meta: Support for external linters

[danieldreier]

Problem Statement

Many advanced users of Terraform leverage a number of external tools, to help them e.g.

• find unused declarations
• enforce best practices or naming conventions
• enforce good security hygiene

Examples of tools

• tflint
• (0.15) terraform test
• terratest
• Sentinel
• Terraform Test Kitchen
• terraform-compliance
• config-lint
• tfsec

Such users would benefit from a more seamless integration with these tools, such that they don't need to leave the comfort of their IDE to run the tool and receive any relevant diagnostics natively - so they're able to address any problems quickly and easily.

Technical Details

While tools may each serve different purpose, they have something in common:

• they all need to be installed and kept updated somehow; or found at a particular location via $PATH
• they all need to be executed with particular file/dir path
• most of them will output some kind of diagnostics associated with particular location in code

User Impact

Any users which use any of the above tools would benefit from closer integration.

See also

• [tracking issue] tflint #337

Expected User Experience

Users will be able to run the configured linters:

• automatically on save
• via the command palette, similar to how they're able to run terraform validate today

Users will receive diagnostics in the Problems pane after the linter runs, similar to how they receive diagnostics for HCL syntax issues e.g.

Proposal

• Store diagnostics in memdb table terraform-ls#856
• Enable users to configure linter settings in VS Code #1035
• Add linter configuration to initialization options terraform-ls#855
• Integrate tflint with focus on a generalized linter/validator interface terraform-ls#334
• Provide diagnostics and quick fixes for invalid configuration #720

It would make sense for the integration to take the above patterns into account and keep in mind possible further integrations that have such commonalities.

There is some potentially relevant prior art to be seen in the Go VS Code extension:

• external tools
• Custom linter