Prohibit branching over file target stems.

[wlandau]

Prework

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Description

Right now, if you branch over a file target, that file target should not be a stem because downstream branches do not invalidate if the files change.

Reproducible example

It is possible to incorrectly map over files. Need to lock that down.

options(crayon.enabled = FALSE, tidyverse.quiet = TRUE)
library(targets)
library(tidyverse)
write_csv(tibble(x1 = 1, x2 = 1), "a.csv")
write_csv(tibble(x1 = 1, x2 = 1), "b.csv")
tar_script({
  options(crayon.enabled = FALSE, tidyverse.quiet = TRUE)
  library(readr)
  tar_pipeline(
    tar_target(paths, c("a.csv", "b.csv"), format = "file"),
    tar_target(data, read_csv(paths, col_types = "dd"), pattern = map(paths))
  )
})
tar_make()
#> ● run target paths
#> ● run branch data_5085a9cd
#> ● run branch data_b7dcad9d
write_csv(tibble(x1 = 2, x2 = 2), "b.csv")
tar_make()
#> ● run target paths
#> ✓ skip branch data_5085a9cd
#> ✓ skip branch data_b7dcad9d

^{Created on 2020-08-07 by the reprex package (v0.3.0)}

Desired result

We have two options.

1. Require paths to already be a pattern.
2. Invalidate all of data.

[wlandau]

Let's go with (1). What users actually want here is to invalidate some sub-targets but not others, and this is impossible in the reprex because of the composite hash.