transform: add allocation alias regression cases

Add allocation diagnostics coverage for pointer-returning helpers, aggregate
slice returns, and conditional pointer returns before changing the allocation
optimizer.

The golden files record the current heap-allocation behavior so later commits
show exactly which diagnostics each optimizer improvement removes.

transform/testdata/allocs2.go

@@ -9,7 +9,6 @@ func main() {
 	n1 := 5
 	derefInt(&n1)
 
-	// This should eventually be modified to not escape.
 	n2 := 6
 	returnIntPtr(&n2)
 
@@ -19,7 +18,6 @@ func main() {
 	s2 := [3]int{}
 	readIntSlice(s2[:])
 
-	// This should also be modified to not escape.
 	s3 := make([]int, 3)
 	returnIntSlice(s3)
 
@@ -78,6 +76,61 @@ func main() {
 	keepAliveNoEscape(unsafe.Pointer(&dmaBuf2[0]))
 }
 
+type vector3 [3]float32
+
+func scaleVector3(vec *vector3, f float32) *vector3 {
+	vec[0] *= f
+	vec[1] *= f
+	vec[2] *= f
+	return vec
+}
+
+func crossVector3(a, b *vector3) vector3 {
+	return vector3{
+		a[1]*b[2] - a[2]*b[1],
+		a[2]*b[0] - a[0]*b[2],
+		a[0]*b[1] - a[1]*b[0],
+	}
+}
+
+func nonEscapingReturnedPointer() vector3 {
+	a := vector3{1, 2, 3}
+	b := vector3{4, 5, 6}
+
+	c := scaleVector3(&b, 0.5)
+	return crossVector3(&a, c)
+}
+
+var escapedSlice []int
+
+func escapingReturnedSlice() {
+	s := make([]int, 3)
+	escapedSlice = returnIntSlice(s)
+}
+
+var escapedVector3 *vector3
+
+func escapingReturnedPointer() {
+	b := vector3{4, 5, 6}
+
+	c := scaleVector3(&b, 0.5)
+	escapedVector3 = c
+}
+
+func recursiveScaleVector3(vec *vector3, n int) *vector3 {
+	if n == 0 {
+		return vec
+	}
+	return recursiveScaleVector3(vec, n-1)
+}
+
+func recursiveReturnedPointer() vector3 {
+	b := vector3{4, 5, 6}
+
+	c := recursiveScaleVector3(&b, 1)
+	return *c
+}
+
 func derefInt(x *int) int {
 	return *x
 }

transform/testdata/allocs2.out.cover

@@ -1,10 +1,14 @@
-testdata/allocs2.go:13.1,13.9 1 0
-testdata/allocs2.go:23.1,23.22 1 0
-testdata/allocs2.go:26.1,26.43 1 0
-testdata/allocs2.go:28.1,28.25 1 0
-testdata/allocs2.go:31.1,31.22 1 0
-testdata/allocs2.go:38.1,38.23 1 0
-testdata/allocs2.go:46.1,46.23 1 0
-testdata/allocs2.go:48.1,48.22 1 0
-testdata/allocs2.go:51.1,51.9 1 0
-testdata/allocs2.go:52.1,52.9 1 0
+testdata/allocs2.go:12.1,12.9 1 0
+testdata/allocs2.go:21.1,21.22 1 0
+testdata/allocs2.go:24.1,24.43 1 0
+testdata/allocs2.go:26.1,26.25 1 0
+testdata/allocs2.go:29.1,29.22 1 0
+testdata/allocs2.go:36.1,36.23 1 0
+testdata/allocs2.go:44.1,44.23 1 0
+testdata/allocs2.go:46.1,46.22 1 0
+testdata/allocs2.go:49.1,49.9 1 0
+testdata/allocs2.go:50.1,50.9 1 0
+testdata/allocs2.go:98.1,98.23 1 0
+testdata/allocs2.go:107.1,107.21 1 0
+testdata/allocs2.go:114.1,114.23 1 0
+testdata/allocs2.go:128.1,128.23 1 0

transform/testdata/allocs2.out.reason

@@ -1,10 +1,14 @@
-testdata/allocs2.go:13:2: object allocated on the heap: escapes at line 14
-testdata/allocs2.go:23:12: object allocated on the heap: escapes at line 24
-testdata/allocs2.go:26:15: object allocated on the heap: size is not constant
-testdata/allocs2.go:28:12: object allocated on the heap: object size 300 exceeds maximum stack allocation size 256
-testdata/allocs2.go:31:12: object allocated on the heap: escapes at line 32
-testdata/allocs2.go:38:21: object allocated on the heap: escapes at line 39
-testdata/allocs2.go:46:22: object allocated on the heap: escapes at line 46
-testdata/allocs2.go:48:13: object allocated on the heap: escapes at line 49
-testdata/allocs2.go:51:2: object allocated on the heap: escapes at line 53
-testdata/allocs2.go:52:2: object allocated on the heap: escapes at line 53
+testdata/allocs2.go:12:2: object allocated on the heap: escapes at line 13
+testdata/allocs2.go:21:12: object allocated on the heap: escapes at line 22
+testdata/allocs2.go:24:15: object allocated on the heap: size is not constant
+testdata/allocs2.go:26:12: object allocated on the heap: object size 300 exceeds maximum stack allocation size 256
+testdata/allocs2.go:29:12: object allocated on the heap: escapes at line 30
+testdata/allocs2.go:36:21: object allocated on the heap: escapes at line 37
+testdata/allocs2.go:44:22: object allocated on the heap: escapes at line 44
+testdata/allocs2.go:46:13: object allocated on the heap: escapes at line 47
+testdata/allocs2.go:49:2: object allocated on the heap: escapes at line 51
+testdata/allocs2.go:50:2: object allocated on the heap: escapes at line 51
+testdata/allocs2.go:98:2: object allocated on the heap: escapes at line 100
+testdata/allocs2.go:107:11: object allocated on the heap: escapes at line 108
+testdata/allocs2.go:114:2: object allocated on the heap: escapes at line 116
+testdata/allocs2.go:128:2: object allocated on the heap: escapes at line 130