x/tools/gopls: add analyzer to simplify slice to array conversions

[dsnet]

Go version

go1.23.1

Output of go env in your module/workspace:

n/a

What did you do?

Run gofmt -s

What did you see happen?

Nothing.

What did you expect to see?

Rewrite of *(*logid.PublicID)(buf) to logid.PublicID(buf). Where logid.PublicID is an [...]byte type and buf is an []byte.

Go 1.17 introduced conversion of slices to array pointers, which resulted in code of the *(*A)(s) pattern. Go 1.20 introduced direct conversion of a slice to an array, thus making the *(*A)(s) pattern redundant.

[mateusz834]

Rewrite of *(*logid.PublicID)(buf) to logid.PublicID(buf). Where logid.PublicID is an [...]byte type and buf is an []byte.

I might be wrong, but this change requires type information, but the gofmt currently is only executed on the AST, so it might not be possible to do so, right?

[dsnet]

Ah, yes. I believe you are correct. This transformation does indeed require type information.

[cherrymui]

From the discussion this sounds infeasible (with the current design)?

cc @griesemer @mvdan to decide.

[zigo101]

*(*logid.PublicID)(buf) is not always equivalent to logid.PublicID(buf), at least when it is used as a L-value.

[zigo101]

I'm not sure whether or not the (*(*[2]int)(x)) expression in the following code should be a L-values.
But the code compiles, which indicates they are L-values.

package main

import (
	"fmt"
)

func main() {
	{
		var x = []int{1, 2}
		for i, v := range (*(*[2]int)(x))[:] {
			x[1] = 9
			if (i == 1) {
				fmt.Println(v) // 9
			}
		}
	}
	{
		var x = []int{1, 2}
		var p = &(*(*[2]int)(x))[1]
		*p = 9
		fmt.Println(x[1]) // 9
	}
}

[adonovan]

While not appropriate for vet, this could be added to one of the "simplify" analyzers within gopls. I'll change the title.

To recap, the pattern is:

var slice []T
const k
*(*[k]T)(slice)  =>  [k]T(slice)  // when used as rvalue