We originally introduced the idea of language experiments as a way to get
early feedback on not-yet-proven feature ideas, ideally as part of the
initial exploration of the solution space rather than only after a
solution has become relatively clear.
Unfortunately, our tradeoff of making them available in normal releases
behind an explicit opt-in in order to make it easier to participate in the
feedback process had the unintended side-effect of making it feel okay
to use experiments in production and endure the warnings they generate.
This in turn has made us reluctant to make use of the experiments feature
lest experiments become de-facto production features which we then feel
compelled to preserve even though we aren't yet ready to graduate them
to stable features.
In an attempt to tweak that compromise, here we make the availability of
experiments _at all_ a build-time flag which will not be set by default,
and therefore experiments will not be available in most release builds.
The intent (not yet implemented in this PR) is for our release process to
set this flag only when it knows it's building an alpha release or a
development snapshot not destined for release at all, which will therefore
allow us to still use the alpha releases as a vehicle for giving feedback
participants access to a feature (without needing to install a Go
toolchain) but will not encourage pretending that these features are
production-ready before they graduate from experimental.
Only language experiments have an explicit framework for dealing with them
which outlives any particular experiment, so most of the changes here are
to that generalized mechanism. However, the intent is that non-language
experiments, such as experimental CLI commands, would also in future
check Meta.AllowExperimentalFeatures and gate the use of those experiments
too, so that we can be consistent that experimental features will never
be available unless you explicitly choose to use an alpha release or
a custom build from source code.
Since there are already some experiments active at the time of this commit
which were not previously subject to this restriction, we'll pragmatically
leave those as exceptions that will remain generally available for now,
and so this new approach will apply only to new experiments started in the
future. Once those experiments have all concluded, we will be left with
no more exceptions unless we explicitly choose to make an exception for
some reason we've not imagined yet.
It's important that we be able to write tests that rely on experiments
either being available or not being available, so here we're using our
typical approach of making "package main" deal with the global setting
that applies to Terraform CLI executables while making the layers below
all support fine-grain selection of this behavior so that tests with
different needs can run concurrently without trampling on one another.
As a compromise, the integration tests in the terraform package will
run with experiments enabled _by default_ since we commonly need to
exercise experiments in those tests, but they can selectively opt-out
if they need to by overriding the loader setting back to false again.
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