Use niche-filling optimization even when multiple variants have data.

[mikebenfield]

Fixes #46213

[rust-highfive]

Some changes occured to the CTFE / Miri engine

cc @rust-lang/miri

[rust-highfive]

r? @michaelwoerister

(rust-highfive has picked a reviewer for you, use r? to override)

[mikebenfield]

Several static size checks needed the size lowered. I'm unsure how to handle this since they'll fail with a bootstrapped compiler, so I've just commented them out for now.

[oli-obk]

This is amazing! Will review in detail later, but starting a perf run now

[oli-obk]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit 31b86437df15d7b335697d3f971721104335d04e with merge dbc0f1b7dd1014d9c7e56714953a0daf1b4f4c35...

[michaelwoerister]

r? @eddyb -- reviewing this is out of my league ;)

Before merging something like this, we need to make sure that debuginfo generation handles this correctly (especially the cpp-like encoding). cc @wesleywiser

[RalfJung]

Wow that's cool. :D

It would probably be good to also have a ui test ensuring specific cases are handled (like some of the examples from the issue). I imagine there would be an existing ui test that can be extended, but I am not sure.

[mikebenfield]

I added a simple test case.

not(bootstrap) doesn't seem to do the trick; I still get assertion failures with that.

[mikebenfield]

Nevermind; not(bootstrap) works fine.

[RalfJung]

This test case seems to have crept in from another PR...?

[joshtriplett]

This is incredible work! I've wanted to see this level of niche optimization for a long time. I think this makes niche optimization work more like people often expect it'll work.

[jdahlstrom]

Hmm, should the following enum be eligible for niche optimization now (courtesy u/gitpy on Reddit)?

enum Data {
    A(f64),
    B(Buf),
}

struct Buf(*const u8, i16);

fn main() {
    println!("{}", std::mem::size_of::<Buf>());  // 16
    println!("{}", std::mem::size_of::<Data>()); // 24 even on nightly
}

[cuviper]

Padding is not used as a niche -- it could contain anything, including the value that you would want to indicate the other variant.

[jdahlstrom]

@cuviper Ah, right. Amusingly you can now offer the compiler a place that works as a niche, though =)

enum Data {
    A(f64),
    B(Buf),
}
struct Buf(*const u8, i16, bool);
//                       ++++++

println!("{}", std::mem::size_of::<Buf>());  // 16
println!("{}", std::mem::size_of::<Data>()); // 16 on nightly!

[cuviper]

Yep! And if you want even more niche, you can use a univariant enum, with a repr to keep it from being a ZST:

#[repr(u8)] // or bigger!
enum Pad { Empty }