New lazyload rdx key type: list(eagerKey=, lazyKeys=)

Prior to R-3.6.0 the keys in the lazyload key files, e.g.
pkg/data/Rdata.rdx or pkg/R/pkg.rdx, seemed to all be 2-long integer
vectors.  Now they can be lists.  The ones I have seen have two components,
"eagerKey" is a 2-long integer vector and "lazyKeys" is a named list of
2-long integer vectors.

rdx <- readRDS(system.file(package="survival", "data", "Rdata.rdx"))
str(Filter(is.list, rdx$references))

List of 2
  $ env::1:List of 2
   ..$ eagerKey: int [1:2] 273691 183
   ..$ lazyKeys:List of 1
   .. ..$ lines: int [1:2] 273874 284
  $ env::2:List of 2
   ..$ eagerKey: int [1:2] 473142 166
   ..$ lazyKeys:List of 1
   .. ..$ lines: int [1:2] 473308 310

or

  rdx <- readRDS(system.file(package="lambda.r", "R", "lambda.r.rdx"))
length(Filter(is.integer, rdx$references))

[1] 4

str(Filter(Negate(is.integer), rdx$references))

List of 5
  $ env::5:List of 2
   ..$ eagerKey: int [1:2] 28278 328
   ..$ lazyKeys:List of 2
   .. ..$ lines    : int [1:2] 28606 80
   .. ..$ parseData: int [1:2] 28686 389
  $ env::6:List of 2
   ..$ eagerKey: int [1:2] 29075 327
   ..$ lazyKeys:List of 2
   .. ..$ lines    : int [1:2] 29402 71
   .. ..$ parseData: int [1:2] 29473 321
  $ env::7:List of 2
   ..$ eagerKey: int [1:2] 29794 325
   ..$ lazyKeys:List of 2
   .. ..$ lines    : int [1:2] 30119 117
   .. ..$ parseData: int [1:2] 30236 752
... many more ...

All the ones I've seen involve the environment in srcref attributes and
most packages do not keep that.  Will these be used for more sorts of
environments in the future?

What is the meaning of the lazyKeys?  Are these stored as promises until
needed or is there some special option to never or always load them?

Bill Dunlap
TIBCO Software
wdunlap tibco.com

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