Add terraform version command

This commit adds a new `module.terraform` command which returns the required and discovered version of Terraform in the currrent workspace.

internal/langserver/handlers/command/terraform.go

@@ -0,0 +1,56 @@
+package command
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/creachadair/jrpc2/code"
+	"github.com/hashicorp/terraform-ls/internal/langserver/cmd"
+	"github.com/hashicorp/terraform-ls/internal/uri"
+)
+
+const terraformVersionRequestVersion = 0
+
+type terraformInfoResponse struct {
+	FormatVersion     int    `json:"v"`
+	RequiredVersion   string `json:"required_version,omitempty"`
+	DiscoveredVersion string `json:"discovered_version,omitempty"`
+	DiscoveredPath    string `json:"discovered_path,omitempty"`
+}
+
+func (h *CmdHandler) TerraformVersionRequestHandler(ctx context.Context, args cmd.CommandArgs) (interface{}, error) {
+	response := terraformInfoResponse{
+		FormatVersion: terraformVersionRequestVersion,
+	}
+
+	modUri, ok := args.GetString("uri")
+	if !ok || modUri == "" {
+		return response, fmt.Errorf("%w: expected module uri argument to be set", code.InvalidParams.Err())
+	}
+
+	if !uri.IsURIValid(modUri) {
+		return response, fmt.Errorf("URI %q is not valid", modUri)
+	}
+
+	modPath, err := uri.PathFromURI(modUri)
+	if err != nil {
+		return response, err
+	}
+
+	mod, _ := h.StateStore.Modules.ModuleByPath(modPath)
+	if mod == nil {
+		return response, nil
+	}
+
+	if mod.TerraformVersion == nil {
+		return response, nil
+	}
+	if mod.Meta.CoreRequirements == nil {
+		return response, nil
+	}
+
+	response.DiscoveredVersion = mod.TerraformVersion.String()
+	response.RequiredVersion = mod.Meta.CoreRequirements.String()
+
+	return response, nil
+}

internal/langserver/handlers/execute_command.go

@@ -24,6 +24,7 @@ func cmdHandlers(svc *service) cmd.Handlers {
 		cmd.Name("terraform.validate"): cmdHandler.TerraformValidateHandler,
 		cmd.Name("module.calls"):       cmdHandler.ModuleCallsHandler,
 		cmd.Name("module.providers"):   cmdHandler.ModuleProvidersHandler,
+		cmd.Name("module.terraform"):   cmdHandler.TerraformVersionRequestHandler,
 	}
 }