library(targets)
list(
  tar_target(
    x,
    warning("run_warning"),
    format = tar_format(
      read = function(path) {
        readRDS(path)
      },
      write = function(object, path) {
        warning("storage_warning")
        saveRDS(object, path)
      }
    )
  )
)

tar_make(reporter = "silent")
tar_meta(x, warnings)
#> # A tibble: 1 × 2
#>   name  warnings                    
#>   <chr> <chr>                       
#> 1 x     run_warning. storage_warning