interp: allow early constant evaluation from builtin call

One builtin has been identified to be used for constant definition:
len(), with a constant string argument. Add support for this.

Fixes #1012.

_test/const23.go

@@ -0,0 +1,12 @@
+package main
+
+const maxlen = len("hello")
+
+var gfm = [maxlen]byte{}
+
+func main() {
+	println(len(gfm))
+}
+
+// Output:
+// 5

_test/const24.go

@@ -0,0 +1,14 @@
+package main
+
+var aa = [...]int{1, 2, 3}
+
+const maxlen = cap(aa)
+
+var gfm = [maxlen]byte{}
+
+func main() {
+	println(len(gfm))
+}
+
+// Output:
+// 3

interp/cfg.go

@@ -565,6 +565,8 @@ func (interp *Interpreter) cfg(root *node, importPath string) ([]*node, error) {
 				switch {
 				case n.action != aAssign:
 					// Do not optimize assign combined with another operator.
+				case src.rval.IsValid():
+					// Do not skip assign operation when setting from a constant value.
 				case isMapEntry(dest):
 					// Setting a map entry needs an additional step, do not optimize.
 					// As we only write, skip the default useless getIndexMap dest action.
@@ -855,13 +857,15 @@ func (interp *Interpreter) cfg(root *node, importPath string) ([]*node, error) {
 			wireChild(n)
 			switch {
 			case interp.isBuiltinCall(n):
-				err = check.builtin(n.child[0].ident, n, n.child[1:], n.action == aCallSlice)
+				c0 := n.child[0]
+				bname := c0.ident
+				err = check.builtin(bname, n, n.child[1:], n.action == aCallSlice)
 				if err != nil {
 					break
 				}
 
-				n.gen = n.child[0].sym.builtin
+				n.gen = c0.sym.builtin
-				n.child[0].typ = &itype{cat: builtinT}
+				c0.typ = &itype{cat: builtinT}
 				if n.typ, err = nodeType(interp, sc, n); err != nil {
 					return
 				}
@@ -872,10 +876,18 @@ func (interp *Interpreter) cfg(root *node, importPath string) ([]*node, error) {
 				case n.anc.kind == returnStmt:
 					// Store result directly to frame output location, to avoid a frame copy.
 					n.findex = 0
+				case bname == "cap" && isInConstOrTypeDecl(n):
+					capConst(n)
+					n.findex = notInFrame
+					n.gen = nop
+				case bname == "len" && isInConstOrTypeDecl(n):
+					lenConst(n)
+					n.findex = notInFrame
+					n.gen = nop
 				default:
 					n.findex = sc.add(n.typ)
 				}
-				if op, ok := constBltn[n.child[0].ident]; ok && n.anc.action != aAssign {
+				if op, ok := constBltn[bname]; ok && n.anc.action != aAssign {
 					op(n) // pre-compute non-assigned constant :
 				}
 			case n.child[0].isType(sc):
@@ -2326,6 +2338,20 @@ func isRecursiveField(n *node) bool {
 	return false
 }
 
+func isInConstOrTypeDecl(n *node) bool {
+	anc := n.anc
+	for anc != nil {
+		switch anc.kind {
+		case constDecl, typeDecl:
+			return true
+		case varDecl, funcDecl:
+			return false
+		}
+		anc = anc.anc
+	}
+	return false
+}
+
 // isNewDefine returns true if node refers to a new definition.
 func isNewDefine(n *node, sc *scope) bool {
 	if n.ident == "_" {

interp/run.go

@@ -2847,6 +2847,21 @@ func _delete(n *node) {
 	})
 }
 
+func capConst(n *node) {
+	n.rval = reflect.New(reflect.TypeOf(int(0))).Elem()
+	// There is no Cap() method for reflect.Type, just return Len() instead.
+	n.rval.SetInt(int64(n.child[1].typ.TypeOf().Len()))
+}
+
+func lenConst(n *node) {
+	n.rval = reflect.New(reflect.TypeOf(int(0))).Elem()
+	if c1 := n.child[1]; c1.rval.IsValid() {
+		n.rval.SetInt(int64(len(vString(c1.rval))))
+	} else {
+		n.rval.SetInt(int64(c1.typ.TypeOf().Len()))
+	}
+}
+
 func _len(n *node) {
 	dest := genValueOutput(n, reflect.TypeOf(int(0)))
 	value := genValue(n.child[1])

interp/type.go

@@ -198,7 +198,7 @@ func nodeType(interp *Interpreter, sc *scope, n *node) (*itype, error) {
 			if sym.kind != constSym {
 				return nil, c0.cfgErrorf("non-constant array bound %q", c0.ident)
 			}
-			if sym.typ == nil || sym.typ.cat != intT {
+			if sym.typ == nil || sym.typ.cat != intT || !sym.rval.IsValid() {
 				t.incomplete = true
 				break
 			}