Remove deprecation on undeclared variable, docs, and summary adjustment (#27795)

* Remove deprecation on undeclared variable

Remove deprecation and add docs specific to the behavior around
undeclared variable values

* Limit full warnings to 2 instances, then summary

This way, the third warning is a summary, rather than the fourth
warning being the summary

backend/unparsed_value.go

@@ -69,21 +69,15 @@ func ParseVariableValues(vv map[string]UnparsedVariableValue, decls map[string]*
 		if !declared {
 			switch val.SourceType {
 			case terraform.ValueFromConfig, terraform.ValueFromAutoFile, terraform.ValueFromNamedFile:
-				// These source types have source ranges, so we can produce
-				// a nice error message with good context.
-				//
-				// This one is a warning for now because there is an existing
-				// pattern of providing a file containing the superset of
-				// variables across all configurations in an organization. This
-				// is deprecated in v0.12.0 because it's more important to give
-				// feedback to users who make typos. Those using this approach
-				// should migrate to using environment variables instead before
-				// this becomes an error in a future major release.
-				if seenUndeclaredInFile < 3 {
+				// We allow undeclared names for variable values from files and warn in case
+				// users have forgotten a variable {} declaration or have a typo in their var name.
+				// Some users will actively ignore this warning because they use a .tfvars file
+				// across multiple configurations.
+				if seenUndeclaredInFile < 2 {
 					diags = diags.Append(tfdiags.Sourceless(
 						tfdiags.Warning,
 						"Value for undeclared variable",
-						fmt.Sprintf("The root module does not declare a variable named %q but a value was found in file %q. To use this value, add a \"variable\" block to the configuration.\n\nUsing a variables file to set an undeclared variable is deprecated and will become an error in a future release. If you wish to provide certain \"global\" settings to all configurations in your organization, use TF_VAR_... environment variables to set these instead.", name, val.SourceRange.Filename),
+						fmt.Sprintf("The root module does not declare a variable named %q but a value was found in file %q. If you meant to use this value, add a \"variable\" block to the configuration.\n\nTo silence these warnings, use TF_VAR_... environment variables to provide certain \"global\" settings to all configurations in your organization. To reduce the verbosity of these warnings, use the -compact-warnings option.", name, val.SourceRange.Filename),
 					))
 				}
 				seenUndeclaredInFile++
@@ -114,7 +108,7 @@ func ParseVariableValues(vv map[string]UnparsedVariableValue, decls map[string]*
 		ret[name] = val
 	}
 
-	if seenUndeclaredInFile >= 3 {
+	if seenUndeclaredInFile > 2 {
 		extras := seenUndeclaredInFile - 2
 		diags = diags.Append(&hcl.Diagnostic{
 			Severity: hcl.DiagWarning,

backend/unparsed_value_test.go

@@ -1,6 +1,7 @@
 package backend
 
 import (
+	"strings"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
@@ -59,7 +60,7 @@ func TestParseVariableValuesUndeclared(t *testing.T) {
 	for _, diag := range diags {
 		t.Logf("%s: %s", diag.Description().Summary, diag.Description().Detail)
 	}
-	if got, want := len(diags), 5; got != want {
+	if got, want := len(diags), 4; got != want {
 		t.Fatalf("wrong number of diagnostics %d; want %d", got, want)
 	}
 
@@ -73,14 +74,14 @@ func TestParseVariableValuesUndeclared(t *testing.T) {
 	if got, want := diags[1].Description().Summary, undeclSingular; got != want {
 		t.Errorf("wrong summary for diagnostic 1\ngot:  %s\nwant: %s", got, want)
 	}
-	if got, want := diags[2].Description().Summary, undeclSingular; got != want {
+	if got, want := diags[2].Description().Summary, undeclPlural; got != want {
 		t.Errorf("wrong summary for diagnostic 2\ngot:  %s\nwant: %s", got, want)
 	}
-	if got, want := diags[3].Description().Summary, undeclPlural; got != want {
-		t.Errorf("wrong summary for diagnostic 3\ngot:  %s\nwant: %s", got, want)
+	if got, want := diags[2].Description().Detail, "3 other variable(s)"; !strings.Contains(got, want) {
+		t.Errorf("wrong detail for diagnostic 2\ngot:  %s\nmust contain: %s", got, want)
 	}
-	if got, want := diags[4].Description().Summary, missingRequired; got != want {
-		t.Errorf("wrong summary for diagnostic 4\ngot:  %s\nwant: %s", got, want)
+	if got, want := diags[3].Description().Summary, missingRequired; got != want {
+		t.Errorf("wrong summary for diagnostic 3\ngot:  %s\nwant: %s", got, want)
 	}
 
 	wantVals := terraform.InputValues{

website/docs/language/values/variables.html.md

@@ -437,6 +437,44 @@ $ export TF_VAR_availability_zone_names='["us-west-1b","us-west-1d"]'
 For readability, and to avoid the need to worry about shell escaping, we
 recommend always setting complex variable values via variable definitions files.
 
+### Values for Undeclared Variables
+
+If you have defined a variable value, but not its corresponding `variable {}`
+definition, you may get an error or warning depending on how you have provided
+that value.
+
+If you provide values for undeclared variables defined as [environment variables](#environment-variables)
+you will not get an error or warning. This is because environment variables may
+be declared but not used in all configurations that might be run.
+
+If you provide values for undeclared variables defined [in a file](#variable-definitions-tfvars-files)
+you will get a warning. This is to help in cases where you have provided a variable
+value _meant_ for a variable declaration, but perhaps there is a mistake in the
+value definition. For example, the following configuration:
+
+```terraform
+variable "moose" {
+  type = string
+}
+```
+
+And the following `.tfvars` file:
+
+```hcl
+mosse = "Moose"
+```
+
+Will cause Terraform to warn you that there is no variable declared `"mosse"`, which can help
+you spot this mistake.
+
+If you use `.tfvars` files across multiple configurations and expect to continue to see this warning,
+you can use the [`-compact-warnings`](https://www.terraform.io/docs/cli/commands/plan.html#compact-warnings)
+option to simplify your output.
+
+If you provide values for undeclared variables on the [command line](#variables-on-the-command-line),
+Terraform will error. To avoid this error, either declare a variable block for the value, or remove
+the variable value from your Terraform call.
+
 ### Variable Definition Precedence
 
 The above mechanisms for setting variables can be used together in any