add tfdiags.GetAttribute

The only contextual information returned by the plugins in a diagnostic
is an attribute path. In order to test the conversions, we need to be
able to extract the cty.Path from a diagnostic if it exists.

tfdiags/contextual.go

@@ -80,6 +80,19 @@ func AttributeValue(severity Severity, summary, detail string, attrPath cty.Path
 	}
 }
 
+// GetAttribute extracts an attribute cty.Path from a diagnostic if it contains
+// one. Normally this is not accessed directly, and instead the config body is
+// added to the Diagnostic to create a more complete message for the user. In
+// some cases however, we may want to know just the name of the attribute that
+// generated the Diagnostic message.
+// This returns a nil cty.Path if it does not exist in the Diagnostic.
+func GetAttribute(d Diagnostic) cty.Path {
+	if d, ok := d.(*attributeDiagnostic); ok {
+		return d.attrPath
+	}
+	return nil
+}
+
 type attributeDiagnostic struct {
 	diagnosticBase
 	attrPath cty.Path

tfdiags/contextual_test.go

@@ -1,6 +1,7 @@
 package tfdiags
 
 import (
+	"reflect"
 	"testing"
 
 	"github.com/go-test/deep"
@@ -137,3 +138,23 @@ baz "b" {
 		t.Error(problem)
 	}
 }
+
+func TestGetAttribute(t *testing.T) {
+	path := cty.Path{
+		cty.GetAttrStep{Name: "foo"},
+		cty.IndexStep{Key: cty.NumberIntVal(0)},
+		cty.GetAttrStep{Name: "bar"},
+	}
+
+	d := AttributeValue(
+		Error,
+		"foo[0].bar",
+		"detail",
+		path,
+	)
+
+	p := GetAttribute(d)
+	if !reflect.DeepEqual(path, p) {
+		t.Fatalf("paths don't match:\nexpected: %#v\ngot: %#v", path, p)
+	}
+}