proposal: crypto/sha256: satisfy io.StringWriter

[hummerd]

Hi, everyone!
Let's implement io.StringWriter for sha256 digest.

// Yes this looks dangerous
// But there is no way for d.Write(p) to change p (now or ever) 
// since this will break backward compatability, also we can guarantee
// imutability of s with test
func (d *digest) WriteString(s string) (nn int, err error) {
	hString := (*reflect.StringHeader)(unsafe.Pointer(&s))
	hSlice := &reflect.SliceHeader{
		Data: hString.Data,
		Len:  hString.Len,
		Cap:  hString.Len,
	}
	p := *(*[]byte)(unsafe.Pointer(hSlice))
	return d.Write(p)
}

This will let us execute this benchmarks 17% faster and with 0 allocs/op (tested with go 13.1)

var testString string = "some very very long string that will cause allocation, when converted to []byte"

// regular string hashing
func BenchmarkHash(b *testing.B) {
	h := sha256.New()

	for i := 0; i < b.N; i++ {
		_, _ = h.Write([]byte(testString))
	}
}

// string hashing with io.StringWriter
func BenchmarkHashString(b *testing.B) {
	h := sha256.New()
	sw := h.(io.StringWriter)

	for i := 0; i < b.N; i++ {
		_, _ = sw.WriteString(testString)
	}
}

[josharian]

Also proposed at #14757

[ainar-g]

This is wildly unsafe:

p := *(*[]byte)(unsafe.Pointer(&s))

See unsafe rule №1:

(1) Conversion of a *T1 to Pointer to *T2.

Provided that T2 is no larger than T1 and that the two share an equivalent memory layout, (…)

In fact, this very same comment shows exactly how this conversion must be done:

var s string
hdr := (*reflect.StringHeader)(unsafe.Pointer(&s)) // case 1
hdr.Data = uintptr(unsafe.Pointer(p))              // case 6 (this case)
hdr.Len = n

[hummerd]

This is wildly unsafe:

p := *(*[]byte)(unsafe.Pointer(&s))

See unsafe rule №1:

(1) Conversion of a *T1 to Pointer to *T2.

Provided that T2 is no larger than T1 and that the two share an
equivalent memory layout, (…)

In fact, this very same comment shows exactly how this conversion must be done:

var s string
hdr := (*reflect.StringHeader)(unsafe.Pointer(&s)) // case 1
hdr.Data = uintptr(unsafe.Pointer(p))              // case 6 (this case)
hdr.Len = n

Changed WriteString implementation to more correct one with proper capacity initialization

[FiloSottile]

Closing as duplicate of #14757. It would be useful to mention real world use cases for it in that issues.