Templating fields

The leaves of the config data structures are either concrete scalar values (such as numbers, strings and booleans) or Fields which is how we support a limited form of type-checked templating within config files.

While hard coded scalar values are parsed directly using serde, Fields are allowed to be in one of two states:

• Pending, where they hold the name of a templating variable that the user must specify as part of their test plan.
• Resolved, where they hold a concrete scalar value, either because a value was provided directly within the config file or following successful templating.

Pending fields are indicated within a config file using "{{ field_name }}" syntax and are parsed into the Field::Pending enum variant directly using serde. When writing new providers you should make use of templating fields where it makes sense for users to be able to dynamically set values when executing a test plan and avoid using them where such flexibility is not required.

For example, we do not allow the inline file provider’s content argument to be templated as the intention is for this to always be provided within the test plan itself.