Allow any statement in loop bodies

[ifreund]

The following snippet gives a compile error while I think it should be valid Zig:

const std = @import("std");
pub fn main() void {
    var list = std.TailQueue(u32).init();
    var count: u32 = 0;
    var it = list.first;
    while (it) |node| : (it = node.next) if (true) count += 1;
}

<source>:7:42: error: invalid left-hand side to assignment
    while (it) |node| : (it = node.next) if (true) count += 1;
                                         ^

Removing the if (true) or using a {} block for the if/while makes the compile error go away.

[Vexu]

According to the grammar this is parsed as:

    while (it) |node| : (it = node.next) ((if (true) count) += 1);

This could be fixed by making the loop statements accept any statement as their body like C does.

[ifreund]

That makes sense. I find the current behavior to be surprising in this case, though I'd assume there is a good reason this was the grammar chosen.

[ifreund]

This affects defer as well, the following won't compile with the same "invalid left-hand side to assignment" error:

    defer if (startup_process) |child| _ = child.kill() catch
        |e| log.err(.server, "failed to terminate startup process: {}", .{e});

wrapping the body of the if in a block fixes things, but is needlessly verbose:

    defer if (startup_process) |child| {
        _ = child.kill() catch
            |e| log.err(.server, "failed to terminate startup process: {}", .{e});
    };

[pascaldekloe]

test case (won't compile):

test "double for on line" {
    var n: usize = 0;
    var in: []const []const u8 = &.{ "foo", "bar" };
    for (in) |s| for (s) |c| n += c;
}

[rohlem]

I just ran into this with if (true) if (true) x = {}; saying if (true) x is an invalid left-hand side,
so it seems this isn't just about loop bodies but any single-statement control flow.
(Originally two optional-unwrapping if-s: if (a) |x| if (b) |y| z = ...;, so combining the conditions with and isn't as concise.)

[zigo101]

Same?

fn foo() u32 {
	var n:u32 = 0;
	defer if (true) n = 8; // error: invalid left-hand side to assignment
	return n;
}