Change SwitchTarget representation in StableMIR

[celinval]

The new structure encodes its invariant, which reduces the likelihood of having an inconsistent representation. It is also more intuitive and user friendly.

I encapsulated the structure for now in case we decide to change it back.

Notes:

1. I had to change the Successors type, since there's a conflict on the iterator type. We could potentially implement an iterator here, but I would prefer keeping it simple for now, and add a successors_iter() method if needed.
2. I removed CoroutineDrop for now since it we never create it. We can add it when we add support to other MIR stages.

[rustbot]

r? @ouz-a

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[rustbot]

This PR changes Stable MIR

cc @oli-obk, @celinval, @spastorino, @ouz-a

[ouz-a]

Thanks! This looks good, have you tested if we are correctly printing otherwise ?

[celinval]

It looks correct. The following function:

pub fn demux(input: u8) -> u8 {
    match input {
        0 => 10,
        1 => 6,
        2 => 8,
        _ => 0,
    }
}

prints the following:

fn demux(_0: u8) -> u8 {
}
    bb0: {
        switchInt(_1) -> [0: bb1, 1: bb2, 2: bb3, otherwise: bb4]
    }
    bb1: {
        _0 = const 0_u8
        goto -> 5
    }
    bb2: {
        _0 = const 10_u8
        goto -> 5
    }
    bb3: {
        _0 = const 6_u8
        goto -> 5
    }
    bb4: {
        _0 = const 8_u8
        goto -> 5
    }
    bb5: {
        return
    }

I believe the issues with this output are unrelated to this change, and you are addressing them in #118364, right?

[ouz-a]

@bors r+ rollup

[bors]

📌 Commit 9242c5e has been approved by ouz-a

It is now in the queue for this repository.

[celinval]

After inspecting the output, I realized the basic blocks order is incorrect.

@bors r-

[celinval]

The same function:

pub fn demux(input: u8) -> u8 {
    match input {
        0 => 10,
        1 => 6,
        2 => 8,
        _ => 0,
    }
}

now prints:

fn demux(_0: u8) -> u8 {
}
    bb0: {
        switchInt(_1) -> [0: bb2, 1: bb3, 2: bb4, otherwise: bb1]
    }
    bb1: {
        _0 = const 0_u8
        goto -> 5
    }
    bb2: {
        _0 = const 10_u8
        goto -> 5
    }
    bb3: {
        _0 = const 6_u8
        goto -> 5
    }
    bb4: {
        _0 = const 8_u8
        goto -> 5
    }
    bb5: {
        return
    }

[ouz-a]

@bors r+ rollup

[bors]

📌 Commit 9d2c923 has been approved by ouz-a

It is now in the queue for this repository.

[bors]

⌛ Testing commit 9d2c923 with merge c263ccf...

[bors]

☀️ Test successful - checks-actions
Approved by: ouz-a
Pushing c263ccf to master...

[rust-timer]

Finished benchmarking commit (c263ccf): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	2.7%	[2.7%, 2.7%]	1
Regressions ❌ (secondary)	2.3%	[2.3%, 2.3%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.1%	[-2.1%, -2.1%]	1
All ❌✅ (primary)	2.7%	[2.7%, 2.7%]	1

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 673.779s -> 673.487s (-0.04%)
Artifact size: 313.43 MiB -> 313.39 MiB (-0.01%)