Proposal: disallow `comptime return` in all functions

[mlugg]

A while ago, I made some changes to the compiler regarding comptime { ... } syntax which led to comptime return being disallowed in runtime functions. This makes logical sense, because you can't return from a runtime function at comptime; that's runtime control flow!

However, this syntax remained allowed for inline functions/calls. At the time, I justified this by the fact that the comptime return value is comptime-known to the caller, so you effectively can do this control flow at comptime.

I have come to realise that my logic was flawed: while the comptime-known result does propagate to the caller, it is not valid to actually perform the control flow at comptime (at least as we implement it today), because this may cause already-analyzed runtime operations to be "skipped", i.e. not actually added to the caller despite having been analyzed. This bug, it turns out, exists in the compiler today:

$ cat repro.zig
pub fn main() void {
    std.debug.print("foo\n", .{});
    inner();
    std.debug.print("qux\n", .{});
}
inline fn inner() void {
    std.debug.print("bar\n", .{});
    comptime return; // the comptime control flow here causes us to lose the runtime operation above
}
const std = @import("std");
$ zig run repro.zig
foo
qux

We could work around this in the compiler; introducing special handling for comptime control flow which pauses to first add instructions to the parent context. However, I believe it would make for a simpler and more logical language specification to simply ban this, as is already the case for non-inline calls.

Note that inline calls are highly analagous to labeled blocks. The equivalent of the above code would be this:

pub fn main() void {
    std.debug.print("foo\n", .{});
    inner: {
        std.debug.print("bar\n", .{});
        comptime break :inner;
    }
    std.debug.print("qux\n", .{});
}
const std = @import("std");

This code actually works today (and prints all three lines of output) due to a quirk in the compiler implementation, but very similar code can easily trigger compiler crashes; this code being accepted is a bug and will eventually become a compile error. I think that's a good demonstration of why the analogous inline fn version should also be a compile error.

[mlugg]

One thing I should clarify: returning in a comptime scope would still be allowed if the function is called at comptime. I'm just proposing extending the ban in normal runtime calls to also include inline runtime calls.