test: emit a nice error message if the user uses ephemeral resource in terrafom test

internal/command/test_test.go

@@ -273,7 +273,7 @@ func TestTest_Runs(t *testing.T) {
 		"ephemeral_resource": {
 			expectedOut: []string{"0 passed, 1 failed."},
 			// TODO: Improve error message, say something about ephemeral resources not being accessible in tests due to their ephemeral nature
-			expectedErr: []string{"Ephemeral resource instance has expired"},
+			expectedErr: []string{"Ephemeral resource instance has expired", "Ephemeral resources not supported in the context of tests"},
 			code:        1,
 		},
 	}

internal/moduletest/eval_context.go

@@ -92,6 +92,12 @@ func (ec *EvalContext) Evaluate() (Status, cty.Value, tfdiags.Diagnostics) {
 		ruleDiags = ruleDiags.Append(moreDiags)
 		refs = append(refs, moreRefs...)
 
+		// We want to emit diagnostics if users are using ephemeral resources in their checks
+		// as they are not supported since they are closed before this is evaluated.
+		// We do not remove the diagnostic about the ephemeral resource being closed already as it
+		// might be useful to the user.
+		ruleDiags = ruleDiags.Append(diagsForEphemeralResources(refs))
+
 		hclCtx, moreDiags := scope.EvalContext(refs)
 		ruleDiags = ruleDiags.Append(moreDiags)
 		if moreDiags.HasErrors() {
@@ -198,6 +204,23 @@ func (ec *EvalContext) Evaluate() (Status, cty.Value, tfdiags.Diagnostics) {
 	return status, cty.ObjectVal(outputVals), diags
 }
 
+func diagsForEphemeralResources(refs []*addrs.Reference) (diags tfdiags.Diagnostics) {
+	for _, ref := range refs {
+		switch v := ref.Subject.(type) {
+		case addrs.ResourceInstance:
+			if v.Resource.Mode == addrs.EphemeralResourceMode {
+				diags = diags.Append(&hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  "Ephemeral resources not supported in the context of tests",
+					Detail:   "Ephemeral resources are not supported in the context of terraform test.",
+					Subject:  ref.SourceRange.ToHCL().Ptr(),
+				})
+			}
+		}
+	}
+	return diags
+}
+
 // evaluationData augments an underlying lang.Data -- presumably resulting
 // from a terraform.Context.PlanAndEval or terraform.Context.ApplyAndEval call --
 // with results from prior runs that should therefore be available when