Parse arbitrary exprs after "import" and "importStr" (#153)

Fixes #138

The spec is not very clear on this but this behavior is consistent with
the C++ and Go implementations.

---------

Co-authored-by: Li Haoyi <32282535+lihaoyi-databricks@users.noreply.github.com>

sjsonnet/src/sjsonnet/Expr.scala

@@ -1,7 +1,6 @@
 package sjsonnet
 
-import java.util.BitSet
-
+import java.util.{Arrays, BitSet}
 import scala.collection.mutable
 
 /**
@@ -116,6 +115,11 @@ object Expr{
   case class AssertExpr(pos: Position, asserted: Member.AssertStmt, returned: Expr) extends Expr
   case class LocalExpr(pos: Position, bindings: Array[Bind], returned: Expr) extends Expr {
     override def toString = s"LocalExpr($pos, ${arrStr(bindings)}, $returned)"
+    override def equals(o: Any): Boolean = o match {
+      case o: LocalExpr =>
+        pos == o.pos && Arrays.equals(bindings.asInstanceOf[Array[AnyRef]], o.bindings.asInstanceOf[Array[AnyRef]]) && returned == o.returned
+      case _ => false
+    }
   }
 
   case class Bind(pos: Position, name: String, args: Params, rhs: Expr) extends Member

sjsonnet/src/sjsonnet/Parser.scala

@@ -237,10 +237,15 @@ class Parser(val currentFile: Path) {
   )
 
   def local[_: P] = P( localExpr )
-  def importStr[_: P](pos: Position) = P( string.map(Expr.ImportStr(pos, _)) )
-  def `import`[_: P](pos: Position) = P( string.map(Expr.Import(pos, _)) )
+  def importStr[_: P](pos: Position) = P( importExpr.map(Expr.ImportStr(pos, _)) )
+  def `import`[_: P](pos: Position) = P( importExpr.map(Expr.Import(pos, _)) )
   def error[_: P](pos: Position) = P(expr.map(Expr.Error(pos, _)) )
 
+  def importExpr[_: P]: P[String] = P(expr.flatMap {
+    case Val.Str(_, s) => Pass(s)
+    case _ => Fail.opaque("string literal (computed imports are not allowed)")
+  })
+
   def unaryOpExpr[_: P](pos: Position, op: Char) = P(
     expr1.map{ e =>
       def k2 = op match{

sjsonnet/test/src/sjsonnet/ParserTests.scala

@@ -27,6 +27,16 @@ object ParserTests extends TestSuite{
     test("givenDuplicateFieldsInListComprehension_expectError") {
       parseErr("""{ ["bar"]: x for x in [1, 2]}""") ==> """Expected no duplicate field: "bar" :1:29, found "}""""
     }
+
+    test("computedImports") {
+      parse("""local foo = import "foo"; 0""") ==>
+        LocalExpr(pos(6), Array(Bind(pos(6), "foo", null, Import(pos(12), "foo"))), Num(pos(26),0.0))
+      parse("""local foo = (import "foo") + bar; 0""") ==>
+        LocalExpr(pos(6), Array(Bind(pos(6), "foo", null, BinaryOp(pos(27), Import(pos(13), "foo"), 3, Id(pos(29), "bar")))), Num(pos(34),0.0))
+      parseErr("""local foo = import ("foo" + bar); 0""") ==> """Expected string literal (computed imports are not allowed):1:33, found "; 0""""
+      parseErr("""local foo = import "foo" + bar; 0""") ==> """Expected string literal (computed imports are not allowed):1:31, found "; 0""""
+    }
+
     test("id starts with number") {
       parseErr("""{1_n: "foo",}""") ==> """Expected "}":1:2, found "1_n: \"foo\"""""
     }