yield-closures

An implementation of MCP-49.

#[test]
fn decode_escape_string() {
    let escaped_text = "Hello,\x20world!\\n";
    let text: String = escaped_text
        .chars()
        .filter_map(co!(|c| {
            loop {
                if c != '\\' {
                    Not escaped
                    yield Some(c);
                    continue;
                }

                Go past the \
                yield None;

                Unescaped-char
                match c {
                    Hexadecimal
                    'x' => {
                        yield None; Go past the x
                        let most = c.to_digit(16);
                        yield None; Go past the first digit
                        let least = c.to_digit(16);
                        Yield the decoded char if valid
                        yield (|| char::from_u32(most? << 4 | least?))()
                    }
                    Simple escapes
                    'n' => yield Some('\n'),
                    'r' => yield Some('\r'),
                    't' => yield Some('\t'),
                    '0' => yield Some('\0'),
                    '\\' => yield Some('\\'),
                    Unnecessary escape
                    _ => yield Some(c),
                }
            }
        }))
        .collect();
    assert_eq!(text, "Hello, world!\n");
}

For the details of the proposal, see https://lang-team.rust-lang.org/design_notes/general_coroutines.html.

Differences between this implementation and the proposal are summarized below:

• This crate offers a macro implementation. It works with the stable Rust.
• No FnPin is provided. Yield closures made with this crate use Box::pin internally and hence FnMut.
• In yield closures, one cannot use return expressions.
• The body of a yield closure must be explosive i.e. must not return and typed by the ! type. Thus it is compatible with both of the two designs of yield closures discussed in the document of MCP-49: poisoning by default or not.