1 files changed, 477 insertions, 0 deletions
diff --git a/src/internal/reflectlite/value.go b/src/internal/reflectlite/value.go
new file mode 100644
index 0000000..b9bca3a
--- /dev/null
+++ b/src/internal/reflectlite/value.go
@@ -0,0 +1,477 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package reflectlite
+
+import (
+	"internal/goarch"
+	"internal/unsafeheader"
+	"runtime"
+	"unsafe"
+)
+
+// Value is the reflection interface to a Go value.
+//
+// Not all methods apply to all kinds of values. Restrictions,
+// if any, are noted in the documentation for each method.
+// Use the Kind method to find out the kind of value before
+// calling kind-specific methods. Calling a method
+// inappropriate to the kind of type causes a run time panic.
+//
+// The zero Value represents no value.
+// Its IsValid method returns false, its Kind method returns Invalid,
+// its String method returns "<invalid Value>", and all other methods panic.
+// Most functions and methods never return an invalid value.
+// If one does, its documentation states the conditions explicitly.
+//
+// A Value can be used concurrently by multiple goroutines provided that
+// the underlying Go value can be used concurrently for the equivalent
+// direct operations.
+//
+// To compare two Values, compare the results of the Interface method.
+// Using == on two Values does not compare the underlying values
+// they represent.
+type Value struct {
+	// typ holds the type of the value represented by a Value.
+	typ *rtype
+
+	// Pointer-valued data or, if flagIndir is set, pointer to data.
+	// Valid when either flagIndir is set or typ.pointers() is true.
+	ptr unsafe.Pointer
+
+	// flag holds metadata about the value.
+	// The lowest bits are flag bits:
+	//	- flagStickyRO: obtained via unexported not embedded field, so read-only
+	//	- flagEmbedRO: obtained via unexported embedded field, so read-only
+	//	- flagIndir: val holds a pointer to the data
+	//	- flagAddr: v.CanAddr is true (implies flagIndir)
+	// Value cannot represent method values.
+	// The next five bits give the Kind of the value.
+	// This repeats typ.Kind() except for method values.
+	// The remaining 23+ bits give a method number for method values.
+	// If flag.kind() != Func, code can assume that flagMethod is unset.
+	// If ifaceIndir(typ), code can assume that flagIndir is set.
+	flag
+
+	// A method value represents a curried method invocation
+	// like r.Read for some receiver r. The typ+val+flag bits describe
+	// the receiver r, but the flag's Kind bits say Func (methods are
+	// functions), and the top bits of the flag give the method number
+	// in r's type's method table.
+}
+
+type flag uintptr
+
+const (
+	flagKindWidth        = 5 // there are 27 kinds
+	flagKindMask    flag = 1<<flagKindWidth - 1
+	flagStickyRO    flag = 1 << 5
+	flagEmbedRO     flag = 1 << 6
+	flagIndir       flag = 1 << 7
+	flagAddr        flag = 1 << 8
+	flagMethod      flag = 1 << 9
+	flagMethodShift      = 10
+	flagRO          flag = flagStickyRO | flagEmbedRO
+)
+
+func (f flag) kind() Kind {
+	return Kind(f & flagKindMask)
+}
+
+func (f flag) ro() flag {
+	if f&flagRO != 0 {
+		return flagStickyRO
+	}
+	return 0
+}
+
+// pointer returns the underlying pointer represented by v.
+// v.Kind() must be Pointer, Map, Chan, Func, or UnsafePointer
+func (v Value) pointer() unsafe.Pointer {
+	if v.typ.size != goarch.PtrSize || !v.typ.pointers() {
+		panic("can't call pointer on a non-pointer Value")
+	}
+	if v.flag&flagIndir != 0 {
+		return *(*unsafe.Pointer)(v.ptr)
+	}
+	return v.ptr
+}
+
+// packEface converts v to the empty interface.
+func packEface(v Value) any {
+	t := v.typ
+	var i any
+	e := (*emptyInterface)(unsafe.Pointer(&i))
+	// First, fill in the data portion of the interface.
+	switch {
+	case ifaceIndir(t):
+		if v.flag&flagIndir == 0 {
+			panic("bad indir")
+		}
+		// Value is indirect, and so is the interface we're making.
+		ptr := v.ptr
+		if v.flag&flagAddr != 0 {
+			// TODO: pass safe boolean from valueInterface so
+			// we don't need to copy if safe==true?
+			c := unsafe_New(t)
+			typedmemmove(t, c, ptr)
+			ptr = c
+		}
+		e.word = ptr
+	case v.flag&flagIndir != 0:
+		// Value is indirect, but interface is direct. We need
+		// to load the data at v.ptr into the interface data word.
+		e.word = *(*unsafe.Pointer)(v.ptr)
+	default:
+		// Value is direct, and so is the interface.
+		e.word = v.ptr
+	}
+	// Now, fill in the type portion. We're very careful here not
+	// to have any operation between the e.word and e.typ assignments
+	// that would let the garbage collector observe the partially-built
+	// interface value.
+	e.typ = t
+	return i
+}
+
+// unpackEface converts the empty interface i to a Value.
+func unpackEface(i any) Value {
+	e := (*emptyInterface)(unsafe.Pointer(&i))
+	// NOTE: don't read e.word until we know whether it is really a pointer or not.
+	t := e.typ
+	if t == nil {
+		return Value{}
+	}
+	f := flag(t.Kind())
+	if ifaceIndir(t) {
+		f |= flagIndir
+	}
+	return Value{t, e.word, f}
+}
+
+// A ValueError occurs when a Value method is invoked on
+// a Value that does not support it. Such cases are documented
+// in the description of each method.
+type ValueError struct {
+	Method string
+	Kind   Kind
+}
+
+func (e *ValueError) Error() string {
+	if e.Kind == 0 {
+		return "reflect: call of " + e.Method + " on zero Value"
+	}
+	return "reflect: call of " + e.Method + " on " + e.Kind.String() + " Value"
+}
+
+// methodName returns the name of the calling method,
+// assumed to be two stack frames above.
+func methodName() string {
+	pc, _, _, _ := runtime.Caller(2)
+	f := runtime.FuncForPC(pc)
+	if f == nil {
+		return "unknown method"
+	}
+	return f.Name()
+}
+
+// emptyInterface is the header for an interface{} value.
+type emptyInterface struct {
+	typ  *rtype
+	word unsafe.Pointer
+}
+
+// mustBeExported panics if f records that the value was obtained using
+// an unexported field.
+func (f flag) mustBeExported() {
+	if f == 0 {
+		panic(&ValueError{methodName(), 0})
+	}
+	if f&flagRO != 0 {
+		panic("reflect: " + methodName() + " using value obtained using unexported field")
+	}
+}
+
+// mustBeAssignable panics if f records that the value is not assignable,
+// which is to say that either it was obtained using an unexported field
+// or it is not addressable.
+func (f flag) mustBeAssignable() {
+	if f == 0 {
+		panic(&ValueError{methodName(), Invalid})
+	}
+	// Assignable if addressable and not read-only.
+	if f&flagRO != 0 {
+		panic("reflect: " + methodName() + " using value obtained using unexported field")
+	}
+	if f&flagAddr == 0 {
+		panic("reflect: " + methodName() + " using unaddressable value")
+	}
+}
+
+// CanSet reports whether the value of v can be changed.
+// A Value can be changed only if it is addressable and was not
+// obtained by the use of unexported struct fields.
+// If CanSet returns false, calling Set or any type-specific
+// setter (e.g., SetBool, SetInt) will panic.
+func (v Value) CanSet() bool {
+	return v.flag&(flagAddr|flagRO) == flagAddr
+}
+
+// Elem returns the value that the interface v contains
+// or that the pointer v points to.
+// It panics if v's Kind is not Interface or Pointer.
+// It returns the zero Value if v is nil.
+func (v Value) Elem() Value {
+	k := v.kind()
+	switch k {
+	case Interface:
+		var eface any
+		if v.typ.NumMethod() == 0 {
+			eface = *(*any)(v.ptr)
+		} else {
+			eface = (any)(*(*interface {
+				M()
+			})(v.ptr))
+		}
+		x := unpackEface(eface)
+		if x.flag != 0 {
+			x.flag |= v.flag.ro()
+		}
+		return x
+	case Pointer:
+		ptr := v.ptr
+		if v.flag&flagIndir != 0 {
+			ptr = *(*unsafe.Pointer)(ptr)
+		}
+		// The returned value's address is v's value.
+		if ptr == nil {
+			return Value{}
+		}
+		tt := (*ptrType)(unsafe.Pointer(v.typ))
+		typ := tt.elem
+		fl := v.flag&flagRO | flagIndir | flagAddr
+		fl |= flag(typ.Kind())
+		return Value{typ, ptr, fl}
+	}
+	panic(&ValueError{"reflectlite.Value.Elem", v.kind()})
+}
+
+func valueInterface(v Value) any {
+	if v.flag == 0 {
+		panic(&ValueError{"reflectlite.Value.Interface", 0})
+	}
+
+	if v.kind() == Interface {
+		// Special case: return the element inside the interface.
+		// Empty interface has one layout, all interfaces with
+		// methods have a second layout.
+		if v.numMethod() == 0 {
+			return *(*any)(v.ptr)
+		}
+		return *(*interface {
+			M()
+		})(v.ptr)
+	}
+
+	// TODO: pass safe to packEface so we don't need to copy if safe==true?
+	return packEface(v)
+}
+
+// IsNil reports whether its argument v is nil. The argument must be
+// a chan, func, interface, map, pointer, or slice value; if it is
+// not, IsNil panics. Note that IsNil is not always equivalent to a
+// regular comparison with nil in Go. For example, if v was created
+// by calling ValueOf with an uninitialized interface variable i,
+// i==nil will be true but v.IsNil will panic as v will be the zero
+// Value.
+func (v Value) IsNil() bool {
+	k := v.kind()
+	switch k {
+	case Chan, Func, Map, Pointer, UnsafePointer:
+		// if v.flag&flagMethod != 0 {
+		// 	return false
+		// }
+		ptr := v.ptr
+		if v.flag&flagIndir != 0 {
+			ptr = *(*unsafe.Pointer)(ptr)
+		}
+		return ptr == nil
+	case Interface, Slice:
+		// Both interface and slice are nil if first word is 0.
+		// Both are always bigger than a word; assume flagIndir.
+		return *(*unsafe.Pointer)(v.ptr) == nil
+	}
+	panic(&ValueError{"reflectlite.Value.IsNil", v.kind()})
+}
+
+// IsValid reports whether v represents a value.
+// It returns false if v is the zero Value.
+// If IsValid returns false, all other methods except String panic.
+// Most functions and methods never return an invalid Value.
+// If one does, its documentation states the conditions explicitly.
+func (v Value) IsValid() bool {
+	return v.flag != 0
+}
+
+// Kind returns v's Kind.
+// If v is the zero Value (IsValid returns false), Kind returns Invalid.
+func (v Value) Kind() Kind {
+	return v.kind()
+}
+
+// implemented in runtime:
+func chanlen(unsafe.Pointer) int
+func maplen(unsafe.Pointer) int
+
+// Len returns v's length.
+// It panics if v's Kind is not Array, Chan, Map, Slice, or String.
+func (v Value) Len() int {
+	k := v.kind()
+	switch k {
+	case Array:
+		tt := (*arrayType)(unsafe.Pointer(v.typ))
+		return int(tt.len)
+	case Chan:
+		return chanlen(v.pointer())
+	case Map:
+		return maplen(v.pointer())
+	case Slice:
+		// Slice is bigger than a word; assume flagIndir.
+		return (*unsafeheader.Slice)(v.ptr).Len
+	case String:
+		// String is bigger than a word; assume flagIndir.
+		return (*unsafeheader.String)(v.ptr).Len
+	}
+	panic(&ValueError{"reflect.Value.Len", v.kind()})
+}
+
+// NumMethod returns the number of exported methods in the value's method set.
+func (v Value) numMethod() int {
+	if v.typ == nil {
+		panic(&ValueError{"reflectlite.Value.NumMethod", Invalid})
+	}
+	return v.typ.NumMethod()
+}
+
+// Set assigns x to the value v.
+// It panics if CanSet returns false.
+// As in Go, x's value must be assignable to v's type.
+func (v Value) Set(x Value) {
+	v.mustBeAssignable()
+	x.mustBeExported() // do not let unexported x leak
+	var target unsafe.Pointer
+	if v.kind() == Interface {
+		target = v.ptr
+	}
+	x = x.assignTo("reflectlite.Set", v.typ, target)
+	if x.flag&flagIndir != 0 {
+		typedmemmove(v.typ, v.ptr, x.ptr)
+	} else {
+		*(*unsafe.Pointer)(v.ptr) = x.ptr
+	}
+}
+
+// Type returns v's type.
+func (v Value) Type() Type {
+	f := v.flag
+	if f == 0 {
+		panic(&ValueError{"reflectlite.Value.Type", Invalid})
+	}
+	// Method values not supported.
+	return v.typ
+}
+
+/*
+ * constructors
+ */
+
+// implemented in package runtime
+func unsafe_New(*rtype) unsafe.Pointer
+
+// ValueOf returns a new Value initialized to the concrete value
+// stored in the interface i. ValueOf(nil) returns the zero Value.
+func ValueOf(i any) Value {
+	if i == nil {
+		return Value{}
+	}
+
+	// TODO: Maybe allow contents of a Value to live on the stack.
+	// For now we make the contents always escape to the heap. It
+	// makes life easier in a few places (see chanrecv/mapassign
+	// comment below).
+	escapes(i)
+
+	return unpackEface(i)
+}
+
+// assignTo returns a value v that can be assigned directly to typ.
+// It panics if v is not assignable to typ.
+// For a conversion to an interface type, target is a suggested scratch space to use.
+func (v Value) assignTo(context string, dst *rtype, target unsafe.Pointer) Value {
+	// if v.flag&flagMethod != 0 {
+	// 	v = makeMethodValue(context, v)
+	// }
+
+	switch {
+	case directlyAssignable(dst, v.typ):
+		// Overwrite type so that they match.
+		// Same memory layout, so no harm done.
+		fl := v.flag&(flagAddr|flagIndir) | v.flag.ro()
+		fl |= flag(dst.Kind())
+		return Value{dst, v.ptr, fl}
+
+	case implements(dst, v.typ):
+		if target == nil {
+			target = unsafe_New(dst)
+		}
+		if v.Kind() == Interface && v.IsNil() {
+			// A nil ReadWriter passed to nil Reader is OK,
+			// but using ifaceE2I below will panic.
+			// Avoid the panic by returning a nil dst (e.g., Reader) explicitly.
+			return Value{dst, nil, flag(Interface)}
+		}
+		x := valueInterface(v)
+		if dst.NumMethod() == 0 {
+			*(*any)(target) = x
+		} else {
+			ifaceE2I(dst, x, target)
+		}
+		return Value{dst, target, flagIndir | flag(Interface)}
+	}
+
+	// Failed.
+	panic(context + ": value of type " + v.typ.String() + " is not assignable to type " + dst.String())
+}
+
+// arrayAt returns the i-th element of p,
+// an array whose elements are eltSize bytes wide.
+// The array pointed at by p must have at least i+1 elements:
+// it is invalid (but impossible to check here) to pass i >= len,
+// because then the result will point outside the array.
+// whySafe must explain why i < len. (Passing "i < len" is fine;
+// the benefit is to surface this assumption at the call site.)
+func arrayAt(p unsafe.Pointer, i int, eltSize uintptr, whySafe string) unsafe.Pointer {
+	return add(p, uintptr(i)*eltSize, "i < len")
+}
+
+func ifaceE2I(t *rtype, src any, dst unsafe.Pointer)
+
+// typedmemmove copies a value of type t to dst from src.
+//
+//go:noescape
+func typedmemmove(t *rtype, dst, src unsafe.Pointer)
+
+// Dummy annotation marking that the value x escapes,
+// for use in cases where the reflect code is so clever that
+// the compiler cannot follow.
+func escapes(x any) {
+	if dummy.b {
+		dummy.x = x
+	}
+}
+
+var dummy struct {
+	b bool
+	x any
+}