spec: load statements may not be nested in any other statement (#276)

This is a potentially breaking language change, but I suspect
few users care. If you are one of them, please report the issue.

Fixes #275

doc/spec.md

@@ -2945,7 +2945,7 @@ load("module.star", "x", "y", "z")       # assigns x, y, and z
 load("module.star", "x", y2="y", "z")    # assigns x, y2, and z
 ```
 
-A load statement within a function is a static error.
+A load statement may not be nested inside any other statement.
 
 
 ## Module execution

resolve/resolve.go

@@ -214,7 +214,8 @@ type resolver struct {
 	// isGlobal may be nil.
 	isGlobal, isPredeclared, isUniversal func(name string) bool
 
-	loops int // number of enclosing for loops
+	loops   int // number of enclosing for/while loops
+	ifstmts int // number of enclosing if statements loops
 
 	errors ErrorList
 }
@@ -497,8 +498,10 @@ func (r *resolver) stmt(stmt syntax.Stmt) {
 			r.errorf(stmt.If, "if statement not within a function")
 		}
 		r.expr(stmt.Cond)
+		r.ifstmts++
 		r.stmts(stmt.True)
 		r.stmts(stmt.False)
+		r.ifstmts--
 
 	case *syntax.AssignStmt:
 		r.expr(stmt.RHS)
@@ -551,8 +554,13 @@ func (r *resolver) stmt(stmt syntax.Stmt) {
 		}
 
 	case *syntax.LoadStmt:
+		// A load statement may not be nested in any other statement.
 		if r.container().function != nil {
 			r.errorf(stmt.Load, "load statement within a function")
+		} else if r.loops > 0 {
+			r.errorf(stmt.Load, "load statement within a loop")
+		} else if r.ifstmts > 0 {
+			r.errorf(stmt.Load, "load statement within a conditional")
 		}
 
 		for i, from := range stmt.From {

resolve/testdata/resolve.star

@@ -142,6 +142,21 @@ load("foo",
      c="_d", ### "load: names with leading underscores are not exported: _d"
      _e="f") # ok
 
+---
+# option:globalreassign
+if M:
+    load("foo", "bar") ### "load statement within a conditional"
+
+---
+# option:globalreassign
+for x in M:
+    load("foo", "bar") ### "load statement within a loop"
+
+---
+# option:recursion option:globalreassign
+while M:
+    load("foo", "bar") ### "load statement within a loop"
+
 ---
 # return statements must be within a function