configs: Accept and minimally validate a "language" argument

configs/experiments.go

@@ -5,6 +5,7 @@ import (
 
 	"github.com/hashicorp/hcl/v2"
 	"github.com/hashicorp/terraform/experiments"
+	"github.com/hashicorp/terraform/version"
 	"github.com/zclconf/go-cty/cty"
 )
 
@@ -25,6 +26,49 @@ func sniffActiveExperiments(body hcl.Body) (experiments.Set, hcl.Diagnostics) {
 		content, _, blockDiags := block.Body.PartialContent(configFileExperimentsSniffBlockSchema)
 		diags = append(diags, blockDiags...)
 
+		if attr, exists := content.Attributes["language"]; exists {
+			// We don't yet have a sense of selecting an edition of the
+			// language, but we're reserving this syntax for now so that
+			// if and when we do this later older versions of Terraform
+			// will emit a more helpful error message than just saying
+			// this attribute doesn't exist. Handling this as part of
+			// experiments is a bit odd for now but justified by the
+			// fact that a future fuller implementation of switchable
+			// languages would be likely use a similar implementation
+			// strategy as experiments, and thus would lead to this
+			// function being refactored to deal with both concerns at
+			// once. We'll see, though!
+			kw := hcl.ExprAsKeyword(attr.Expr)
+			currentVersion := version.SemVer.String()
+			const firstEdition = "TF2021"
+			switch {
+			case kw == "": // (the expression wasn't a keyword at all)
+				diags = diags.Append(&hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  "Invalid language edition",
+					Detail: fmt.Sprintf(
+						"The language argument expects a bare language edition keyword. Terraform %s supports only language edition %s, which is the default.",
+						currentVersion, firstEdition,
+					),
+					Subject: attr.Expr.Range().Ptr(),
+				})
+			case kw != firstEdition:
+				rel := "different"
+				if kw > firstEdition { // would be weird for this not to be true, but it's user input so anything goes
+					rel = "newer"
+				}
+				diags = diags.Append(&hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  "Unsupported language edition",
+					Detail: fmt.Sprintf(
+						"Terraform v%s only supports language edition %s. This module requires a %s version of Terraform CLI.",
+						currentVersion, firstEdition, rel,
+					),
+					Subject: attr.Expr.Range().Ptr(),
+				})
+			}
+		}
+
 		attr, exists := content.Attributes["experiments"]
 		if !exists {
 			continue

configs/parser_config.go

@@ -58,8 +58,8 @@ func (p *Parser) loadConfigFile(path string, override bool) (*File, hcl.Diagnost
 			content, contentDiags := block.Body.Content(terraformBlockSchema)
 			diags = append(diags, contentDiags...)
 
-			// We ignore the "terraform_version" and "experiments" attributes
-			// here because sniffCoreVersionRequirements and
+			// We ignore the "terraform_version", "language" and "experiments"
+			// attributes here because sniffCoreVersionRequirements and
 			// sniffActiveExperiments already dealt with those above.
 
 			for _, innerBlock := range content.Blocks {
@@ -244,6 +244,7 @@ var terraformBlockSchema = &hcl.BodySchema{
 	Attributes: []hcl.AttributeSchema{
 		{Name: "required_version"},
 		{Name: "experiments"},
+		{Name: "language"},
 	},
 	Blocks: []hcl.BlockHeaderSchema{
 		{
@@ -283,8 +284,7 @@ var configFileVersionSniffBlockSchema = &hcl.BodySchema{
 // to decode a single attribute from inside a "terraform" block.
 var configFileExperimentsSniffBlockSchema = &hcl.BodySchema{
 	Attributes: []hcl.AttributeSchema{
-		{
-			Name: "experiments",
-		},
+		{Name: "experiments"},
+		{Name: "language"},
 	},
 }

configs/testdata/error-files/invalid_language_edition.tf

@@ -0,0 +1,4 @@
+terraform {
+  # The language argument expects a bare keyword, not a string.
+  language = "TF2021" # ERROR: Invalid language edition
+}

configs/testdata/error-files/unsupported_language_edition.tf

@@ -0,0 +1,6 @@
+terraform {
+  # If a future change in this repository happens to make TF2038 a valid
+  # edition then this will start failing; in that case, change this file to
+  # select a different edition that isn't supported.
+  language = TF2038 # ERROR: Unsupported language edition
+}

configs/testdata/valid-files/valid-language-edition.tf

@@ -0,0 +1,8 @@
+terraform {
+  # If we drop support for TF2021 in a future Terraform release then this
+  # test will fail. In that case, update this to a newer edition that is
+  # still supported, because the purpose of this test is to verify that
+  # we can successfully decode the language argument, not specifically
+  # that we support TF2021.
+  language = TF2021
+}