implement std::process::Command::args_clear()

see #87379

Only unix implementation yet.

Author: cehteh

library/std/src/lib.rs

@@ -306,6 +306,7 @@
 #![feature(min_specialization)]
 #![feature(mixed_integer_ops)]
 #![feature(must_not_suspend)]
+#![feature(mutate_command_args)]
 #![feature(needs_panic_runtime)]
 #![feature(negative_impls)]
 #![feature(never_type)]

library/std/src/process.rs

@@ -648,6 +648,38 @@ impl Command {
         self
     }
 
+    /// Clears the argument array.
+    ///
+    /// 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;
+    ///
+    /// let mut lsdir = Command::new("ls");
+    ///
+    /// lsdir
+    ///     .arg("target")
+    ///     .spawn()
+    ///     .expect("ls command failed to start");
+    ///
+    /// lsdir
+    ///     .args_clear()
+    ///     .arg("tests")
+    ///     .spawn()
+    ///     .expect("ls command failed to start");
+    /// ```
+    #[unstable(feature = "mutate_command_args", issue = "87379")]
+    pub fn args_clear(&mut self) -> &mut Command {
+        self.inner.args_clear();
+        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

@@ -211,6 +211,32 @@ fn test_wait_with_output_once() {
     assert_eq!(stderr, Vec::new());
 }
 
+#[test]
+fn test_args_clear() {
+    let mut prog = Command::new("echo");
+
+    if cfg!(target_os = "windows") {
+        prog.args(&["/C", "echo reset_me"])
+    } else {
+        prog.arg("reset_me")
+    };
+
+    prog.args_clear();
+
+    if cfg!(target_os = "windows") {
+        prog.args(&["/C", "echo hello"]);
+    } else {
+        prog.arg("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());
+}
+
 #[cfg(all(unix, not(target_os = "android")))]
 pub fn env_cmd() -> Command {
     Command::new("env")

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

@@ -190,6 +190,15 @@ impl Command {
         self.args.push(arg);
     }
 
+    pub fn args_clear(&mut self) {
+        // resize `argv` to 2 (argv[0] and NULL).
+        self.argv.0.truncate(2);
+        self.argv.0[1] = ptr::null();
+
+        // drop all excess args.
+        self.args.truncate(1);
+    }
+
     pub fn cwd(&mut self, dir: &OsStr) {
         self.cwd = Some(os2c(dir, &mut self.saw_nul));
     }