compiler: add //go:noheap pragma

testdata/errors/noheap.go

@@ -0,0 +1,69 @@
+package main
+
+//go:noheap
+func main() {
+	// This object is optimized away, and won't cause a linker failure.
+	var a int
+	add(&a)
+
+	// This object is not optimized away since its address is captured.
+	var b int
+	escape(&b)
+
+	// A simple defer won't cause a heap allocation.
+	defer func() {
+		println("once")
+	}()
+
+	// A defer in a loop does causes a heap allocation.
+	for range 3 {
+		defer func() {
+			println("many")
+		}()
+	}
+
+	// This slice should not be heap-allocated.
+	s1 := make([]int, 5)
+	sum(s1)
+
+	// But this slice is.
+	s2 := make([]int, 5)
+	escape(&s2[0])
+
+	// Allocated at compile time as a global, so won't escape at runtime.
+	escape(globalInt)
+}
+
+func add(n *int) {
+	*n++
+}
+
+func sum(slice []int) (result int) {
+	for n := range slice {
+		result += n
+	}
+	return result
+}
+
+var globalInt = globalNewInt()
+
+var globalAssign *int
+
+//go:noheap
+func globalNewInt() *int {
+	return new(int)
+}
+
+func escape(n *int) {
+	// Do some stuff that will definitely force a heap allocation.
+	// (This might be optimized in the future, in which case we need to change
+	// the function again).
+	n2 := func(n *int) *int {
+		return n
+	}(n)
+	println(n2)
+}
+
+// ERROR: noheap.go:10: object allocated on the heap in //go:noheap function
+// ERROR: noheap.go:20: object allocated on the heap in //go:noheap function
+// ERROR: noheap.go:30: object allocated on the heap in //go:noheap function