Switch to some alternatives for `impl Executor for Exec`

[weihanglo]

The Executor trait was never meant to use as a public API. A pull request from who wrote that tries to remove it (rust-lang/cargo#11428).

I looked into the usage in this repo. It seems that the most outstanding usage is suppressing some message from rustc. Really sorry that at this moment I cannot foresee any possible alternatives. Is it good for you if Cargo removes the API in the next few versions?

cargo-c/src/build.rs

Lines 825 to 835 in 8421f2d

	if msg.level == "note" {
	if msg.message.starts_with("Link against the following native artifacts when linking against this static library") {
	Ok(())
	} else if let Some(link_line) = msg.message.strip_prefix("native-static-libs:") {
	self.link_line.lock().unwrap().insert(id, link_line.to_string());
	Ok(())
	} else {
	on_stderr_line(s)
	}
	} else {
	on_stderr_line(s)

[lu-zero]

It would break completely what I'm doing there and it is not suppressing messages but recover the native-static-libs from cargo.

[weihanglo]

I see. Thank you for your promptly response! I'll keep it there until we find a better approach.