internal/unsafeheader: consolidate stringHeader and sliceHeader declarations into an internal package

The new package "internal/unsafeheader" depends only on "unsafe", and
provides declarations equivalent to reflect.StringHeader and
reflect.SliceHeader but with Data fields of the proper unsafe.Pointer
type (instead of uintptr).

Unlike the types it replaces, the "internal/unsafeheader" package has
a regression test to ensure that its header types remain equivalent to
the declarations provided by the "reflect" package.

Since "internal/unsafeheader" has almost no dependencies, it can be
used in other low-level packages such as "syscall" and "reflect".

This change is based on the corresponding x/sys change in CL 231177.

Fixes #37805
Updates #19367

Change-Id: I7a6d93ef8dd6e235bcab94e7c47270aad047af31
Reviewed-on: https://go-review.googlesource.com/c/go/+/231223
Reviewed-by: Ian Lance Taylor <iant@golang.org>

3 files changed, 11 insertions, 19 deletions

diff --git a/src/internal/reflectlite/swapper.go b/src/internal/reflectlite/swapper.go
index 4594fb5ee2..6330ab2d34 100644
--- a/src/internal/reflectlite/swapper.go
+++ b/src/internal/reflectlite/swapper.go
@@ -4,7 +4,10 @@
 
 package reflectlite
 
-import "unsafe"
+import (
+	"internal/unsafeheader"
+	"unsafe"
+)
 
 // Swapper returns a function that swaps the elements in the provided
 // slice.
@@ -58,7 +61,7 @@ func Swapper(slice interface{}) func(i, j int) {
 		}
 	}
 
-	s := (*sliceHeader)(v.ptr)
+	s := (*unsafeheader.Slice)(v.ptr)
 	tmp := unsafe_New(typ) // swap scratch space
 
 	return func(i, j int) {
diff --git a/src/internal/reflectlite/type.go b/src/internal/reflectlite/type.go
index 49a03ac1e1..eb7f1a4b78 100644
--- a/src/internal/reflectlite/type.go
+++ b/src/internal/reflectlite/type.go
@@ -7,6 +7,7 @@
 package reflectlite
 
 import (
+	"internal/unsafeheader"
 	"unsafe"
 )
 
@@ -338,7 +339,7 @@ func (n name) name() (s string) {
 	}
 	b := (*[4]byte)(unsafe.Pointer(n.bytes))
 
-	hdr := (*stringHeader)(unsafe.Pointer(&s))
+	hdr := (*unsafeheader.String)(unsafe.Pointer(&s))
 	hdr.Data = unsafe.Pointer(&b[3])
 	hdr.Len = int(b[1])<<8 | int(b[2])
 	return s
@@ -350,7 +351,7 @@ func (n name) tag() (s string) {
 		return ""
 	}
 	nl := n.nameLen()
-	hdr := (*stringHeader)(unsafe.Pointer(&s))
+	hdr := (*unsafeheader.String)(unsafe.Pointer(&s))
 	hdr.Data = unsafe.Pointer(n.data(3+nl+2, "non-empty string"))
 	hdr.Len = tl
 	return s
diff --git a/src/internal/reflectlite/value.go b/src/internal/reflectlite/value.go
index 6a493938f5..85beea606c 100644
--- a/src/internal/reflectlite/value.go
+++ b/src/internal/reflectlite/value.go
@@ -5,6 +5,7 @@
 package reflectlite
 
 import (
+	"internal/unsafeheader"
 	"runtime"
 	"unsafe"
 )
@@ -335,10 +336,10 @@ func (v Value) Len() int {
 		return maplen(v.pointer())
 	case Slice:
 		// Slice is bigger than a word; assume flagIndir.
-		return (*sliceHeader)(v.ptr).Len
+		return (*unsafeheader.Slice)(v.ptr).Len
 	case String:
 		// String is bigger than a word; assume flagIndir.
-		return (*stringHeader)(v.ptr).Len
+		return (*unsafeheader.String)(v.ptr).Len
 	}
 	panic(&ValueError{"reflect.Value.Len", v.kind()})
 }
@@ -379,19 +380,6 @@ func (v Value) Type() Type {
 	return v.typ
 }
 
-// stringHeader is a safe version of StringHeader used within this package.
-type stringHeader struct {
-	Data unsafe.Pointer
-	Len  int
-}
-
-// sliceHeader is a safe version of SliceHeader used within this package.
-type sliceHeader struct {
-	Data unsafe.Pointer
-	Len  int
-	Cap  int
-}
-
 /*
  * constructors
  */