reflect: allow conversion from slice to array ptr

Note that this removes an invariant:

v.Type().ConvertibleTo(t) might return true,
yet v.Convert(t) might panic nevertheless.

This is a fairly unavoidable consequence of the decision
to add the first-ever conversion that can panic.

ConvertibleTo describes a relationship between types,
but whether the conversion panics now depends on the value,
not just the type.

If this turns out to be a problem, we can add v.ConvertibleTo(t),
or something similar, to allow callers to avoid the panic.

This is the last of the changes needed to complete the implementation.

Fixes #395

Change-Id: I79b7e4dd87a67a47723e00a65d0b1ac6090371b7
Reviewed-on: https://go-review.googlesource.com/c/go/+/301652
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

1 files changed, 17 insertions, 1 deletions

diff --git a/src/reflect/value.go b/src/reflect/value.go
index 6f1a3c02d6..418dff781f 100644
--- a/src/reflect/value.go
+++ b/src/reflect/value.go
@@ -6,6 +6,7 @@ package reflect
 
 import (
 	"internal/abi"
+	"internal/itoa"
 	"internal/unsafeheader"
 	"math"
 	"runtime"
@@ -2770,7 +2771,7 @@ func (v Value) assignTo(context string, dst *rtype, target unsafe.Pointer) Value
 
 // Convert returns the value v converted to type t.
 // If the usual Go conversion rules do not allow conversion
-// of the value v to type t, Convert panics.
+// of the value v to type t, or if converting v to type t panics, Convert panics.
 func (v Value) Convert(t Type) Value {
 	if v.flag&flagMethod != 0 {
 		v = makeMethodValue("Convert", v)
@@ -2841,6 +2842,11 @@ func convertOp(dst, src *rtype) func(Value, Type) Value {
 				return cvtRunesString
 			}
 		}
+		// "x is a slice, T is a pointer-to-array type,
+		// and the slice and array types have identical element types."
+		if dst.Kind() == Ptr && dst.Elem().Kind() == Array && src.Elem() == dst.Elem().Elem() {
+			return cvtSliceArrayPtr
+		}
 
 	case Chan:
 		if dst.Kind() == Chan && specialChannelAssignability(dst, src) {
@@ -3034,6 +3040,16 @@ func cvtStringRunes(v Value, t Type) Value {
 	return makeRunes(v.flag.ro(), []rune(v.String()), t)
 }
 
+// convertOp: []T -> *[N]T
+func cvtSliceArrayPtr(v Value, t Type) Value {
+	n := t.Elem().Len()
+	h := (*unsafeheader.Slice)(v.ptr)
+	if n > h.Len {
+		panic("reflect: cannot convert slice with length " + itoa.Itoa(h.Len) + " to array pointer with length " + itoa.Itoa(n))
+	}
+	return Value{t.common(), h.Data, v.flag&^(flagIndir|flagAddr|flagKindMask) | flag(Ptr)}
+}
+
 // convertOp: direct copy
 func cvtDirect(v Value, typ Type) Value {
 	f := v.flag