command: validate must set values for root variables

Since the intent of the validate command is to check config validity
regardless of context (input variables, state, etc), we use unknown values
of the requested type here, which will then allow us to complete type
checking against the specified types of the variables without assuming
any particular values.

command/validate.go

@@ -6,6 +6,8 @@ import (
 	"path/filepath"
 	"strings"
 
+	"github.com/zclconf/go-cty/cty"
+
 	"github.com/hashicorp/terraform/terraform"
 	"github.com/hashicorp/terraform/tfdiags"
 )
@@ -119,8 +121,27 @@ func (c *ValidateCommand) validate(dir string) tfdiags.Diagnostics {
 		return diags
 	}
 
+	// "validate" is to check if the given module is valid regardless of
+	// input values, current state, etc. Therefore we populate all of the
+	// input values with unknown values of the expected type, allowing us
+	// to perform a type check without assuming any particular values.
+	varValues := make(terraform.InputValues)
+	for name, variable := range cfg.Module.Variables {
+		ty := variable.Type
+		if ty == cty.NilType {
+			// Can't predict the type at all, so we'll just mark it as
+			// cty.DynamicVal (unknown value of cty.DynamicPseudoType).
+			ty = cty.DynamicPseudoType
+		}
+		varValues[name] = &terraform.InputValue{
+			Value:      cty.UnknownVal(ty),
+			SourceType: terraform.ValueFromCLIArg,
+		}
+	}
+
 	opts := c.contextOpts()
 	opts.Config = cfg
+	opts.Variables = varValues
 
 	tfCtx, ctxDiags := terraform.NewContext(opts)
 	diags = diags.Append(ctxDiags)