Linter warnings bubble up as config formatting errors with confusing message

[Bisaloo]

See the following reprex package: https://github.com/Bisaloo/testlintrconfigerror/:

linters: linters_with_defaults(
    backport_linter("2.10.0")
  )

The config file is correctly formatted but the backport_linter() setting will generate a warning:

lintr/R/backport_linter.R

Lines 116 to 119 in 1493c5e

	if (r_version < "3.0.0") {
	warning("It is not recommended to depend on an R version older than 3.0.0. Resetting 'r_version' to 3.0.0.")
	r_version <- R_system_version("3.0.0")
	}

This warning is totally unrelated with the config formatting but will bubble up when config is read and produce a red herring message:

lintr::lint_package()

Error: Malformed config file, ensure it ends in a newline
Malformed config file, ensure it ends in a newline
It is not recommended to depend on an R version older than 3.0.0. Resetting 'r_version' to 3.0.0.

I looked again and again for the missing newline and couldn't figure out what was wrong until I realized the error came from the warning added at the end of the error message.