New Code Action: Extract expression into a variable

[radeksimko]

Background

When refactoring existing configuration, one of the actions users may want to take is to extract an expression into a variable, to make the configuration more flexible/reusable and better communicate intentions. This is especially relevant if the configuration represents a published module or the refactoring has the intentions of turning it into one.

For example, they may start with:

resource "azurerm_app_service_environment" "name" {
  name                = "foo"
  resource_group_name = "toot"
  subnet_id           = "noot"
}

and want to end up with:

variable "resource_group_name" {
  type    = string
  default = "toot"
}

resource "azurerm_app_service_environment" "name" {
  name                = "foo"
  resource_group_name = var.resource_group_name
  subnet_id           = "noot"
}

Proposal

• Implement refactor.extract code action for any interpolatable expression inside of resource
• Implement refactor.extract code action for any interpolatable expression inside of data
• Implement refactor.extract code action for any interpolatable expression (inputs) inside of module
• Implement refactor.extract code action for any interpolatable expression (value) inside of output
• Implement refactor.extract code action for any interpolatable expression inside of provider
• Implement refactor.extract code action for any expression inside of locals
• Implement refactor.extract code action for id inside of import
• Implement refactor.extract code action for any interpolatable expression inside of check

Implementation Notes

There will be edge cases related to pre-existing interpolation involving function calls or references (which are not allowed inside of variable default).

For example, the user starts with:

resource "azurerm_app_service_environment" "name" {
  name                = "foo"
  resource_group_name = lower("Toot-${var.foot}")
  subnet_id           = "noot"
}

We can consider different solutions for such edge cases:

1. Extract it as variable with corresponding constraint but lose the value in the process
2. Avoid offering that code action in this context entirely

Related: Extraction into a local value may be a better choice here.