detect when rearrangement changes the size class

[Emmanuel T Odeke]

Coming here from a suggestion from Josh Bleecher Snyder, we should detect when the rearrangement changes the size class of the struct.

[Cuong Manh Le]

@odeke-em do you refer to the size class that runtime use to allocate memory for struct? https://golang.org/src/runtime/mksizeclasses.go

[Emmanuel T Odeke]

That's right! Please see also see golang/go#42412 (comment)

[Cuong Manh Le]

Look cool!

So when size class changed, what should we report? Or we will report the actual size class instead of the struct size?

[Emmanuel T Odeke]

We should report all of them. Basically:

<LINENO_POS>: struct has size 112 (size class <CLASS_SIZE>), rearrange to <REARRANGEMENT> for optimal size

[Emmanuel T Odeke]

Also we should make sure that the optimal size suggested is an actual size class.

[Emmanuel T Odeke]

For brevity here are the size classes from the Go runtime https://github.com/golang/go/blob/ecc3f5112ebaf23c4b1ac4c5eedfa406d82ecc9a/src/runtime/sizeclasses.go#L6-L73