fix: Do not try to connect via postcard to proc-macro-srv

[Veykril]

Closes #21130

[Veykril]

cc @Shourya742 seems like if we try to connect to a proc-macro server that does not support postcard we will just hang on it now crashing as expected, we'll need to figure out a different approach here.

[Shourya742]

cc @Shourya742 seems like if we try to connect to a proc-macro server that does not support postcard we will just hang on it now crashing as expected, we'll need to figure out a different approach here.

I guess I know the reason for this, the json framing uses read_line, which blocks until it sees a newline. That means if the server doesn’t send one, we end up stuck waiting indefinitely. The problematic bit is here:

fn read<'a, R: BufRead>(
    inp: &mut R,
    buf: &'a mut String,
) -> io::Result<Option<&'a mut String>> {
    loop {
        buf.clear();

        inp.read_line(buf)?;
        buf.pop(); // Remove trailing '\n'

When we intialize the client with postcard, we don't write new line as the delimitor. I’ll send a patch.

[Veykril]

So looking at things, the --format flag does seem to validate its inputs on the latest nightly, so we might just want to hold off a bit. Do a toolchain version check and then retry things here. The postcard protocol not using lineendings seems fine to me after all

[Shourya742]

So looking at things, the --format flag does seem to validate its inputs on the latest nightly, so we might just want to hold off a bit. Do a toolchain version check and then retry things here. The postcard protocol not using lineendings seems fine to me after all

I was thinking that if we don’t receive a response within a specified time, we should abort the process and fall back.

[Veykril]

that would work too and is definitely something we should implement.

I'll revert some parts here for now to unblock the pre-releases. And I'll also include #21132. It was a mistake not naming both of these flags legacy i think since we do want to replace the protocol entirely at some point.

[Shourya742]

ACK