terraform: test helpers for printing diagnostics in a useful way

We often want to bail out of a test if diagnostics are present, and it's
easiest to debug that when the diagnostics are printed in a compact but
complete manner that is non-trivial to produce.

Rather than duplicating that diagnostic formatting in every test, these
helpers allow us to succinctly print diagnostics and bail out when they
are present.

terraform/context_test.go

@@ -14,7 +14,6 @@ import (
 
 	"github.com/google/go-cmp/cmp"
 	"github.com/google/go-cmp/cmp/cmpopts"
-
 	"github.com/hashicorp/go-version"
 	"github.com/hashicorp/hil"
 	"github.com/zclconf/go-cty/cty"
@@ -31,6 +30,7 @@ import (
 	"github.com/hashicorp/terraform/provisioners"
 	"github.com/hashicorp/terraform/states"
 	"github.com/hashicorp/terraform/states/statefile"
+	"github.com/hashicorp/terraform/tfdiags"
 	tfversion "github.com/hashicorp/terraform/version"
 )
 
@@ -944,6 +944,64 @@ func legacyDiffComparisonString(changes *plans.Changes) string {
 	return buf.String()
 }
 
+// assertNoDiagnostics fails the test in progress (using t.Fatal) if the given
+// diagnostics is non-empty.
+func assertNoDiagnostics(t *testing.T, diags tfdiags.Diagnostics) {
+	t.Helper()
+	if len(diags) == 0 {
+		return
+	}
+	logDiagnostics(t, diags)
+	t.FailNow()
+}
+
+// assertNoDiagnostics fails the test in progress (using t.Fatal) if the given
+// diagnostics has any errors.
+func assertNoErrors(t *testing.T, diags tfdiags.Diagnostics) {
+	t.Helper()
+	if !diags.HasErrors() {
+		return
+	}
+	logDiagnostics(t, diags)
+	t.FailNow()
+}
+
+// logDiagnostics is a test helper that logs the given diagnostics to to the
+// given testing.T using t.Log, in a way that is hopefully useful in debugging
+// a test. It does not generate any errors or fail the test. See
+// assertNoDiagnostics and assertNoErrors for more specific helpers that can
+// also fail the test.
+func logDiagnostics(t *testing.T, diags tfdiags.Diagnostics) {
+	for _, diag := range diags {
+		desc := diag.Description()
+		rng := diag.Source()
+
+		var severity string
+		switch diag.Severity() {
+		case tfdiags.Error:
+			severity = "ERROR"
+		case tfdiags.Warning:
+			severity = "WARN"
+		default:
+			severity = "???" // should never happen
+		}
+
+		if subj := rng.Subject; subj != nil {
+			if desc.Detail == "" {
+				t.Logf("[%s@%s] %s", severity, subj.StartString(), desc.Summary)
+			} else {
+				t.Logf("[%s@%s] %s: %s", severity, subj.StartString(), desc.Summary, desc.Detail)
+			}
+		} else {
+			if desc.Detail == "" {
+				t.Logf("[%s] %s", severity, desc.Summary)
+			} else {
+				t.Logf("[%s] %s: %s", severity, desc.Summary, desc.Detail)
+			}
+		}
+	}
+}
+
 const testContextGraph = `
 root: root
 aws_instance.bar