compiler: mark the source argument to the append builtin as not escaping

compiler/symbol.go

@@ -163,6 +163,8 @@ func (c *compilerContext) getFunction(fn *ssa.Function) (llvm.Type, llvm.Value)
 		// > allocsize argument to signify "not present."
 		llvmFn.AddFunctionAttr(c.ctx.CreateEnumAttribute(llvm.AttributeKindID("allocsize"), 0x0000_0000_ffff_ffff))
 	case "runtime.sliceAppend":
+		// The slice doesn't escape.
+		llvmFn.AddAttributeAtIndex(1, c.ctx.CreateEnumAttribute(llvm.AttributeKindID("nocapture"), 0))
 		// Appending a slice will only read the to-be-appended slice, it won't
 		// be modified.
 		llvmFn.AddAttributeAtIndex(2, c.ctx.CreateEnumAttribute(llvm.AttributeKindID("nocapture"), 0))

transform/testdata/allocs2.go

@@ -23,7 +23,7 @@ func main() {
 	s4 := make([]byte, 300) // OUT: object allocated on the heap: object size 300 exceeds maximum stack allocation size 256
 	readByteSlice(s4)
 
-	s5 := make([]int, 4) // OUT: object allocated on the heap: escapes at line 27
+	s5 := make([]int, 4)
 	_ = append(s5, 5)
 
 	s6 := make([]int, 3)