schema/decoder: Support multiple `OfType`/`OfScopeId` in `Reference`

[radeksimko]

Context

Whilst working on #241, #232 (f2c8ee0 specifically) and investigating #170 it became clear that we need some more effective way to express that an attribute takes multiple types and/or scopes.

This function also expresses the problem:

hcl-lang/decoder/expr_one_of_ref_origins.go

Lines 28 to 59 in d2c7ba3

	func appendOrigins(origins, newOrigins reference.Origins) reference.Origins {
	// Deduplicating origins like this is probably not ideal
	// from performance perspective (N^2) but improving it would
	// require redesign of the schema.Reference constraint,
	// such that it doesn't necessitate the need of OneOf for multiple ScopeIds
	// and maintains all possible ScopeIds & Types as a *single* slice.
	for _, newOrigin := range newOrigins {
	newMatchableOrigin, ok := newOrigin.(reference.MatchableOrigin)
	if !ok {
	origins = append(origins, newOrigin)
	continue
	}
	foundMatch := false
	for i, origin := range origins {
	existingOrigin, ok := origin.(reference.MatchableOrigin)
	if ok &&
	existingOrigin.Address().Equals(newMatchableOrigin.Address()) &&
	rangesEqual(existingOrigin.OriginRange(), newMatchableOrigin.OriginRange()) {
	origins[i] = existingOrigin.AppendConstraints(newMatchableOrigin.OriginConstraints())
	foundMatch = true
	break
	}
	}
	if !foundMatch {
	origins = append(origins, newOrigin)
	}
	}
	return origins
	}

There are currently some known use cases in Terraform schema:

• for_each - map or set
• depends_on - multiple scopes (data, module, resource)

hcl-lang/decoder/extension_schemas.go

Lines 23 to 33 in d2c7ba3

	func forEachAttributeSchema() *schema.AttributeSchema {
	return &schema.AttributeSchema{
	IsOptional: true,
	Constraint: schema.OneOf{
	schema.AnyExpression{OfType: cty.Map(cty.DynamicPseudoType)},
	schema.AnyExpression{OfType: cty.Set(cty.String)},
	},
	Description: lang.Markdown("A meta-argument that accepts a map or a set of strings, and creates an instance for each item in that map or set.\n\n" +
	"**Note**: A given block cannot use both `count` and `for_each`."),
	}
	}

See also https://github.com/search?q=repo%3Ahashicorp%2Fterraform-schema+OfScopeId&type=code

Proposal

We should explore some more elegant ways of expressing such a constraint.

One possible solution would be to just introduce plural version OfType and OfScopeId, where validation ensures that either singular or plural is used (not both).