1. Conceptually revert #20276
The feature was implemented for the `responders` gem. In the end,
they did not need that feature, and have found a better fix (see
plataformatec/responders#131).
`ImplicitRender` is the place where Rails specifies our default
policies for the case where the user did not explicitly tell us
what to render, essentially describing a set of heuristics. If
the gem (or the user) knows exactly what they want, they could
just perform the correct `render` to avoid falling through to
here, as `responders` did (the user called `respond_with`).
Reverting the patch allows us to avoid exploding the complexity
and defining “the fallback for a fallback” policies.
2. `respond_to` and templates are considered exhaustive enumerations
If the user specified a list of formats/variants in a `respond_to`
block, anything that is not explicitly included should result
in an `UnknownFormat` error (which is then caught upstream to
mean “406 Not Acceptable” by default). This is already how it
works before this commit.
Same goes for templates – if the user defined a set of templates
(usually in the file system), that set is now considered exhaustive,
which means that “missing” templates are considered `UnknownFormat`
errors (406).
3. To keep API endpoints simple, the implicit render behavior for
actions with no templates defined at all (regardless of formats,
locales, variants, etc) are defaulted to “204 No Content”. This
is a strictly narrower version of the feature landed in #19036 and
#19377.
4. To avoid confusion when interacting in the browser, these actions
will raise an `UnknownFormat` error for “interactive” requests
instead. (The precise definition of “interactive” requests might
change – the spirit here is to give helpful messages and avoid
confusions.)
Closes#20666, #23062, #23077, #23564
[Godfrey Chan, Jon Moss, Kasper Timm Hansen, Mike Clark, Matthew Draper]
