cmd/gc: implement len(array) / cap(array) rule

The spec is looser than the current implementation.
The spec edit was made in CL 4444050 (May 2011)
but I never implemented it.

Fixes #3244.

R=ken2
CC=golang-dev
https://golang.org/cl/5785049

src/cmd/gc/typecheck.c

@@ -190,6 +190,46 @@ typecheck(Node **np, int top)
 	return n;
 }
 
+/*
+ * does n contain a call or receive operation?
+ */
+static int callrecvlist(NodeList*);
+
+static int
+callrecv(Node *n)
+{
+	if(n == nil)
+		return 0;
+	
+	switch(n->op) {
+	case OCALL:
+	case OCALLMETH:
+	case OCALLINTER:
+	case OCALLFUNC:
+	case ORECV:
+		return 1;
+	}
+
+	return callrecv(n->left) ||
+		callrecv(n->right) ||
+		callrecv(n->ntest) ||
+		callrecv(n->nincr) ||
+		callrecvlist(n->ninit) ||
+		callrecvlist(n->nbody) ||
+		callrecvlist(n->nelse) ||
+		callrecvlist(n->list) ||
+		callrecvlist(n->rlist);
+}
+
+static int
+callrecvlist(NodeList *l)
+{
+	for(; l; l=l->next)
+		if(callrecv(l->n))
+			return 1;
+	return 0;
+}
+
 static void
 typecheck1(Node **np, int top)
 {
@@ -995,12 +1035,14 @@ reswitch:
 			}
 			break;
 		case TARRAY:
-			if(t->bound >= 0 && l->op == ONAME) {
-				r = nod(OXXX, N, N);
-				nodconst(r, types[TINT], t->bound);
-				r->orig = n;
-				n = r;
-			}
+			if(t->bound < 0) // slice
+				break;
+			if(callrecv(l)) // has call or receive
+				break;
+			r = nod(OXXX, N, N);
+			nodconst(r, types[TINT], t->bound);
+			r->orig = n;
+			n = r;
 			break;
 		}
 		n->type = types[TINT];

test/const4.go

@@ -0,0 +1,77 @@
+// run
+
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test len constants and non-constants, http://golang.org/issue/3244.
+
+package main
+
+var b struct {
+	a[10]int
+}
+
+var m map[string][20]int
+
+var s [][30]int
+
+const (
+	n1 = len(b.a)
+	n2 = len(m[""])
+	n3 = len(s[10])
+)
+
+// Non-constants (see also const5.go).
+var (
+	n4 = len(f())
+	n5 = len(<-c)
+	n6 = cap(g())
+	n7 = cap(<-c1)
+)
+
+var calledF = false
+
+func f() *[40]int {
+	calledF = true
+	return nil
+}
+
+var c = func() chan *[50]int {
+	c := make(chan *[50]int, 2)
+	c <- nil
+	c <- new([50]int)
+	return c
+}()
+
+var calledG = false
+
+func g() *[60]int {
+	calledG = true
+	return nil
+}
+
+var c1 = func() chan *[70]int {
+	c := make(chan *[70]int, 2)
+	c <- nil
+	c <- new([70]int)
+	return c
+}()
+
+func main() {
+	if n1 != 10 || n2 != 20 || n3 != 30 || n4 != 40 || n5 != 50 || n6 != 60 || n7 != 70 {
+		println("BUG:", n1, n2, n3, n4, n5, n6, n7)
+	}
+	if !calledF {
+		println("BUG: did not call f")
+	}
+	if <-c == nil {
+		println("BUG: did not receive from c")
+	}
+	if !calledG {
+		println("BUG: did not call g")
+	}
+	if <-c1 == nil {
+		println("BUG: did not receive from c1")
+	}
+}

test/const5.go

@@ -0,0 +1,33 @@
+// errorcheck
+
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test that len non-constants are not constants, http://golang.org/issue/3244.
+
+package p
+
+var b struct {
+	a[10]int
+}
+
+var m map[string][20]int
+
+var s [][30]int
+
+func f() *[40]int
+var c chan *[50]int
+
+const (
+	n1 = len(b.a)
+	n2 = len(m[""])
+	n3 = len(s[10])
+
+	n4 = len(f())  // ERROR "must be constant"
+	n5 = len(<-c) // ERROR "must be constant"
+
+	n6 = cap(f())  // ERROR "must be constant"
+	n7 = cap(<-c) // ERROR "must be constant"
+)
+