implement std::process::Command::arg_set()

Only unix implementation yet.

Author: cehteh

library/std/src/process.rs

@@ -680,6 +680,40 @@ impl Command {
         self
     }
 
+    /// Sets the argument at index to a new value.
+    ///
+    /// When one wants to restart a Command again with different
+    /// arguments the argument list can to be cleared first.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```no_run
+    /// #![feature(mutate_command_args)]
+    /// use std::process::Command;
+    ///
+    /// // Prepare a command
+    /// let mut command = Command::new("ls");
+    /// command.args(["-l", "-a", "FILE"]);
+    ///
+    /// // Run it with mutated 3rd parameter
+    /// ["foo", "bar", "baz"].iter()
+    ///     .for_each(
+    ///         |file| {
+    ///             command
+    ///                 .arg_set(3, file)
+    ///                 .spawn()
+    ///                 .unwrap();
+    ///         }
+    ///     );
+    /// ```
+    #[unstable(feature = "mutate_command_args", issue = "87379")]
+    pub fn arg_set<S: AsRef<OsStr>>(&mut self, index: usize, value: S) -> &mut Command {
+        self.inner.arg_set(index, value.as_ref());
+        self
+    }
+
     /// Inserts or updates an environment variable mapping.
     ///
     /// Note that environment variable names are case-insensitive (but case-preserving) on Windows,

library/std/src/process/tests.rs

@@ -237,6 +237,42 @@ fn test_args_clear() {
     assert_eq!(stderr, Vec::new());
 }
 
+#[test]
+fn test_arg_set() {
+    let mut prog = Command::new("echo");
+
+    if cfg!(target_os = "windows") {
+        prog.args(&["/C", "echo set_me"])
+    } else {
+        prog.arg("set_me")
+    };
+
+    if cfg!(target_os = "windows") {
+        prog.arg_set(2, "echo hello");
+    } else {
+        prog.arg_set(1, "hello");
+    };
+
+    let Output { status, stdout, stderr } = prog.output().unwrap();
+    let output_str = str::from_utf8(&stdout).unwrap();
+
+    assert!(status.success());
+    assert_eq!(output_str.trim().to_string(), "hello");
+    assert_eq!(stderr, Vec::new());
+}
+
+#[test]
+#[should_panic]
+fn test_arg_set_fail() {
+    let mut prog = Command::new("echo");
+
+    if cfg!(target_os = "windows") {
+        prog.arg_set(1, "echo hello");
+    } else {
+        prog.arg_set(1, "hello");
+    };
+}
+
 #[cfg(all(unix, not(target_os = "android")))]
 pub fn env_cmd() -> Command {
     Command::new("env")

library/std/src/sys/unix/process/process_common.rs

@@ -199,6 +199,13 @@ impl Command {
         self.args.truncate(1);
     }
 
+    pub fn arg_set(&mut self, index: usize, arg: &OsStr) {
+        debug_assert!(index >= 1 && index < self.args.len(), "Index out of range");
+        let arg = os2c(arg, &mut self.saw_nul);
+        self.argv.0[index] = arg.as_ptr();
+        self.args[index] = arg;
+    }
+
     pub fn cwd(&mut self, dir: &OsStr) {
         self.cwd = Some(os2c(dir, &mut self.saw_nul));
     }