all: use unsafe.{Slice, SliceData, String, StringData} to simplify code

Because most of these APIs are recently supported, we can only do some
advancement work as much as possible under the premise of compatibility.

For #54854.

Change-Id: Id15d11288bf23902570d54eaf2704a5264210b2e
Reviewed-on: https://go-review.googlesource.com/c/go/+/429115
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: hopehook <hopehook@golangcn.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/cmd/go/internal/modindex/read.go

@@ -14,7 +14,6 @@ import (
 	"go/token"
 	"internal/godebug"
 	"internal/goroot"
-	"internal/unsafeheader"
 	"path"
 	"path/filepath"
 	"runtime"
@@ -948,14 +947,7 @@ func (sf *sourceFile) embeds() []embed {
 }
 
 func asString(b []byte) string {
-	p := (*unsafeheader.Slice)(unsafe.Pointer(&b)).Data
-
-	var s string
-	hdr := (*unsafeheader.String)(unsafe.Pointer(&s))
-	hdr.Data = p
-	hdr.Len = len(b)
-
-	return s
+	return unsafe.String(unsafe.SliceData(b), len(b))
 }
 
 // A decoder helps decode the index format.

src/hash/maphash/maphash.go

@@ -13,7 +13,6 @@
 package maphash
 
 import (
-	"internal/unsafeheader"
 	"unsafe"
 )
 
@@ -72,11 +71,11 @@ func String(seed Seed, s string) uint64 {
 		panic("maphash: use of uninitialized Seed")
 	}
 	for len(s) > bufSize {
-		p := (*byte)((*unsafeheader.String)(unsafe.Pointer(&s)).Data)
+		p := (*byte)(unsafe.StringData(s))
 		state = rthash(p, bufSize, state)
 		s = s[bufSize:]
 	}
-	p := (*byte)((*unsafeheader.String)(unsafe.Pointer(&s)).Data)
+	p := (*byte)(unsafe.StringData(s))
 	return rthash(p, len(s), state)
 }
 
@@ -190,7 +189,7 @@ func (h *Hash) WriteString(s string) (int, error) {
 	if len(s) > bufSize {
 		h.initSeed()
 		for len(s) > bufSize {
-			ptr := (*byte)((*unsafeheader.String)(unsafe.Pointer(&s)).Data)
+			ptr := (*byte)(unsafe.StringData(s))
 			h.state.s = rthash(ptr, bufSize, h.state.s)
 			s = s[bufSize:]
 		}

src/internal/fuzz/counters_supported.go

@@ -7,7 +7,6 @@
 package fuzz
 
 import (
-	"internal/unsafeheader"
 	"unsafe"
 )
 
@@ -18,12 +17,5 @@ import (
 func coverage() []byte {
 	addr := unsafe.Pointer(&_counters)
 	size := uintptr(unsafe.Pointer(&_ecounters)) - uintptr(addr)
-
-	var res []byte
-	*(*unsafeheader.Slice)(unsafe.Pointer(&res)) = unsafeheader.Slice{
-		Data: addr,
-		Len:  int(size),
-		Cap:  int(size),
-	}
-	return res
+	return unsafe.Slice((*byte)(addr), int(size))
 }