Github user ueshin commented on a diff in the pull request:

    https://github.com/apache/spark/pull/21193#discussion_r186681857
  
    --- Diff: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Cast.scala
 ---
    @@ -623,8 +624,14 @@ case class Cast(child: Expression, dataType: DataType, 
timeZoneId: Option[String
       override def doGenCode(ctx: CodegenContext, ev: ExprCode): ExprCode = {
         val eval = child.genCode(ctx)
         val nullSafeCast = nullSafeCastFunction(child.dataType, dataType, ctx)
    +
    +    // Below the code comment including `eval.value` and `eval.isNull` is 
a trick. It makes the two
    +    // expr values are referred by this code block.
         ev.copy(code = eval.code +
    -      castCode(ctx, eval.value, eval.isNull, ev.value, ev.isNull, 
dataType, nullSafeCast))
    +      code"""
    +        // Cast from ${eval.value}, ${eval.isNull}
    +        ${castCode(ctx, eval.value, eval.isNull, ev.value, ev.isNull, 
dataType, nullSafeCast)}
    --- End diff --
    
    I might miss something, but don't we need to pass `ExprValue` or `Block` 
and return `Block` from `castCode()` to keep track of code snippets and inputs?
    We can say the same thing anywhere we create a smaller code snippet using 
`s` interpolation.


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