ephemeral: improve diagnostic message through compactValueStr

internal/backend/local/backend_apply.go

@@ -16,6 +16,7 @@ import (
 	"github.com/hashicorp/terraform/internal/addrs"
 	"github.com/hashicorp/terraform/internal/backend/backendrun"
 	"github.com/hashicorp/terraform/internal/command/views"
+	viewsjson "github.com/hashicorp/terraform/internal/command/views/json"
 	"github.com/hashicorp/terraform/internal/configs"
 	"github.com/hashicorp/terraform/internal/logging"
 	"github.com/hashicorp/terraform/internal/plans"
@@ -356,7 +357,7 @@ func (b *Local) opApply(
 						diags = diags.Append(&hcl.Diagnostic{
 							Severity: hcl.DiagError,
 							Summary:  "Can't set 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 %q as the value whereas during apply the value %q was %s. To declare an ephemeral variable which is not saved in the plan file, use ephemeral = true.", varName, v.Value.GoString(), val.GoString(), v.SourceType.DiagnosticLabel()),
+							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, viewsjson.CompactValueStr(v.Value), viewsjson.CompactValueStr(val), v.SourceType.DiagnosticLabel()),
 							Subject:  rng,
 						})
 					}

internal/command/views/json/diagnostic.go

@@ -438,13 +438,17 @@ func parseRange(src []byte, rng hcl.Range) (*hcl.File, int) {
 	return file, offset
 }
 
-// compactValueStr produces a compact, single-line summary of a given value
+// CompactValueStr produces a compact, single-line summary of a given value
 // that is suitable for display in the UI.
 //
 // For primitives it returns a full representation, while for more complex
 // types it instead summarizes the type, size, etc to produce something
 // that is hopefully still somewhat useful but not as verbose as a rendering
 // of the entire data structure.
+func CompactValueStr(val cty.Value) string {
+	return compactValueStr(val)
+}
+
 func compactValueStr(val cty.Value) string {
 	// This is a specialized subset of value rendering tailored to producing
 	// helpful but concise messages in diagnostics. It is not comprehensive