x/tools/go/analysis/passes/modernize: replace unsafe pointer arithmetic with unsafe.Add(ptr, n)

[adonovan]

@drchase had the idea of a modernizer to replace unsafe pointer arithmetic expressions such as

(*T)(unsafe.Pointer(uintptr(unsafe.Pointer(ptr)) + uintptr(n)))

by calls to helper functions in the unsafe package, such as

unsafe.Add(ptr, n)

We should implement it. Other functions include String, StringData, Slice, and SliceData.

[gopherbot]

Change https://go.dev/cl/725680 mentions this issue: go/analysis/passes/modernize: unsafefuncs: ptr+int => unsafe.Add

[adonovan]

@drchase mentioned these additional examples of desired transformations:

func BytesToReadOnlyString(b []byte) string {
    return *(*string)(unsafe.Pointer(&b))
}

should be
return unsafe.String(unsafe.SliceData(b), b.len())

------------------------------------------------------------

func StringToReadOnlyBytes(s string) (bs []byte) {
    sh := (*reflect.StringHeader)(unsafe.Pointer(&s))
    bh := (*reflect.SliceHeader)(unsafe.Pointer(&bs))
    bh.Data = sh.Data
    bh.Cap = sh.Len
    bh.Len = sh.Len
    return
}

should be
return unsafe.Slice(unsafe.StringData(s), s.len())

------------------------------------------------------------

func bytesToStr(data []byte) string {
    h := (*reflect.SliceHeader)(unsafe.Pointer(&data))
    shdr := reflect.StringHeader{Data: h.Data, Len: h.Len}
    return *(*string)(unsafe.Pointer(&shdr))
}

should be
return unsafe.String(unsafe.SliceData(data), data.len())

------------------------------------------------------------

... // using uintptr and len from string headers to see if data overlaps
    c := (*reflect.StringHeader)(unsafe.Pointer(&p.source.Contents))
    n := (*reflect.StringHeader)(unsafe.Pointer(&name))

    if n.Data >= c.Data && n.Data+uintptr(n.Len) < c.Data+uintptr(c.Len) {
        return js_ast.Ref{SourceIndex: -uint32(n.Len), InnerIndex: uint32(n.Data - c.Data)}
    } else {
        ...
    }