Fix diagnostic & avoid variable override via environment

Author: radeksimko

internal/backend/local/backend_apply.go

@@ -340,16 +340,49 @@ func (b *Local) opApply(
 					})
 				} else {
 					// The user can't override the planned variables, so we
-					// error when possible to avoid confusion. If the parsed
-					// variables comes from an auto-file however, it's not input
-					// directly by the user so we have to ignore it.
-					if parsedVar.Value.Equals(plannedVar).False() && parsedVar.SourceType != terraform.ValueFromAutoFile {
-						diags = diags.Append(&hcl.Diagnostic{
-							Severity: hcl.DiagError,
-							Summary:  "Can't change variable when applying a saved plan",
-							Detail:   fmt.Sprintf("The variable %s cannot be set using the -var and -var-file options when applying a saved plan file, because a saved plan includes the variable values that were set when it was created. The saved plan specifies %s as the value whereas during apply the value %s was %s. To declare an ephemeral variable which is not saved in the plan file, use ephemeral = true.", varName, tfdiags.CompactValueStr(parsedVar.Value), tfdiags.CompactValueStr(plannedVar), parsedVar.SourceType.DiagnosticLabel()),
-							Subject:  rng,
-						})
+					// error when possible to avoid confusion.
+					if parsedVar.Value.Equals(plannedVar).False() {
+						switch parsedVar.SourceType {
+						case terraform.ValueFromAutoFile:
+							// If the parsed variables comes from an auto-file,
+							// it's not input directly by the user so we have to ignore it.
+							continue
+						case terraform.ValueFromEnvVar:
+							diags = diags.Append(&hcl.Diagnostic{
+								Severity: hcl.DiagWarning,
+								Summary:  "Ignoring variable when applying a saved plan",
+								Detail: fmt.Sprintf("The variable %s cannot be overriden when applying a saved plan file, "+
+									"because a saved plan includes the variable values that were set when it was created. "+
+									"The saved plan specifies %s as the value whereas during apply the value %s was %s. "+
+									"To declare an ephemeral variable which is not saved in the plan file, use ephemeral = true.",
+									varName, tfdiags.CompactValueStr(plannedVar), tfdiags.CompactValueStr(parsedVar.Value),
+									parsedVar.SourceType.DiagnosticLabel()),
+								Subject: rng,
+							})
+						case terraform.ValueFromCLIArg, terraform.ValueFromNamedFile:
+							diags = diags.Append(&hcl.Diagnostic{
+								Severity: hcl.DiagError,
+								Summary:  "Can't change variable when applying a saved plan",
+								Detail: fmt.Sprintf("The variable %s cannot be set using the -var and -var-file options when "+
+									"applying a saved plan file, because a saved plan includes the variable values that were "+
+									"set when it was created. The saved plan specifies %s as the value whereas during apply "+
+									"the value %s was %s. To declare an ephemeral variable which is not saved in the plan "+
+									"file, use ephemeral = true.",
+									varName, tfdiags.CompactValueStr(plannedVar), tfdiags.CompactValueStr(parsedVar.Value),
+									parsedVar.SourceType.DiagnosticLabel()),
+								Subject: rng,
+							})
+						default:
+							// Other SourceTypes should never reach this point because
+							//  - ValueFromConfig - supplied plan already contains the original configuration
+							//  - ValueFromInput - we disable prompt when plan file is supplied
+							//  - ValueFromCaller - only used in tests
+							panic(fmt.Sprintf("Attempted to change variable %s when applying a saved plan. "+
+								"The saved plan specifies %s as the value whereas during apply the value %s was %s. "+
+								"This is a bug in Terraform, please report it.",
+								varName, tfdiags.CompactValueStr(plannedVar), tfdiags.CompactValueStr(parsedVar.Value),
+								parsedVar.SourceType.DiagnosticLabel()))
+						}
 					}
 				}
 			}