feature request: warn when using if: ${{ ... }} with anything else around it

[antdking]

A few have encountered this issue in the past too: https://github.com/orgs/community/discussions/25641

These end up being equivalent, which is unexpected behaviour at first glance.

if: |
  ${{ false }}

if: format('{0}\n', false)

It seems that when ${{ }} is encountered in if:, the value is treated as a string to interpolate, which is then used for the truthy check directly. No trimming, no recursive evaluation.

That means this is also a string:

if: true && ${{ false }}
# equiv: format('true && {0}', false)

Spaces also trigger this behaviour:

if: ' ${{ false }}'
# same as
if: format(' {0}', false)

As an experiment, these were also tried, to force the string 'false':

if: ${{ 'fa' }}${{ 'lse' }}
if: format('{0}{1}', 'fa', 'lse')
if: format('{0}', false)
if: ${{ false }}${{ '' }}

Which is not converted to the boolean false, but is instead tested as the string 'false'; again being truthy in the final evaluation.

While it'd be great to fully unravel expressions, I think there's a shortcut that can be taken:

If there is anything before or after the first ${{ }}, it's a string that will always be truthy

---

Edgecase

Technically, placing '' and null next to each other will collapse to '', so is falsy.
But in reality, you should be using join(a.*.c, '') or chaining ||.

if: ${{ '' }}${{ null }}${{ null }}${{ '' }}${{ '' }}
# equiv: format('{0}{1}{2}{3}{4}', '', null, null, '', '')
# equiv: ''

• reference workflow where much of this was tested: https://github.com/antdking/actions-scratch-pad/actions/runs/4089246585/jobs/7051744589