reflect: fix ArenaNew to match documentation

Currently ArenaNew expects the type passed in to be a *T and it returns
a *T. This does not match the function's documentation.

Since this is an experiment, change ArenaNew to match the documentation.
This more closely aligns ArenaNew with arena.New. (Takes a type T,
returns a *T value.)

Note that this is a breaking change. However, as far as pkg.go.dev can
tell, there's exactly one package using it in the open source world.

Also, add smoke test for the exported API, which is just a wrapper
around the internal API. Clearly there's enough room for error here that
it should be tested, but we don't need thorough tests at this layer
because that already exists in the runtime. We just need to make sure it
basically works.

Fixes #60528.

Change-Id: I673cc4609378380ef80648b0c2eb2928e73f49c9
Reviewed-on: https://go-review.googlesource.com/c/go/+/501860
Run-TryBot: Michael Knyszek <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: mknyszek

src/cmd/internal/testdir/testdir_test.go

@@ -64,7 +64,7 @@ var (
 
 	// dirs are the directories to look for *.go files in.
 	// TODO(bradfitz): just use all directories?
-	dirs = []string{".", "ken", "chan", "interface", "syntax", "dwarf", "fixedbugs", "codegen", "runtime", "abi", "typeparam", "typeparam/mdempsky"}
+	dirs = []string{".", "ken", "chan", "interface", "syntax", "dwarf", "fixedbugs", "codegen", "runtime", "abi", "typeparam", "typeparam/mdempsky", "arenas"}
 )
 
 // Test is the main entrypoint that runs tests in the GOROOT/test directory.

src/reflect/arena.go

@@ -12,7 +12,7 @@ import "arena"
 // specified type, allocating storage for it in the provided arena. That is,
 // the returned Value's Type is PointerTo(typ).
 func ArenaNew(a *arena.Arena, typ Type) Value {
-	return ValueOf(arena_New(a, typ))
+	return ValueOf(arena_New(a, PointerTo(typ)))
 }
 
 func arena_New(a *arena.Arena, typ any) any

test/arenas/smoke.go

@@ -0,0 +1,65 @@
+// build -goexperiment arenas
+
+// Copyright 2023 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 main
+
+import (
+	"arena"
+	"log"
+	"reflect"
+)
+
+func main() {
+	a := arena.NewArena()
+	defer a.Free()
+
+	const iValue = 10
+
+	i := arena.New[int](a)
+	*i = iValue
+
+	if *i != iValue {
+		// This test doesn't reasonably expect this to fail. It's more likely
+		// that *i crashes for some reason. Still, why not check it.
+		log.Fatalf("bad i value: got %d, want %d", *i, iValue)
+	}
+
+	const wantLen = 125
+	const wantCap = 1912
+
+	sl := arena.MakeSlice[*int](a, wantLen, wantCap)
+	if len(sl) != wantLen {
+		log.Fatalf("bad arena slice length: got %d, want %d", len(sl), wantLen)
+	}
+	if cap(sl) != wantCap {
+		log.Fatalf("bad arena slice capacity: got %d, want %d", cap(sl), wantCap)
+	}
+	sl = sl[:cap(sl)]
+	for j := range sl {
+		sl[j] = i
+	}
+	for j := range sl {
+		if *sl[j] != iValue {
+			// This test doesn't reasonably expect this to fail. It's more likely
+			// that sl[j] crashes for some reason. Still, why not check it.
+			log.Fatalf("bad sl[j] value: got %d, want %d", *sl[j], iValue)
+		}
+	}
+
+	t := reflect.TypeOf(int(0))
+	v := reflect.ArenaNew(a, t)
+	if want := reflect.PointerTo(t); v.Type() != want {
+		log.Fatalf("unexpected type for arena-allocated value: got %s, want %s", v.Type(), want)
+	}
+	i2 := v.Interface().(*int)
+	*i2 = iValue
+
+	if *i2 != iValue {
+		// This test doesn't reasonably expect this to fail. It's more likely
+		// that *i crashes for some reason. Still, why not check it.
+		log.Fatalf("bad i2 value: got %d, want %d", *i2, iValue)
+	}
+}