be translated to a Windows-style path using
+`cygpath` before being executed.
+
+If the interpreter path does not contain a `/` and does not start with a `~`, it will be executed without being translated. This is useful if `cygpath` is not available, or you wish to use a Windows style path to the interpreter.
+
 === Multi-line Constructs
 
 Recipes without an initial shebang are evaluated and run line-by-line, which means that multi-line constructs probably won't do what you want.

src/platform.rs

@@ -53,19 +53,28 @@ impl PlatformInterface for Platform {
     command: &str,
     argument: Option<&str>,
   ) -> Result<Command, OutputError> {
-    // Translate path to the interpreter from unix style to windows style
-    let mut cygpath = Command::new("cygpath");
-    cygpath.current_dir(working_directory);
-    cygpath.arg("--windows");
-    cygpath.arg(command);
-
-    let mut cmd = Command::new(output(cygpath)?);
+    // If the path contains forward slashes…
+    let command = if command.contains('/') || command.starts_with('~') {
+      // …translate path to the interpreter from unix style to windows style
+      let mut cygpath = Command::new("cygpath");
+      cygpath.current_dir(working_directory);
+      cygpath.arg("--windows");
+      cygpath.arg(command);
+
+      Cow::Owned(output(cygpath)?)
+    } else {
+      // otherwise use it as-is
+      Cow::Borrowed(command)
+    };
+
+    let mut cmd = Command::new(command.as_ref());
 
     cmd.current_dir(working_directory);
 
     if let Some(argument) = argument {
       cmd.arg(argument);
     }
+
     cmd.arg(path);
     Ok(cmd)
   }

tests/integration.rs

@@ -2635,3 +2635,15 @@ test! {
   status: 127,
   shell: false,
 }
+
+#[cfg(windows)]
+test! {
+  name: windows_interpreter_path_no_base
+  justfile: r#"
+    foo:
+      #!powershell
+
+      exit 0
+  "#,
+  args: (),
+}