implement `@expect` builtin

[Rexicon226]

closes #489

Why @expect takes a bool:

• @expect affects branching codegen and you can only branch on a bool.
• Expecting a specific value can be done with unreachable with respect to the optimizer.

Why @expect doesn't have a probability:

• Probabilities don't make a practical difference in codegen vs. binary.
• The switch usecase can't reliably predict codegen (could be a jump table, some could be comparisons, or it could be a mixture of both)

[daurnimator]

Probabilities don't make a practical difference in codegen vs. binary.

Are you saying that LLVM's expect.with.probability doesn't do anything?

[kprotty]

Expect with probability seems to affect switch cases but it's unclear how it does so practically compared to if.

[Techatrix]

When there is no expected value nor probability, wouldn't a name like @likely instead of @expect be more fitting so that this builtin doesn't get mixed up to be an assertion or function like std.testing.expect?

[notcancername]

For purposes of consistency with C (compiler __builtin_expect vs the common #define LIKELY(x) __builtin_expect(x, 1)), it seems better to name it likely.

[Rexicon226]

I don't want likely as it implies an unlikely. Note that the only real purpose of this builtin is the unlikely version, and unlikely is a bit ugly imo.

I will change it up to accept a second field for true or false instead and keep expect as per @kprotty suggestion when we talked about it.

[notcancername]

Since the user can implement a likely or unlikely pretty easily themselves when a second field is added:

inline fn likely(x: bool) bool {
    return @expect(x, true);
}
inline fn unlikely(x: bool) bool {
    return @expect(x, false);
}

I think this would be the best solution. Thanks! :)

[notcancername]

Should likely and unlikely functions be part of the standard library? On one hand, they're one-liners any user can easily write, on the other hand, everyone writing their likely and unlikely functions is worse, a likely/unlikely micropackage would also suck. Discuss.

[Rexicon226]

Thought about that yesterday, but I think making @expect take a second "expected" bool supports both likely and unlikely without a wrapper.

[Rexicon226]

What about a nice sugary @expect(operand: anytype, expected: anytype) and add a check for @typeOf(operand) == @typeOf(expected)?

No, this will defeat the purpose of this builtin.

[Snektron]

nobody @expects the Spanish inquisition!!

[andrewrk]

Reverted in 9be8a9000faead40b1aec4877506ff10b066659c. I would like a chance to review this please.