spec: add unsafe.Add and unsafe.Slice

Updates #19367.
Updates #40481.

Change-Id: I578066ad68d2cd6bea50df1a534cf799e4404a7f
Reviewed-on: https://go-review.googlesource.com/c/go/+/312212
Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

doc/go_spec.html

@@ -1,6 +1,6 @@
 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of Apr 20, 2021",
+	"Subtitle": "Version of Apr 21, 2021",
 	"Path": "/ref/spec"
 }-->
 
@@ -6711,6 +6711,10 @@ type Pointer *ArbitraryType
 func Alignof(variable ArbitraryType) uintptr
 func Offsetof(selector ArbitraryType) uintptr
 func Sizeof(variable ArbitraryType) uintptr
+
+type IntegerType int  // shorthand for an integer type; it is not a real type
+func Add(ptr Pointer, len IntegerType) Pointer
+func Slice(ptr *ArbitraryType, len IntegerType) []ArbitraryType
 </pre>
 
 <p>
@@ -6767,6 +6771,32 @@ Calls to <code>Alignof</code>, <code>Offsetof</code>, and
 <code>Sizeof</code> are compile-time constant expressions of type <code>uintptr</code>.
 </p>
 
+<p>
+The function <code>Add</code> adds <code>len</code> to <code>ptr</code>
+and returns the updated pointer <code>unsafe.Pointer(uintptr(ptr) + uintptr(len))</code>.
+The <code>len</code> argument must be of integer type or an untyped <a href="#Constants">constant</a>.
+A constant <code>len</code> argument must be <a href="#Representability">representable</a> by a value of type <code>int</code>;
+if it is an untyped constant it is given type <code>int</code>.
+The rules for <a href="/pkg/unsafe#Pointer">valid uses</a> of <code>Pointer</code> still apply.
+</p>
+
+<p>
+The function <code>Slice</code> returns a slice whose underlying array starts at <code>ptr</code>
+and whose length and capacity are <code>len</code>:
+</p>
+
+<pre>
+(*[len]ArbitraryType)(unsafe.Pointer(ptr))[:]
+</pre>
+
+<p>
+The <code>len</code> argument must be of integer type or an untyped <a href="#Constants">constant</a>.
+A constant <code>len</code> argument must be non-negative and <a href="#Representability">representable</a> by a value of type <code>int</code>;
+if it is an untyped constant it is given type <code>int</code>.
+If <code>ptr</code> is <code>nil</code> or <code>len</code> is negative at run time,
+a <a href="#Run_time_panics">run-time panic</a> occurs.
+</p>
+
 <h3 id="Size_and_alignment_guarantees">Size and alignment guarantees</h3>
 
 <p>

src/unsafe/unsafe.go

@@ -14,6 +14,10 @@ package unsafe
 // part of the unsafe package. It represents the type of an arbitrary Go expression.
 type ArbitraryType int
 
+// IntegerType is here for the purposes of documentation only and is not actually
+// part of the unsafe package. It represents any arbitrary integer type.
+type IntegerType int
+
 // Pointer represents a pointer to an arbitrary type. There are four special operations
 // available for type Pointer that are not available for other types:
 //	- A pointer value of any type can be converted to a Pointer.
@@ -203,3 +207,22 @@ func Offsetof(x ArbitraryType) uintptr
 // value returned by reflect.TypeOf(s.f).FieldAlign().
 // The return value of Alignof is a Go constant.
 func Alignof(x ArbitraryType) uintptr
+
+// The function Add adds len to ptr and returns the updated pointer
+// Pointer(uintptr(ptr) + uintptr(len)).
+// The len argument must be of integer type or an untyped constant.
+// A constant len argument must be representable by a value of type int;
+// if it is an untyped constant it is given type int.
+// The rules for valid uses of Pointer still apply.
+func Add(ptr Pointer, len IntegerType) Pointer
+
+// The function Slice returns a slice whose underlying array starts at ptr
+// and whose length and capacity are len:
+//
+//	(*[len]ArbitraryType)(unsafe.Pointer(ptr))[:]
+//
+// The len argument must be of integer type or an untyped constant.
+// A constant len argument must be non-negative and representable by a value of type int;
+// if it is an untyped constant it is given type int.
+// If ptr is nil or len is negative at run time, a run-time panic occurs.
+func Slice(ptr *ArbitraryType, len IntegerType) []ArbitraryType