stdout/stderr should be flushed when reading from stdin

[barosl]

As stdout is flushed only when a newline character is printed, the following program does not work properly:

print!("Input a text: ");

let mut text = String::new();
stdin().read_line(&mut text).unwrap();

println!("Text: {}", text);

Therefore, it is required to flush explicitly like stdout().flush().unwrap(). #23818 suggested always calling flush if anything is printed, but it is probably not acceptable due to the performance concerns.

However, at least C and Python seem to have an implicit flushing behavior when scanf()/input() is called, so the following works as expected:

print('Input a text: ', end='')

text = input()

print('Text: {}'.format(text))

printf("Input a text: ");

char text[1024];
fgets(text, sizeof(text), stdin);

printf("Text: %s", text);

It would also be a good idea to adopt a similar strategy in Rust.

[emberian]

This seems like a decent "Do What I Mean" fix.

[nagisa]

We used to do this, but do not anymore (#23087) because it used to be a very easy way to cause a deadlock. Since stdio now uses reentrant mutexes (#24029), I believe this functionality can could be introduced again.

[alexcrichton]

The associated PR for this issue was closed, so I'm going to close this as well.