meta: Support all Terraform expression types

[radeksimko]

Problem Statement

Terraform supports many different expressions via HCL. Some expressions might be more commonly used (e.g. static strings, numbers, booleans, scoped references...) and some less.

Users expect an LS-supported editor such as VS Code extension to provide them completion or hover and highlight arbitrarily complex and nested valid expressions.

Technical Details

The following HCL expressions are already supported in some form or shape:

• LiteralValueExpr
• ScopeTraversalExpr
• ObjectConsExpr
• ObjectConsKeyExpr
• TupleConsExpr

The following are not yet supported:

• splat expression
  • AnonSymbolExpr
  • SplatExpr
• for loop
  • ForExpr
  • TemplateJoinExpr
• template expression
  • TemplateExpr
  • TemplateWrapExpr
• operators
  • BinaryOpExpr
  • UnaryOpExpr
  • ParenthesesExpr
• ConditionalExpr
• FunctionCallExpr
• IndexExpr
• RelativeTraversalExpr

User Impact

Pretty much all users would benefit from this, some expressions may only be used by more advanced users, but supporting all expressions is pre-requisite for being able to fully and reliably evaluate any expression.

Expected User Experience

Completion

User is provided with relevant completion for and inside any expression.

For example:

• User is able to complete function names, i.e. attr = HERE
• User is able to complete relevant references as arguments to function calls, i.e. attr = max( HERE )
• User is able to complete references within a template, i.e. "${ HERE }"
• User is able to complete references alongside operators, i.e. attr = 3 + HERE
• etc.

Hover

User can see details about any expression on hover

For example:

• attr = max(1, 3) when hovered over max signature of the function and description comes up
• attr = "hello ${var.person}" when hovered over var.person, type and description of the variable comes up
• etc.

Semantic Highlighting

All expressions are represented as highlightable tokens where appropriate.

For example:

• max(1, 3) - max is reported as function call
• type = list(string) - list and string is reported as type declarations
• "hello ${var.person}" - "hello is reported as string and var & person as traversal steps
• etc.

Proposal

• UX around expression types

  • Support step-based reference completion terraform-ls#492
  • Recognize individual expressions when nested terraform-ls#496
  • Provide function signature help terraform-ls#857
  • Reflect convertible types in type comparison terraform-ls#583
  • Show interpolated expression values on hover terraform-ls#495
  • Reflect nullable variables in Terraform 1.1.0+ terraform-ls#894
  • Reflect new resource. address and _ escaping block in 0.15.4+ terraform-ls#893

• Local References

  • schema: Introduce Extensions to BodySchema hcl-lang#118
  • Support self.* references in provisioner, connection and lifecycle blocks terraform-ls#859
  • Support count.index references in blocks w/ count terraform-ls#860
  • Support each.* references in blocks w/ for_each terraform-ls#861
  • Support for dynamic block terraform-ls#530
  • Go-to-definition/go-to-references for count.index/count terraform-ls#1093
  • Go-to-definition/go-to-references for each.*/for_each terraform-ls#1095
  • Go-to-definition/go-to-references for self.* in provisioner blocks etc. terraform-ls#1096

• Expression Types Support

  • Support for type declarations (for variable type) terraform-ls#490
  • Solidify handling of optional/default object attributes terraform-ls#917
  • Support built-in functions terraform-ls#37
  • Support ${...} template expressions in attribute values terraform-ls#522
  • Support splat expressions (*) terraform-ls#526
  • Support for-in-if expressions terraform-ls#527
  • Support conditional expressions (condition ? true : false) terraform-ls#528
  • Support operator expressions (+, -, !=, !...) and parentheses terraform-ls#529
  • Support more complex index expressions (elem[key]) terraform-ls#531
  • Support relative references / traversals terraform-ls#532