Introduce deprecated_function_linter()

[IndrajeetPatil]

As discussed in #1377.

This linter will mostly centre around packages that assign lifecycle stages for functions (testthat, tidyverse, etc.).

[AshesITR]

Is there maybe a way to programmatically extract this lifecycle information from packages?

[IndrajeetPatil]

Yeah, I was thinking the same.

Probably via .Rd files because a function is marked as deprecated using the following code in the docs:

#' `r lifecycle::badge("deprecated")`

[AshesITR]

It seems the first call in the function should also be to lifecycle.

[AshesITR]

Seems like if we loadNamespace() the required packages, we can inspect the functions to see if their call graph unconditionally hits a (known) deprecation function:

body(testthat::expect_that)[[2]][[1]]
#> edition_deprecate
body(survival::survConcordance)[[3]][[1]]
#> .Deprecated
body(tibble::data_frame)[[2]][[1]]
#> deprecate_warn

[AshesITR]

I just discovered lifecycle::lint_lifecycle() Maybe we can convince lifecycle to export lifecycle_linter()?
That means the deprecated_function_linter() could focus on other lifecycle management styles, e.g. .Deprecated() or the style used in testthat.

[IndrajeetPatil]

A few resources to consider here, which would help us with implementation:

• {pkgdown} has a utility to extract lifecycle badge:

https://github.com/r-lib/pkgdown/blob/0ba8cb6c68c098a0d1d780052758da5ac9f83e3a/R/package.R#L188

• {now} detects and removes functions of given lifecycle from the namespace’s exports

https://github.com/moodymudskipper/now