reflect: make Value.MapRange inlineable

This allows the caller to decide whether MapIter should be
stack allocated or heap allocated based on whether it escapes.
In most cases, it does not escape and thus removes the utility
of MapIter.Reset (#46293). In fact, use of sync.Pool with MapIter
and calling MapIter.Reset is likely to be slower.

Change-Id: Ic93e7d39e5dd4c83e7fca9e0bdfbbcd70777f0e1
Reviewed-on: https://go-review.googlesource.com/c/go/+/400675
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

1 files changed, 5 insertions, 3 deletions

diff --git a/src/reflect/all_test.go b/src/reflect/all_test.go
index 5a35d98b51..f7adf2fa1a 100644
--- a/src/reflect/all_test.go
+++ b/src/reflect/all_test.go
@@ -370,9 +370,11 @@ func TestMapIterSet(t *testing.T) {
 			e.SetIterValue(iter)
 		}
 	}))
-	// Making a *MapIter allocates. This should be the only allocation.
-	if got != 1 {
-		t.Errorf("wanted 1 alloc, got %d", got)
+	// Calling MapRange should not allocate even though it returns a *MapIter.
+	// The function is inlineable, so if the local usage does not escape
+	// the *MapIter, it can remain stack allocated.
+	if got != 0 {
+		t.Errorf("wanted 0 alloc, got %d", got)
 	}
 }